Christian Kästner

Assistant Professor · Carnegie Mellon University · Institute for Software Research

 
 
 

Publications Publication feed

Key publications highlighted in yellow.

J. Al-Kofahi, T. Nguyen, and C. Kästner. Escaping AutoHell: A Vision For Automated Analysis and Migration of Autotools Build Systems. In Proceedings of the 4rd International Workshop on Release Engineering (Releng), New York, NY: ACM Press, November 2016. [ .pdf, bib ]

GNU Autotools is a widely used build tool in the open source community. As open source projects grow more complex, maintaining their build systems becomes more challenges, due to the lack of tool support. Here we propose a platform to mitigate this problem, and aid developers by providing a platform to build support tools for GNU Autotools build systems. The platform would provide an abstract approximation for the build system to be used in different analysis techniques.

J. Meinicke, C. Wong, C. Kästner, T. Thüm, and G. Saake. On Essential Configuration Complexity: Measuring Interactions In Highly-Configurable Systems. In Proceedings of the 31st IEEE/ACM International Conference on Automated Software Engineering (ASE), New York, NY: ACM Press, September 2016. Acceptance rate: 19 % (57/298). [ .pdf, bib ]

Quality assurance for highly-configurable systems is challenging due to the exponentially growing configuration space. Interactions among multiple options can lead to surprising behaviors, bugs, and security vulnerabilities. Analyzing all configurations systematically might be possible though if most options do not interact or interactions follow specific patterns that can be exploited by analysis tools. To better understand interactions in practice, we analyze program traces to identify where interactions occur on control flow and data. To this end, we developed a dynamic analysis for Java based on variability-aware execution and monitor executions of multiple mid-sized real-world programs. We find that the essential configuration complexity of these programs is indeed much lower than the combinatorial explosion of the configuration space indicates, but also that the interaction characteristics that allow scalable and complete analyses are more nuanced than what is exploited by existing state-of-the-art quality assurance strategies.

P. Kawthekar, and C. Kästner. Sensitivity Analysis For Building Evolving & Adaptive Robotic Software. In Proceedings of the IJCAI Workshop on Autonomous Mobile Service Robots (WSR), July 2016. [ .pdf, bib ]

There has been a considerable growth in research and development of service robots in recent years. For deployment in diverse environment conditions for a wide range of service tasks, novel features and algorithms are developed and existing ones undergo change. However, developing and evolving the robot software requires making and revising many design decisions that can affect the quality of performance of the robots and that are non-trivial to reason about intuitively because of interactions among them. We propose to use sensitivity analysis to build models of the quality of performance to the different design decisions to ease design and evolution. Moreover, we envision these models to be used for run-time adaptation in response to changing goals or environment conditions. Constructing these models is challenging due to the exponential size of the decision space. We build on previous work on performance influence models of highly-configurable software systems using a machine-learning-based approach to construct influence models for robotic software.

C. Bogart, C. Kästner, J. Herbsleb, and F. Thung. How to Break an API: Cost Negotiation and Community Values in Three Software Ecosystems. In Proceedings of the ACM SIGSOFT Symposium on the Foundations of Software Engineering (FSE), New York, NY: ACM Press, November 2016. Acceptance rate: 27 % (74/273). [ http, .pdf, bib ]

Change introduces conflict into software ecosystems: breaking changes may ripple through the ecosystem and trigger rework for users of a package, but often developers can invest additional effort or accept opportunity costs to alleviate or delay downstream costs. We performed a multiple case study of three software ecosystems with different tooling and philosophies toward change, Eclipse, R/CRAN, and Node.js/npm, to understand how developers make decisions about change and change-related costs and what practices, tooling, and policies are used. We found that all three ecosystems differ substantially in their practices and expectations toward change and that those differences can be explained largely by different community values in each ecosystem. Our results illustrate that there is a large design space in how to build an ecosystem, its policies and its supporting infrastructure; and there is value in making community values and accepted tradeoffs explicit and transparent in order to resolve conflicts and negotiate change-related costs.

G. Ferreira, M. Malik, C. Kästner, J. Pfeffer, and S. Apel. Do #ifdefs Influence the Occurrence of Vulnerabilities? An Empirical Study of the Linux Kernel. In Proceedings of the 20th International Software Product Line Conference (SPLC), New York, NY: ACM Press, September 2016. Acceptance rate: 39 % (17/44). [ .pdf, bib ]

Preprocessors support the diversification of software products with #ifdefs, but also require additional effort from developers to maintain and understand variable code. We conjecture that #ifdefs cause developers to produce more vulnerable code because they are required to reason about multiple features simultaneously and maintain complex mental models of dependencies of configurable code. We extracted a variational call graph across all configurations of the Linux kernel, and used configuration complexity metrics to compare vulnerable and non-vulnerable functions considering their vulnerability history. Our goal was to learn about whether we can observe a measurable influence of configuration complexity on the occurrence of vulnerabilities. Our results suggest, among others, that vulnerable functions have higher variability than non-vulnerable ones and are also constrained by fewer configuration options. This suggests that developers are inclined to notice functions appear in frequently-compiled product variants. We aim to raise developers’ awareness to address variability more systematically, since configuration complexity is an important, but often ignored aspect of software product lines.

W. Ahmad, C. Kästner, J. Sunshine, and J. Aldrich. Inter-app Communication in Android: Developer Challenges. In Proceedings of the 13th International Conference on Mining Software Repositories (MSR), New York, NY: ACM Press, May 2016. Acceptance rate: 27 % (36/133). [ .pdf, bib ]

The Android platform is designed to support mutually untrusted third-party apps, which run as isolated processes but may interact via platform-controlled mechanisms, called Intents. Interactions among third-party apps are intended and can contribute to a rich user experience, for example, the ability to share pictures from one app with another. The Android platform presents an interesting point in a design space of module systems that is biased toward isolation, extensibility, and untrusted contributions. The Intent mechanism essentially provides message channels among modules, in which the set of message types is extensible. However, the module system has design limitations including the lack of consistent mechanisms to document message types, very limited checking that a message conforms to its specification, the inability to explicitly declare dependencies on other modules, and the lack of checks for backward compatibility as message types evolve over time. In order to understand the degree to which these design limitations result in real issues, we studied a broad corpus of apps and cross-validated our results against app documentation and Android support forums. Our findings suggest that design limitations do indeed cause development problems. Based on our results, we outline further research questions and propose possible mitigation strategies..

F. Medeiros, C. Kästner, M. Ribeiro, R. Gheyi, and S. Apel. A Comparison of 10 Sampling Algorithms for Configurable Systems. In Proceedings of the 38th International Conference on Software Engineering (ICSE), New York, NY: ACM Press, May 2016. Acceptance rate: 19 % (101/530). [ .pdf, bib ]

Almost every software system provides configuration options to tailor the system to the target platform and application scenario. Often, this configurability renders the analysis of every individual system configuration infeasible. To address this problem, researchers proposed a diverse set of sampling algorithms. We present a comparative study of 10 state-of-the-art sampling algorithms regarding their fault-detection capability and size of sample sets. The former is important to improve software quality and the latter to reduce the time of analysis. In a nutshell, we found that the sampling algorithms with larger sample sets detected higher numbers of faults. Furthermore, we observed that the limiting assumptions made in previous work influence the number of detected faults, the size of sample sets, and the ranking of algorithms. Finally, we identified a number of technical challenges when trying to avoid the limiting assumptions, which question the practicality of certain sampling algorithms.

J. Herbsleb, C. Kästner, and C. Bogart. Intelligently Transparent Software Ecosystems. IEEE Software (IEEE-Sw), 33(1):89--96, 2015. [ http, doi, .pdf, bib ]

Today’s social coding tools foreshadow a transformation of the software industry, as it increasingly relies on open libraries, frameworks, and code fragments. Our vision calls for new “intelligently transparent” services that support rapid development of innovative products while managing risk and receiving early warnings of looming failures. Intelligent transparency is enabled by an infrastructure that applies analytics to data from all phases of the lifecycle of open source projects, from development to deployment, bringing stakeholders the information they need when they need it.

C. Bogart, C. Kästner, and J. Herbsleb. When it Breaks, it Breaks: How Ecosystem Developers Reason About the Stability of Dependencies. In Proceedings of the ASE Workshop on Software Support for Collaborative and Global Software Engineering (SCGSE), November 2015. [ .pdf, bib ]

Dependencies among software projects and libraries are an indicator of the often implicit collaboration among many developers in software ecosystems. Negotiating change can be tricky: changes to one module may cause ripple effects to many other modules that depend on it, yet insisting on only backward-compatible changes may incur significant opportunity cost and stifle change. We argue that awareness mechanisms based on various notions of stability can enable developers to make decisions that are independent yet wise and provide stewardship rather than disruption to the ecosystem. In ongoing interviews with developers in two software ecosystems (CRAN and Node.js), we are finding that developers in fact struggle with change, that they often use adhoc mechanisms to negotiate change, and that existing awareness mechanisms like Github notification feeds are rarely used due to information overload. We study the state of the art and current information needs and outline a vision toward a change-based awareness system.

W. Ahmad, J. Sunshine, C. Kästner, and A. Wynne. Enforcing Fine-Grained Security and Privacy Policies in an Ecosystem within an Ecosystem. In Proceedings of the 3rd International Workshop on Mobile Development Lifecycle (MobileDeLi), pages 28--34, October 2015. [ .pdf, doi, bib ]

Smart home automation and IoT promise to bring many advantages but they also expose their users to certain security and privacy vulnerabilities. For example, leaking the information about the absence of a person from home or the medicine somebody is taking may have serious security and privacy consequences for home users and potential legal implications for providers of home automation and IoT platforms. We envision that a new ecosystem within an existing smartphone ecosystem will be a suitable platform for distribution of apps for smart home and IoT devices. Android is increasingly becoming a popular platform for smart home and IoT devices and applications. Built-in security mechanisms in ecosystems such as Android have limitations that can be exploited by malicious apps to leak users’ sensitive data to unintended recipients. For instance, Android enforces that an app requires the Internet permission in order to access a web server but it does not control which servers the app talks to or what data it shares with other apps. Therefore, sub-ecosystems that enforce additional fine-grained custom policies on top of existing policies of the smartphone ecosystems are necessary for smart home or IoT platforms. To this end, we have built a tool that enforces additional policies on inter-app interactions and permissions of Android apps. We have done preliminary testing of our tool on three proprietary apps developed by a future provider of a home automation platform. Our initial evaluation demonstrates that it is possible to develop mechanisms that allow definition and enforcement of custom security policies appropriate for ecosystems of the like smart home automation and IoT.

H. Nguyen, M. Nguyen, S. Dang, C. Kästner, and T. Nguyen. Detecting Semantic Merge Conflicts With Variability-Aware Execution. In Proceedings of the International Symposium on Foundations of Software Engineering -- New Ideas Track (ESEC/FSE-NIER), pages 926--929, New York, NY: ACM Press, August 2015. [ .pdf, doi, bib ]

In collaborative software development, when two or more developers incorporate their changes, a merge conflict may arise if the changes are incompatible. Previous research has shown that such conflicts are common and occur as textual conflicts or build/test failure, i.e., semantic conflicts. When a merge conflict occurs for a large number of parallel changes, it is desirable to identify the actual (minimum) set of changes that directly results in the conflict. Pinpointing the specific conflicting changes directly leading to test failure facilitates quick accountability and correction from developers. For semantic conflicts, to identify such subset of the changes is challenging. A naive approach trying all possible subsets would not scale due to the exponential number of possible combinations. We propose Semex, a novel approach to detect semantic conflicts using variability-aware execution. In the first step, we encode all parallel changes into a single program with variability in which we use symbolic variables to represent whether a given change is applied to the original program. In the second step, we run the test cases via variability-aware execution. Variability-aware execution explores all possible executions of the combined program with regard to all possible values of the symbolic variables representing all changes, and returns a propositional formula over the set of variables repre- senting the condition in which a test case fails. Due to our encoding algorithm, such a set corresponds to the minimum set of changes that are responsible for the conflict. In our preliminary experimental study on seven PHP applications with a total of 50 test cases and 19 semantic conflicts, Semex correctly detected all 19 conflicts.

N. Siegmund, A. Grebhahn, C. Kästner, and S. Apel. Performance-Influence Models for Highly Configurable Systems. In Proceedings of the European Software Engineering Conference and ACM SIGSOFT Symposium on the Foundations of Software Engineering (ESEC/FSE), pages 284--294, New York, NY: ACM Press, August 2015. Acceptance rate: 25 % (74/291). [ .pdf, bib ]

Almost every complex software system today is configurable. While configurability has many benefits, it challenges performance prediction, optimization, and debugging. Often, the influences of the individual configurations options on performance is unknown. Worse, configuration options may interact, giving rise to a configuration space of possibly exponential size. Addressing this challenge, we propose an approach that derives a performance-influence model for a given configurable system, describing all relevant influences of configuration options and their interactions. Such a model shall be useful for automatic performance prediction and optimization, on the one hand, and performance debugging for developers, on the other hand. Our approach combines machine-learning and sampling technique in a novel way. Our approach improves over standard techniques in that it (1) represents influences of options and their interactions explicitly (which eases debugging), (2) smoothly integrates binary and numeric configuration options for the first time, (3) incorporates domain knowledge, if available (which eases learning and increases accuracy), (4) considers complex constraints among options, and (5) systematically reduces the solution space to a tractable size. A series of experiments demonstrates the feasibility of our approach in terms of the accuracy of the models learned as well as the accuracy of the performances predictions one can make with them. Using our approach, we were able to identify a number of real performance bugs and other problems in real-world systems.

H. Nguyen, C. Kästner, and T. Nguyen. Cross-language Program Slicing for Dynamic Web Applications. In Proceedings of the European Software Engineering Conference and ACM SIGSOFT Symposium on the Foundations of Software Engineering (ESEC/FSE), pages 369--380, New York, NY: ACM Press, August 2015. Acceptance rate: 25 % (74/291). [ .pdf, bib ]

During software maintenance, program slicing is a useful technique to assist developers in understanding the impact of their changes. While different program-slicing techniques have been proposed for traditional software systems, program slicing for dynamic web applications is challenging since the client-side code is generated from the server-side code and data entities are referenced across different languages and are often embedded in string literals in the server-side program. To address those challenges, we introduce WebSlice, an approach to compute program slices across different languages for web applications. We first identify data-flow dependencies among data entities for PHP code based on symbolic execution. We also compute SQL queries and a conditional DOM that represents client-code variations and construct the data flows for embedded languages: SQL, HTML, and JavaScript. Next, we connect the data flows across different languages and those across PHP pages. Finally, we compute a program slice for any given entity based on the established data flows. Running WebSlice on five real-world PHP systems, we found that out of 40,670 program slices, 10 % cross languages, 38 % cross files, and 13 % cross string fragments, demonstrating the potential benefit of tool support for cross-language program slicing in web applications.

S. Nadi, T. Berger, C. Kästner, and K. Czarnecki. Where do Configuration Constraints Stem From? An Extraction Approach and an Empirical Study. IEEE Transactions on Software Engineering (TSE), 41(8):820--841, 2015. [ .pdf, doi, bib ]

Highly configurable systems allow users to tailor software to specific needs. Valid combinations of configuration options are often restricted by intricate constraints. Describing options and constraints in a variability model allows reasoning about the supported configurations. To automate creating and verifying such models, we need to identify the origin of such constraints. We propose a static analysis approach, based on two rules, to extract configuration constraints from code. We apply it on four highly configurable systems to evaluate the accuracy of our approach and to determine which constraints are recoverable from the code. We find that our approach is highly accurate (93 % and 77 % respectively) and that we can recover 28 % of existing constraints. We complement our approach with a qualitative study to identify constraint sources, triangulating results from our automatic extraction, manual inspections, and interviews with 27 developers. We find that, apart from low-level implementation dependencies, configuration constraints enforce correct runtime behavior, improve users’ configuration experience, and prevent corner cases. While the majority of constraints is extractable from code, our results indicate that creating a complete model requires further substantial domain knowledge and testing. Our results aim at supporting researchers and practitioners working on variability model engineering, evolution, and verification techniques.

F. Medeiros, C. Kästner, M. Ribeiro, S. Nadi, and R. Gheyi. The Love/Hate Relationship with The C Preprocessor: An Interview Study. In Proceedings of the 29th European Conference on Object-Oriented Programming (ECOOP), volume 37 of Leibniz International Proceedings in Informatics, pages 495--518, Dagstuhl, Germany: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2015. [ .pdf, doi, bib ]

The C preprocessor has received strong criticism in academia, among others regarding separation of concerns, error proneness, and code obfuscation, but is widely used in practice. Many (mostly academic) alternatives to the preprocessor exist, but have not been adopted in practice. Since developers continue to use the preprocessor despite all criticism and research, we ask how practitioners perceive the C preprocessor. We performed interviews with 40 developers, used grounded theory to analyze the data, and cross-validated the results with data from a survey among 202 developers, repository mining, and results from previous studies. In particular, we investigated four research questions related to why the preprocessor is still widely used in practice, common problems, alternatives, and the impact of undisciplined annotations. Our study shows that developers are aware of the criticism the C preprocessor receives, but use it nonetheless, mainly for portability and variability. They indicate that they regularly face preprocessor-related problems and preprocessor-related bugs. The majority of our interviewees do not see any current C-native technologies that can entirely replace the C preprocessor. However, developers tend to mitigate problems with guidelines, but those guidelines are not enforced consistently. We report the key insights gained from our study and discuss implications for practitioners and researchers on how to better use the C preprocessor to minimize its negative impact.

S. Zhou, J. Al-Kofahi, T. Nguyen, C. Kästner, and S. Nadi. Extracting Configuration Knowledge from Build Files with Symbolic Analysis. In Proceedings of the 3rd International Workshop on Release Engineering (Releng), pages 20--23, New York, NY: ACM Press, May 2015. [ .pdf, doi, bib ]

Build systems contain a lot of configuration knowledge about a software system, such as under which conditions specific files are compiled. Extracting such configuration knowledge is important for many tools analyzing highly-configurable systems, but very challenging due to the complex nature of build systems. We design an approach, based on SYMake, that symbolically evaluates Makefiles and extracts configuration knowledge in terms of file presence conditions and conditional parameters. We implement an initial prototype and demonstrate feasibility on small examples.

H. Nguyen, C. Kästner, and T. Nguyen. Varis: IDE Support for Embedded Client Code in PHP Web Applications. In Proceedings of the 37th International Conference on Software Engineering (Volume 2) (ICSE), pages 693--696, May 2015. Formal Demonstration paper, Best Demonstration Award. [ .pdf, doi, bib ]

In software development, IDE services such as syntax highlighting, code completion, and “jump to declara- tion” are used to assist developers in programming tasks. In dynamic web applications, however, since the client-side code is dynamically generated from the server-side code and is embedded in the server-side program as string literals, providing IDE services for such embedded code is challenging. In this work, we introduce Varis, a tool that provides editor services on the client-side code of a PHP-based web application, while it is still embedded within server-side code. Technically, we first perform symbolic execution on a PHP program to approximate all possible variations of the generated client-side code and subsequently parse this client code into a VarDOM that compactly represents all its variations. Finally, using the VarDOM, we implement various types of IDE services for embedded client code including syntax highlighting, code completion, and “jump to declaration”.

S. Nadi, T. Berger, C. Kästner, and K. Czarnecki. Where do Configuration Constraints Stem From? An Extraction Approach and an Empirical Study. Technical Report GSDLAB-TR 2015-01-27, Waterloo, ON, Canada: Generative Software Development Laboratory, University of Waterloo, January 2015. [ .pdf, http, bib ]
C. Hunsen, J. Siegmund, O. Leßenich, S. Apel, B. Zhang, C. Kästner, and M. Becker. Preprocessor-Based Variability in Open-Source and Industrial Software Systems: An Empirical Study. Empirical Software Engineering (EMSE), Special Issue on Empirical Evidence on Software Product Line Engineering, 1--34, 2015. [ .pdf, doi, bib ]

Almost every sufficiently complex software system today is configurable. Conditional compilation is a simple variability-implementation mechanism that is widely used in open-source projects and industry. Especially, the C preprocessor (cpp) is very popular in practice, but it is also gaining (again) interest in academia. Although there have been several attempts to understand and improve cpp, there is a lack of understanding of how it is used in open-source and industrial systems and whether different usage patterns have emerged. The background is that much research on configurable systems and product lines concentrates on open-source systems, simply because they are available for study in the first place. This leads to the potentially problematic situation that it is unclear whether the results obtained from these studies are transferable to industrial systems. We aim at lowering this gap by comparing the use of cpp in open-source projects and industry—especially from the embedded-systems domain—, based on a substantial set of subject systems and well-known variability metrics, including size, scattering, and tangling metrics. A key result of our empirical study is that, regarding almost all aspects we studied, the analyzed open-source systems and the considered embedded systems from industry have comparable distributions regarding most metrics, including systems that have been developed in industry and made open source at some point. So, our study indicates that, regarding cpp as variability-implementation mechanism, insights, methods, and tools developed based on studies of open- source systems are transferable to industrial systems—at least, with respect to the metrics we considered.

M. Lillack, C. Kästner, and E. Bodden. Tracking Load-time Configuration Options. In Proceedings of the 29th IEEE/ACM International Conference on Automated Software Engineering (ASE), pages 445--456, Los Alamitos, CA: IEEE Computer Society, September 2014. Acceptance rate: 20 % (55/276). [ .pdf, doi, bib ]

Highly-configurable software systems are pervasive, although configuration options and their interactions raise complexity of the program and increase maintenance effort. Especially load-time configuration options, such as parameters from command-line options or configuration files, are used with standard programming constructs such as variables and if statements intermixed with the program’s implementation; manually tracking configuration options from the time they are loaded to the point where they may influence control-flow decisions is tedious and error prone. We design and implement Lotrack, an extended static taint analysis to automatically track configuration options. Lotrack derives a configuration map that explains for each code fragment under which configurations it may be executed. An evaluation on Android applications shows that Lotrack yields high accuracy with reasonable performance. We use Lotrack to empirically characterize how much of the implementation of Android apps depends on the platform’s configuration options or interactions of these options.

H. Nguyen, C. Kästner, and T. Nguyen. Building Call Graphs for Embedded Client-Side Code in Dynamic Web Applications. In Proceedings of the ACM SIGSOFT Symposium on the Foundations of Software Engineering (FSE), pages 518--529, New York, NY: ACM Press, November 2014. Acceptance rate: 22 % (61/273). [ .pdf, doi, bib ]

When developing and maintaining a software system, programmers often rely on IDEs to provide editor services such as syntax highlighting, auto-completion, and “jump to declaration”. In dynamic web applications, such tool support is currently limited to either the server-side code or to hand-written or generated client-side code. Our goal is to build a call graph for providing editor services on client-side code while it is still embedded as string literals within server-side code. First, we symbolically execute the server-side code to identify all possible client-side code variations. Subsequently, we parse the generated client-side code with all its variations into a VarDOM that compactly represents all DOM variations for further analysis. Based on VarDOM, we build conditional call graphs for embedded HTML, CSS, and JS. Our empirical evaluation on real-world web applications show that our analysis achieves 100 % precision in identifying call-graph edges. 62 % of the edges cross PHP strings, and 17 % of them cross files—in both situations, navigation without tool support is tedious and error prone.

E. Walkingshaw, C. Kästner, M. Erwig, S. Apel, and E. Bodden. Variational Data Structures: Exploring Tradeoffs in Computing with Variability. In Proceedings of the 13rd SIGPLAN Symposium on New Ideas in Programming and Reflections on Software at SPLASH (Onward!), pages 213--226, New York, NY: ACM Press, 2014. [ .pdf, doi, bib ]

Variation is everywhere, but in the construction and analysis of customizable software it is paramount. In this context, there arises a need for variational data structures for efficiently representing and computing with related variants of an underlying data type. So far, variational data structures have been explored and developed ad hoc. This paper is a first attempt and a call to action for systematic and foundational research in this area. Research on variational data structures will benefit not only customizable software, but the many other application domains that must cope with variability. In this paper, we show how support for variation can be understood as a general and orthogonal property of data types, data structures, and algorithms. We begin a systematic exploration of basic variational data structures, exploring the tradeoffs between different implementations. Finally, we retrospectively analyze the design decisions in our own previous work where we have independently encountered problems requiring variational data structures.

Z. Coker, S. Hasan, J. Overbey, M. Hafiz, and C. Kästner. Integers In C: An Open Invitation to Security Attacks? Technical Report CSSE14-01, Auburn, AL: College of Engineering, Auburn University, February 2014. [ .pdf, bib ]

We performed an empirical study to explore how closely well-known, open source C programs follow the safe C standards for integer behavior, with the goal of understanding how difficult it is to migrate legacy code to these stricter standards. We performed an automated analysis on fifty-two releases of seven C programs (6 million lines of preprocessed C code), as well as releases of Busybox and Linux (nearly one billion lines of partially-preprocessed C code). We found that integer issues, that are allowed by the C standard but not by the safer C standards, are ubiquitous—one out of four integers were inconsistently declared, and one out of eight integers were inconsistently used. Integer issues did not improve over time as the programs evolved. Also, detecting the issues is complicated by a large number of integers whose types vary under different preprocessor configurations. Most of these issues are benign, but the chance of finding fatal errors and exploitable vulnerabilities among these many issues remains significant. A preprocessor-aware, tool-assisted approach may be the most viable way to migrate legacy C code to comply with the standards for secure programming.

T. Thüm, S. Apel, C. Kästner, I. Schaefer, and G. Saake. A Classification and Survey of Analysis Strategies for Software Product Lines. ACM Computing Surveys (CSUR), 47(1):Article 6, June 2014. [ .pdf, http, doi, bib ]

Software-product-line engineering has gained considerable momentum in recent years, both in industry and in academia. A software product line is a set of software products that share a common set of features. Software product lines challenge traditional analysis techniques, such as type checking, model checking, and theorem proving, in their quest of ensuring correctness and reliability of software. Simply creating and analyzing all products of a product line is usually not feasible, due to the potentially exponential number of valid feature combinations. Recently, researchers began to develop analysis techniques that take the distinguishing properties of software product lines into account, for example, by checking feature-related code in isolation or by exploiting variability information during analysis. The emerging field of product-line analyses is both broad and diverse, such that it is difficult for researchers and practitioners to understand their similarities and differences. We propose a classification of product-line analyses to enable systematic research and application. Based on our insights with classifying and comparing a corpus of 76 articles, we infer a research agenda to guide future research on product-line analyses.

J. Siegmund, C. Kästner, S. Apel, C. Parnin, A. Bethmann, T. Leich, G. Saake, and A. Brechmann. Understanding Understanding Source Code with Functional Magnetic Resonance Imaging. In Proceedings of the 36th International Conference on Software Engineering (ICSE), pages 378--389, June 2014. Acceptance rate: 20 % (99/495). [ .pdf, doi, bib ]

Program comprehension is an important cognitive process that inherently eludes direct measurement. Thus, researchers are struggling with providing optimal programming languages, tools, or coding conventions to support developers in their everyday work. With our approach, we explore whether functional magnetic resonance imaging (fMRI), which is well established in cognitive neuroscience, is feasible to directly measure program comprehension. To this end, we observed 17 participants inside an fMRI scanner while comprehending short source-code snippets, which we contrasted with locating syntax errors. We found a clear, distinct activation pattern of five brain regions, which are related to working memory, attention, and language processing—all processes that fit well to our understanding of program comprehension. Based on the results, we propose a model of program comprehension. Our results encourage us to use fMRI in future studies to measure program comprehension and, in the long run, answer questions, such as: Can we predict whether someone will be an excellent programmer? How effective are new languages and tools for program understanding? How do we train someone to become an excellent programmer?

H. Nguyen, C. Kästner, and T. Nguyen. Exploring Variability-Aware Execution for Testing Plugin-Based Web Applications. In Proceedings of the 36th International Conference on Software Engineering (ICSE), pages 907--918, June 2014. Acceptance rate: 20 % (99/495). [ .pdf, doi, bib ]

In plugin-based systems, plugin conflicts may occur when two or more plugins interfere with one another, changing their expected behaviors. It is highly challenging to detect plugin conflicts due to the exponential explosion of the combinations of plugins (i.e., configurations). In this paper, we address the challenge of executing a test case over many configurations. Leveraging the fact that many executions of a test are similar, our variability-aware execution runs common code once. Only when encountering values that are different depending on specific configurations will the execution split to run for each of them. To evaluate the scalability of variability-aware execution on a large real-world setting, we built a prototype PHP interpreter called Varex and ran it on the popular WordPress blogging Web application. The results show that while plugin interactions exist, there is a significant amount of sharing that allows variability-aware execution to scale to 2^50 configurations within seven minutes of running time. During our study, with Varex, we were able to detect two plugin conflicts: one was recently reported on WordPress forum, and another one is not yet discovered.

S. Nadi, T. Berger, C. Kästner, and K. Czarnecki. Mining Configuration Constraints: Static Analyses and Empirical Results. In Proceedings of the 36th International Conference on Software Engineering (ICSE), pages 140--151, June 2014. Acceptance rate: 20 % (99/495). [ .pdf, doi, bib ]

Highly-configurable systems allow users to tailor the software to their specific needs. Not all combinations of configuration options are valid though, and constraints arise for technical or non-technical reasons. Explicitly describing these constraints in a variability model allows reasoning about the supported configurations. To automate creating variability models, we need to identify the origin of such configuration constraints. We propose an approach which uses build-time errors and a novel feature-effect heuristic to automatically extract configuration constraints from C code. We conduct an empirical study on four highly-configurable open-source systems with existing variability models having three objectives in mind: evaluate the accuracy of our approach, determine the recoverability of existing variability-model constraints using our analysis, and classify the sources of variability-model constraints. We find that both our extraction heuristics are highly accurate (93 % and 77 % respectively), and that we can recover 19 % of the existing variability-models using our approach. However, we find that many of the remaining constraints require expert knowledge or more expensive analyses. We argue that our approach, tooling, and experimental results support researchers and practitioners working on variability model re-engineering, evolution, and consistency-checking techniques.

M. Ribeiro, P. Borba, and C. Kästner. Feature Maintenance with Emergent Interfaces. In Proceedings of the 36th International Conference on Software Engineering (ICSE), pages 989--1000, June 2014. Acceptance rate: 20 % (99/495). [ .pdf, doi, bib ]

Hidden code dependencies are responsible for many complications in maintenance tasks. With the introduction of variable features in product lines, dependencies may even cross feature boundaries and related problems are prone to be detected late. Many current implementation techniques for product lines lack proper interfaces, which could make such dependencies explicit. As alternative to changing the implementation approach, we provide a comprehensive tool-based solution to support developers in recognizing and dealing with feature dependencies: emergent interfaces. Emergent interfaces are computed on demand, based on feature-sensitive interprocedural data-flow analysis. They emerge in the IDE and emulate benefits of modularity not available in the host language. To evaluate the potential of emergent interfaces, we conducted and replicated a controlled experiment, and found, in the studied context, that emergent interfaces can improve performance of code change tasks by up to 3 times while also reducing the number of errors.

J. Feigenspan, C. Kästner, J. Liebig, S. Apel, and S. Hanenberg. Measuring and Modeling Programming Experience. Empirical Software Engineering (EMSE), 19(5):1299--1334, October 2014. [ .pdf, doi, bib ]

Programming experience is an important confounding parameter in controlled experiments regarding program comprehension. In literature, ways to measure or control programming experience vary. Often, researchers neglect it or do not specify how they controlled for it. We set out to find a well-defined understanding of programming experience and a way to measure it. From published comprehension experiments, we extracted questions that assess programming experience. In a controlled experiment, we compare the answers of computer-science students to these questions with their performance in solving program-comprehension tasks. We found that self estimation seems to be a reliable way to measure programming experience. Furthermore, we applied exploratory and confirmatory factor analyses to extract and evaluate a model of programming experience. With our analysis, we initiate a path toward validly and reliably measuring and describing programming experience to better understand and control its influence in program-comprehension experiments.

S. Apel, S. Kolesnikov, N. Siegmund, C. Kästner, and B. Garvin. Exploring Feature Interactions in the Wild: The New Feature-Interaction Challenge. In Proceedings of the 5th International Workshop on Feature-Oriented Software Development (FOSD), pages 1--8, New York, NY: ACM Press, October 2013. Acceptance rate: 75 % (6/8). [ .pdf, doi, bib ]

The feature-interaction problem has been keeping researchers and practitioners in suspense for years. Although there has been substantial progress in developing approaches for modeling, detecting, managing, and resolving feature interactions, we lack sufficient knowledge on the kind of feature interactions that occur in real-world systems. In this position paper, we set out the goal to explore the nature of feature interactions systematically and comprehensively, classified in terms of order and visibility. Understanding this nature will have significant implications on research in this area, for example, on the efficiency of interaction-detection or performance-prediction techniques. A set of preliminary results as well as a discussion of possible experimental setups and corresponding challenges give us confidence that this endeavor is within reach but requires a collaborative effort of the community.

C. Kästner, A. Dreiling, and K. Ostermann. Variability Mining: Consistent Semiautomatic Detection of Product-Line Features. IEEE Transactions on Software Engineering (TSE), 40(1):67--82, 2014. [ .pdf, doi, http, bib ]

Software product line engineering is an efficient means to generate a set of tailored software products from a common implementation. However, adopting a product-line approach poses a major challenge and significant risks, since typically legacy code must be migrated toward a product line. Our aim is to lower the adoption barrier by providing semiautomatic tool support—called variability mining—to support developers in locating, documenting, and extracting implementations of product-line features from legacy code. Variability mining combines prior work on concern location, reverse engineering, and variability-aware type systems, but is tailored specifically for the use in product lines. Our work pursues three technical goals: (1) we provide a consistency indicator based on a variability-aware type system, (2) we mine features at a fine level of granularity, and (3) we exploit domain knowledge about the relationship between features when available. With a quantitative study, we demonstrate that variability mining can efficiently support developers in locating features.

J. Liebig, A. von Rhein, C. Kästner, S. Apel, J. Dörre, and C. Lengauer. Scalable Analysis of Variable Software. In Proceedings of the European Software Engineering Conference and ACM SIGSOFT Symposium on the Foundations of Software Engineering (ESEC/FSE), pages 81--91, New York, NY: ACM Press, August 2013. Acceptance rate: 20 % (51/251). [ .pdf, doi, http, bib ]

The advent of proper variability management and generator technology enables users to derive individual variants from a variable code base solely based on a selection of desired configuration options. This approach gives rise to a huge configuration space, but the high degree of variability comes at a cost: classic analysis methods do not scale any more; there are simply too many potential variants to analyze. To address this issue, researchers and practitioners usually apply sampling techniques—only a subset of all possible variants is analyzed. While sampling promises to reduce the analysis effort significantly, the information obtained is necessarily incomplete. Furthermore, it is unknown whether sampling strategies scale to billions of variants, because even samples may be huge and expensive to compute. Recently, researchers have begun to develop variability-aware analyses that analyze the variable code base directly with the goal to exploit the similarities among individual variants to reduce analysis effort. However, while being promising, so far, variability-aware analyses have been applied mostly only to small academic systems. To learn about the mutual strengths and weaknesses of variability-aware and sampling-based analyses of large-scale, real-world software systems, we compared the two by means of two concrete analysis implementations (type checking and liveness analysis) applied to three subject systems: the Busybox tool suite, the x86 Linux kernel, and the cryptographic library OpenSSL. A key result is that in these settings already setting up sampling techniques is challenging while variability-aware analysis even outperforms most sampling approximations with respect to analysis time.

S. Apel, D. Batory, C. Kästner, and G. Saake. Feature-Oriented Software Product Lines: Concepts and Implementation. Berlin/Heidelberg: Springer-Verlag, 2013. 308 pages, ISBN 978-3-642-37520-0. [ http, bib ]

While standardization has empowered the software industry to substantially scale software development and to provide affordable software to a broad market, it often does not address smaller market segments, nor the needs and wishes of individual customers. Software product lines reconcile mass production and standardization with mass customization in software engineering. Ideally, based on a set of reusable parts, a software manufacturer can generate a software product based on the requirements of its customer. The concept of features is central to achieving this level of automation, because features bridge the gap between the requirements the customer has and the functionality a product provides. Thus features are a central concept in all phases of product-line development. The authors take a developer’s viewpoint, focus on the development, maintenance, and implementation of product-line variability, and especially concentrate on automated product derivation based on a user’s feature selection. The book consists of three parts. Part I provides a general introduction to feature-oriented software product lines, describing the product-line approach and introducing the product-line development process with its two elements of domain and application engineering. The pivotal Part II covers a wide variety of implementation techniques including design patterns, frameworks, components, feature-oriented programming, and aspect-oriented programming, as well as tool-based approaches including preprocessors, build systems, version-control systems, and virtual separation of concerns. Finally, Part III is devoted to advanced topics related to feature-oriented product lines like refactoring, feature interaction, and analysis tools specific to product lines. In addition, an Appendix lists various helpful tools for software product-line development, along with a description of how they relate to the topics covered in this book. To tie the book together, the authors use two running examples that are well documented in the product-line literature: data management for embedded systems, and variations of graph data structures. They start every chapter by explicitly stating the respective learning goals and finish it with a set of exercises; additional teaching material is also available online. All these features make the book ideally suited for teaching – both for academic classes and for professionals interested in self-study.

S. Apel, A. von Rhein, T. Thüm, and C. Kästner. Feature-Interaction Detection based on Feature-Based Specifications. Computer Networks (COMNET), Special Issue on Feature Interaction, 57(12):2399--2409, August 2013. [ .pdf, doi, bib ]

Formal specification and verification techniques have been used successfully to detect feature interactions. We investigate whether feature-based specifications can be used for this task. Feature-based specifications are a special class of specifications that aim at modularity in open-world, feature-oriented systems. The question we address is whether modularity of specifications impairs the ability to detect feature interactions, which cut across feature boundaries. In an exploratory study on 10 feature-oriented systems, we found that the majority of feature interactions could be detected based on feature-based specifications, but some specifications have not been modularized properly and require undesirable workarounds to modularization. Based on the study, we discuss the merits and limitations of feature-based specifications, as well as open issues and perspectives. A goal that underlies our work is to raise awareness of the importance and challenges of feature-based specification.

J. Siegmund, C. Kästner, S. Apel, A. Brechmann, and G. Saake. Experience from Measuring Program Comprehension -- Toward a General Framework. In Proceedings of the Software Engineering 2013 -- Fachtagung des GI-Fachbereichs Softwaretechnik (SE), volume P-213 of Lecture Notes in Informatics, pages 239--257, Bonn, Germany: Gesellschaft für Informatik (GI), February 2013. [ .pdf, http, bib ]

Program comprehension plays a crucial role during the software-development life cycle: Maintenance programmers spend most of their time with comprehending source code, and maintenance is the main cost factor in software development. Thus, if we can improve program comprehension, we can save considerable amount of time and cost. To improve program comprehension, we have to measure it first. However, program comprehension is a complex, internal cognitive process that we cannot observe directly. Typically, we need to conduct controlled experiments to soundly measure program comprehension. However, empirical research is applied only reluctantly in software engineering. To close this gap, we set out to support researchers in planning and conducting experiments regarding program comprehension. We report our experience with experiments that we conducted and present the resulting framework to support researchers in planning and conducting experiments. Additionally, we discuss the role of teaching for the empirical researchers of tomorrow.

P. Giarrusso, K. Ostermann, M. Eichberg, R. Mitschke, T. Rendel, and C. Kästner. Reify Your Collection Queries for Modularity and Speed! In Proceedings of the 12th ACM International Conference on Aspect-Oriented Software Development (AOSD), pages 1--12, New York, NY: ACM Press, March 2013. Acceptance rate: 24 % (17/72). [ .pdf, doi, bib ]

The collections API of a programming language forms an embedded domain-specific language to express queries and operations on collections. Unfortunately, the ordinary style of implementing such APIs does not allow automatic domain-specific analyses and optimizations such as fusion of collection traversals, usage of indexing, or reordering of filters. Performance-critical code using collections must instead be hand-optimized, leading to non-modular, brittle, and redundant code. We propose SQuOpt, the Scala Query Optimizer—a deep embedding of the Scala collections API that allows such analyses and optimizations to be defined and executed within Scala, with- out relying on external tools or compiler extensions. SQuOpt provides the same “look and feel” (syntax and static typing guar- antees) as the standard collections API. We evaluate SQuOpt by re-implementing several code analyses of the Findbugs tool using SQuOpt and demonstrate that SQuOpt can reconcile modularity and efficiency in real-world applications.

L. Passos, K. Czarnecki, S. Apel, A. Wąsowski, C. Kästner, and J. Guo. Feature Oriented Software Evolution. In Proceedings of the 7th Int'l Workshop on Variability Modelling of Software-Intensive Systems (VaMoS), pages 17:1--17:8, New York, NY: ACM Press, January 2013. Acceptance rate: 42 % (19/45). [ doi, .pdf, bib ]

Software product-line engineering aims at the development of families of related products that share common assets. An important aspect is that customers are often interested not only in particular functionalities (i.e., features), but also in non-functional quality attributes such as performance, reliability, and footprint. A naive approach is to measure quality attributes of every single product, and to deliver the products that fit the customers' needs. However, as product lines may consist of millions of products, this approach does not scale. In this research-in-progress report, we propose a systematic approach for the efficient and scalable prediction of quality attributes of products that consists of two steps. First, we generate predictors for certain categories of quality attributes (e.g., a predictor for low performance) based on software and network measures, and receiver operating characteristic analysis. Second, we use these predictors to guide a sampling process that takes the asset base of a product line as input and efficiently determines the products that fall into the category denoted by a given predictor (e.g., products with low performance). In other words, we use predictors to make the process of finding “acceptable” products more efficient. We discuss and compare several strategies to incorporate predictors in the sampling process.

S. Kolesnikov, S. Apel, N. Siegmund, S. Sobernig, C. Kästner, and S. Senkaya. Predicting Quality Attributes of Software Product Lines Using Software and Network Measures and Feature Sampling. In Proceedings of the 7th Int'l Workshop on Variability Modelling of Software-Intensive Systems (VaMoS), pages 6:1--6:5, New York, NY: ACM Press, January 2013. Acceptance rate: 42 % (19/45). [ doi, .pdf, bib ]

Software product-line engineering aims at the development of families of related products that share common assets. An important aspect is that customers are often interested not only in particular functionalities (i.e., features), but also in non-functional quality attributes such as performance, reliability, and footprint. A naive approach is to measure quality attributes of every single product, and to deliver the products that fit the customers' needs. However, as product lines may consist of millions of products, this approach does not scale. In this research-in-progress report, we propose a systematic approach for the efficient and scalable prediction of quality attributes of products that consists of two steps. First, we generate predictors for certain categories of quality attributes (e.g., a predictor for low performance) based on software and network measures, and receiver operating characteristic analysis. Second, we use these predictors to guide a sampling process that takes the asset base of a product line as input and efficiently determines the products that fall into the category denoted by a given predictor (e.g., products with low performance). In other words, we use predictors to make the process of finding “acceptable” products more efficient. We discuss and compare several strategies to incorporate predictors in the sampling process.

A. von Rhein, S. Apel, C. Kästner, T. Thüm, and I. Schaefer. The PLA Model: On the Combination of Product-Line Analyses. In Proceedings of the 7th Int'l Workshop on Variability Modelling of Software-Intensive Systems (VaMoS), pages 14:1--14:8, New York, NY: ACM Press, January 2013. Acceptance rate: 42 % (19/45). [ doi, .pdf, bib ]

Product-line analysis has received considerable attention in the past. As it is often infeasible to analyze each product of a product line individually, researchers have developed analyses, called variability-aware analyses, that consider and exploit variability manifested in a code base. Variability-aware analyses are often significantly more efficient than traditional analyses, but each of them has certain weaknesses regarding applicability or scalability, as we discuss in this paper. We present the Product-Line-Analysis Model, a formal model for the classification and comparison of existing analyses, including traditional and variability-aware analyses, and lay a foundation for formulating and exploring further, combined analyses. As a proof of concept, we discuss different examples of analyses in the light of our model, and demonstrate its benefits for systematic comparison and exploration of product-line analyses.

J. Liebig, A. von Rhein, C. Kästner, S. Apel, J. Dörre, and C. Lengauer. Large-Scale Variability-Aware Type Checking and Dataflow Analysis. Technical Report MIP-1212, Passau, Germany: Department of Informatics and Mathematics, University of Passau, November 2012. [ .pdf, bib ]
J. Siegmund, A. Brechmann, S. Apel, C. Kästner, J. Liebig, T. Leich, and G. Saake. Toward Measuring Program Comprehension with Functional Magnetic Resonance Imaging. In Proceedings of the 20th International Symposium on Foundations of Software Engineering -- New Ideas Track (FSE-NIER), pages 24:1--24:4, November 2012. Acceptance rate: 20 % (12/59). [ .pdf, doi, bib ]

Program comprehension is an often evaluated, internal cognitive process. In neuroscience, functional magnetic resonance (fMRI) imaging is used to visualize such internal cognitive processes. We propose an experimental design to measure program comprehension based on fMRI. In the long run, we hope to answer questions like What distinguishes good programmers from bad programmers? or What makes a good programmer?

C. Kästner, A. von Rhein, S. Erdweg, J. Pusch, S. Apel, T. Rendel, and K. Ostermann. Toward Variability-Aware Testing. In Proceedings of the 4th International Workshop on Feature-Oriented Software Development (FOSD), pages 1--8, New York, NY: ACM Press, September 2012. Acceptance rate: 57 % (8/14). [ .pdf, doi, bib ]

We investigate how to execute a unit test in all configurations of a product line without generating each product in isolation in a brute-force fashion. Learning from variability-aware analyses, we (a) design and implement a variability-aware interpreter and (b) reencode variability of the product line to simulate the test cases with a model checker. The interpreter internally reasons about variability, executing paths not affected by variability only once for the whole product line. The model checker achieves similar results by reusing powerful off-the-shelf analyses. We experimented with a prototype implementation for each strategy. We compare both strategies and discuss trade-offs and future directions.

J. Siegmund, C. Kästner, J. Liebig, and S. Apel. Comparing Program Comprehension of Physically and Virtually Separated Concerns. In Proceedings of the 4th International Workshop on Feature-Oriented Software Development (FOSD), pages 17--24, New York, NY: ACM Press, September 2012. Acceptance rate: 57 % (8/14). [ .pdf, doi, bib ]

It is common believe that separating source code along concerns or features improves program comprehension of source code. However, empirical evidence is mostly missing. In this paper, we design a controlled experiment to evaluate that believe for feature-oriented programming based on maintenance tasks with human participants. We validate our experiment with a pilot study, which already preliminarily confirms that students use different strategies to complete maintenance tasks.

S. Erdweg, T. Rendel, C. Kästner, and K. Ostermann. Layout-Sensitive Generalized Parsing. In Proceedings of the International Conference on Software Language Engineering (SLE), pages 244--263, Berlin/Heidelberg: Springer-Verlag, September 2012. Acceptance rate: 32 % (20/62). [ .pdf, doi, bib ]

The theory of context-free languages is well-understood and context-free parsers can be used as off-the-shelf tools in practice. In particular, to use a context-free parser framework, a user does not need to understand its internals but can specify a language declaratively as a grammar. However, many languages in practice are not context-free. One particularly important class of such languages is layout-sensitive languages, in which the structure of code depends on indentation and whitespace. For example, Python, Haskell, F\#, and Markdown use indentation instead of curly braces to determine the block structure of code. Their parsers (and lexers) are not declaratively specified but hand-tuned to account for layout-sensitivity. To support declarative specifications of layout-sensitive languages, we propose a parsing framework in which a user can annotate layout in a grammar as constraints on the relative positioning of tokens in the parsed subtrees. For example, a user can declare that a block consists of statements that all start on the same column. We have integrated layout constraints into SDF and implemented a layout-sensitive generalized parser as an extension of generalized LR parsing. We evaluate the correctness and performance of our parser by parsing 33290 open-source Haskell files. Layout-sensitive generalized parsing is easy to use, and its performance overhead compared to layout-insensitive parsing is small enough for most practical applications.

N. Siegmund, M. Rosenmüller, C. Kästner, P. Giarrusso, S. Apel, and S. Kolesnikov. Scalable Prediction of Non-functional Properties in Software Product Lines: Footprint and Memory Consumption. Information and Software Technology (IST), Special Issue on Software Reuse and Product Lines, 55(3):491--507, March 2013. [ .pdf, doi, http, bib ]

Context: A software product line is a family of related software products, typically created from a set of common assets. Users select features to derive a product that fulfills their needs. Users often expect a product to have specific non-functional properties, such as a small footprint or a bounded response time. Because a product line may have an exponential number of products with respect to its features, it is usually not feasible to generate and measure non-functional properties for each possible product. Objective: Our overall goal is to derive optimal products with respect to non-functional requirements by showing customers which features must be selected. Method: We propose an approach to predict a product’s non-functional properties based on the product’s feature selection. We aggregate the influence of each selected feature on a non-functional property to predict a product’s properties. We generate and measure a small set of products and, by comparing measurements, we approximate each feature’s influence on the non-functional property in question. As a research method, we conducted controlled experiments and evaluated prediction accuracy for the non-functional properties footprint and main-memory consumption. But, in principle, our approach is applicable for all quantifiable non-functional properties. Results: With nine software product lines, we demonstrate that our approach predicts the footprint with an average accuracy of 94\,\%, and an accuracy of over 99\,\% on average if feature interactions are known. In a further series of experiments, we predicted main memory consumption of six customizable programs and achieved an accuracy of 89\,\% on average. Conclusion: Our experiments suggest that, with only few measurements, it is possible to accurately predict non-functional properties of products of a product line. Furthermore, we show how already little domain knowledge can improve predictions and discuss trade-offs between accuracy and required number of measurements. With this technique, we provide a basis for many reasoning and product-derivation approaches.

P. Giarrusso, K. Ostermann, M. Eichberg, T. Rendel, and C. Kästner. Reifying and Optimizing Collection Queries for Modularity. In Proceedings of the Companion of the 26th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 77--78, New York, NY: ACM Press, 2012. Poster. [ .pdf, bib ]
C. Kästner, and S. Apel. Feature-Oriented Software Development: A Short Tutorial on Feature-Oriented Programming, Virtual Separation of Concerns, and Variability-Aware Analysis. In GTTSE Summer School: Generative & Transformational Techniques in Software Engineering, volume 7680 of Lecture Notes in Computer Science, pages 346--382, Berlin/Heidelberg: Springer-Verlag, 2011. [ .pdf, http, bib ]

Feature-oriented software development is a paradigm for the construction, customization, and synthesis of large-scale and variable software systems, focusing on structure, reuse and variation. In this tutorial, we provide a gentle introduction to software product lines, feature oriented programming, virtual separation of concerns, and variability- aware analysis. We provide an overview, show connections between the different lines of research, and highlight possible future research directions.

T. Thüm, C. Kästner, F. Benduhn, J. Meinicke, G. Saake, and T. Leich. FeatureIDE: An Extensible Framework for Feature-Oriented Software Development. Science of Computer Programming (SCP), Special Issue on Experimental Software and Toolkits, 79:70--85, 2014. [ doi, .pdf, bib ]

FeatureIDE is an open-source framework for feature-oriented software development (FOSD) based on Eclipse. FOSD is a paradigm for the construction, customization, and synthesis of software systems. Code artifacts are mapped to features, and a customized software system can be generated given a selection of features. The set of software systems that can be generated is called a software product line (SPL). FeatureIDE supports several FOSD implementation techniques such as feature-oriented programming, aspect-oriented programming, delta-oriented programming, and preprocessors. All phases of FOSD are supported in FeatureIDE, namely domain analysis, requirements analysis, domain implementation, and software generation.

C. Kästner, K. Ostermann, and S. Erdweg. A Variability-Aware Module System. In Proceedings of the 27th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 773--792, New York, NY: ACM Press, October 2012. Acceptance rate: 25 % (57/228). [ .pdf, doi, bib ]

Module systems enable a divide and conquer strategy to software development. To implement compile-time variability in software product lines, modules can be composed in different combinations. However, this way variability dictates a dominant decomposition. Instead, we introduce a variability-aware module system that supports compile-time variability inside a module and its interface. This way, each module can be considered a product line that can be type checked in isolation. Variability can crosscut multiple modules. The module system breaks with the antimodular tradition of a global variability model in product-line development and provides a path toward software ecosystems and product lines of product lines developed in an open fashion. We discuss the design and implementation of such a module system on a core calculus and provide an implementation for C, which we use to type check the open source product line Busybox with 811 compile-time options.

C. Kästner, K. Ostermann, and S. Erdweg. A Variability-Aware Module System. Technical Report 01/2012, Marburg, Germany: Department of Mathematics and Computer Science, Philipps University Marburg, April 2012. [ .pdf, bib ]

Module systems enable a divide and conquer strategy to software development. To implement compile-time variability in software product lines, modules can be composed in different combinations. However, this way variability dictates a dominant decomposition. Instead, we introduce a variability-aware module system that supports compile-time variability inside a module and its interface. This way, each module can be considered a product line that can be type checked in isolation. Variability can crosscut multiple modules. The module system breaks with the antimodular tradition of a global variability model in product-line development and provides a path toward software ecosystems and product lines of product lines developed in an open fashion. We discuss the design and implementation of such a module system on a core calculus and provide an implementation for C, which we use to type check the open source product line Busybox with 811 compile-time options.

T. Thüm, S. Apel, C. Kästner, M. Kuhlemann, I. Schaefer, and G. Saake. Analysis Strategies for Software Product Lines. Technical Report FIN-2012-04, Magdeburg, Germany: University of Magdeburg, April 2012. [ .pdf, bib ]

Software-product-line engineering has gained considerable momentum in recent years, both in industry and in academia. A software product line is a set of software products that share a common set of features. Software product lines challenge traditional analysis techniques, such as type checking, testing, and formal verification, in their quest of ensuring correctness and reliability of software. Simply creating and analyzing all products of a product line is usually not feasible, due to the potentially exponential number of valid feature combinations. Recently, researchers began to develop analysis techniques that take the distinguishing properties of software product lines into account, for example, by checking feature-related code in isolation or by exploiting variability information during analysis. The emerging field of product-line analysis techniques is both broad and diverse such that it is difficult for researchers and practitioners to understand their similarities and differences (e.g., with regard to variability awareness or scalability), which hinders systematic research and application. We classify the corpus of existing and ongoing work in this field, we compare techniques based on our classification, and we infer a research agenda. A short-term benefit of our endeavor is that our classification can guide research in product-line analysis and, to this end, make it more systematic and efficient. A long-term goal is to empower developers to choose the right analysis technique for their needs out of a pool of techniques with different strengths and weaknesses.

J. Feigenspan, C. Kästner, S. Apel, J. Liebig, M. Schulze, R. Dachselt, M. Papendieck, T. Leich, and G. Saake. Do Background Colors Improve Program Comprehension in the #ifdef Hell? Empirical Software Engineering (EMSE), 18(4):699--745, 2012. [ .pdf, http, doi, bib ]

Software-product-line engineering aims at the development of variable and reusable software systems. In practice, software product lines are often implemented with preprocessors. Preprocessor directives are easy to use, and many mature tools are available for practitioners. However, preprocessor directives have been heavily criticized in academia and even referred to as “#ifdef hell”, because they introduce threats to program comprehension and correctness. There are many voices that suggest to use other implementation techniques instead, but these voices ignore the fact that a transition from preprocessors to other languages and tools is tedious, erroneous, and expensive in practice. Instead, we and others propose to increase the readability of preprocessor directives by using background colors to highlight source code annotated with ifdef directives. In three controlled experiments with over 70 subjects in total, we evaluate whether and how background colors improve program comprehension in preprocessor-based implementations. Our results demonstrate that background colors have the potential to improve program comprehension, independently of size and programming language of the underlying product. Additionally, we found that subjects generally favor background colors. We integrate these and other findings in a tool called FeatureCommander, which facilitates program comprehension in practice and which can serve as a basis for further research.

J. Feigenspan, M. Schulze, M. Papendieck, C. Kästner, R. Dachselt, V. Köppen, M. Frisch, and G. Saake. Supporting Program Comprehension in Large Preprocessor-Based Software Product Lines. IET Software, 6(6):488--501, December 2012. [ .pdf, doi, bib ]

Background: Software product line engineering provides an effective mechanism to implement variable software. However, the usage of preprocessors to realize variability, which is typical in industry, is heavily criticized, because it often leads to obfuscated code. Using background colours to highlight preprocessor statements to support comprehensibility has shown effective, however, scalability to large software product lines (SPLs) is questionable. Aim: Our goal is to implement and evaluate scalable usage of background colours for industrial-sized SPLs. Method: We designed and implemented scalable concepts in a tool called FeatureCommander. To evaluate its effectiveness, we conducted a controlled experiment with a large real-world SPL with over 99,000 lines of code and 340 features. We used a within-subjects design with treatments colours and no colours. We compared correctness and response time of tasks for both treatments. Results: For certain kinds of tasks, background colours improve program comprehension. Furthermore, subjects generally favour background colours compared to no background colours. Additionally, subjects who worked with background colours had to use the search functions less frequently. Conclusion: We show that background colours can improve program comprehension in large SPLs. Based on these encouraging results, we will continue our work on improving program comprehension in large SPLs.

J. Feigenspan, C. Kästner, J. Liebig, S. Apel, and S. Hanenberg. Measuring Programming Experience. In Proceedings of the 20th International Conference on Program Comprehension (ICPC), pages 73--82, Los Alamitos, CA: IEEE Computer Society, 2012. Acceptance rate: 41 % (21/51). [ .pdf, bib ]

Programming experience is an important confounding parameter in controlled experiments regarding program comprehension. In literature, ways to measure or control programming experience vary. Often, researchers neglect it or do not specify how they controlled it. We set out to find a well-defined understanding of programming experience and a way to measure it. From published comprehension experiments, we extracted questions that assess programming experience. In a controlled experiment, we compare the answers of 128 students to these questions with their performance in solving program-comprehension tasks. We found that self estimation seems to be a reliable way to measure programming experience. Furthermore, we applied exploratory factor analysis to extract a model of programming experience. With our analysis, we initiate a path toward measuring programming experience with a valid and reliable tool, so that we can control its influence on program comprehension.

C. Kästner. Virtual Separation of Concerns: Toward Preprocessors 2.0. Information Technology (it), 54(1):42--46, 2012. [ doi, http, .pdf, bib ]
N. Siegmund, S. Kolesnikov, C. Kästner, S. Apel, D. Batory, M. Rosenmüller, and G. Saake. Predicting Performance via Automated Feature-Interaction Detection. In Proceedings of the 34th International Conference on Software Engineering (ICSE), pages 167--177, Los Alamitos, CA: IEEE Computer Society, 2012. Acceptance rate: 21 % (87/408). [ .pdf, bib ]

Customizable programs and program families provide user-selectable features to tailor a program to an application scenario. Knowing in advance which feature selection yields the best performance is difficult because a direct measurement of all possible feature combinations is infeasible. Our work aims at predicting program performance based on selected features. The challenge is predicting performance accurately when features interact. An interaction occurs when a feature combination has an unexpected influence on performance. We present a method that automatically detects performance feature interactions to improve prediction accuracy. To this end, we propose three heuristics to reduce the number of measurements required to detect interactions. Our evaluation consists of six real-world case studies from varying domains (e.g. databases, compression libraries, and web server) using different configuration techniques (e.g., configuration files and preprocessor flags). Results show, on average, a prediction accuracy of 95 %.

S. Apel, C. Kästner, and C. Lengauer. Language-Independent and Automated Software Composition: The FeatureHouse Experience. IEEE Transactions on Software Engineering (TSE), 39(1):63--79, 2013. [ .pdf, http, bib ]

Superimposition is a composition technique that has been applied successfully in many areas of software development. Although superimposition is a general-purpose concept, it has been (re)invented and implemented individually for various kinds of software artifacts. We unify languages and tools that rely on superimposition by using the language-independent model of feature structure trees (FSTs). On the basis of the FST model, we propose a general approach to the composition of software artifacts written in different languages. Furthermore, we offer a supporting framework and tool chain, called FeatureHouse. We use attribute grammars to automate the integration of additional languages. In particular, we have integrated Java, C#, C, Haskell, Alloy, and JavaCC. A substantial number of case studies demonstrate the practicality and scalability of our approach and reveal insights into the properties that a language must have in order to be ready for superimposition. We discuss perspectives of our approach and demonstrate how we extended FeatureHouse with support for XML languages (in particular, XHTML, XMI/UML, and Ant) and alternative composition approaches (in particular, aspect weaving). Rounding off our previous work, we provide here a holistic view of the FeatureHouse approach based on rich experience with numerous languages and case studies and reflections on several years of research.

M. Pukall, C. Kästner, W. Cazzola, S. Götz, A. Grebhahn, R. Schröter, and G. Saake. JavAdaptor: Flexible Runtime Updates of Java Applications. Software: Practice and Experience (SPE), 43(2):153--185, February 2013. [ .pdf, http, doi, bib ]

Software is changed frequently during its life cycle. New requirements come and bugs must be fixed. To update an application it usually must be stopped, patched, and restarted. This causes time periods of unavailability which is always a problem for highly available applications. Even for the development of complex applications restarts to test new program parts can be time consuming and annoying. Thus, we aim at dynamic software updates to update programs at runtime. There is a large body of research on dynamic software updates, but so far, existing approaches have shortcomings either in terms of flexibility or performance. In addition, some of them depend on specific runtime environments and dictate the program’s architecture. We present JavAdaptor, the first runtime update approach based on Java that (a) offers flexible dynamic software updates, (b) is platform independent, (c) introduces only minimal performance overhead, and (d) does not dictate the program architecture. JavAdaptor combines schema changing class replacements by class renaming and caller updates with Java HotSwap using containers and proxies. It runs on top of all major standard Java virtual machines. We evaluate our approach’s applicability and performance in non-trivial case studies and compare it to existing dynamic software update approaches.

C. Kästner, A. Dreiling, and K. Ostermann. Variability Mining with LEADT. Technical Report 01/2011, Marburg, Germany: Department of Mathematics and Computer Science, Philipps University Marburg, September 2011. [ .pdf, http, bib ]

Software product line engineering is an efficient means to generate a set of tailored software products from a common implementation. However, adopting a product-line approach poses a major challenge and significant risks, since typically legacy code must be migrated toward a product line. Our aim is to lower the adoption barrier by providing semiautomatic tool support—called variability mining—to support developers in locating, documenting, and extracting implementations of product-line features from legacy code. Variability mining combines prior work on concern location, reverse engineering, and variability-aware type systems, but is tailored specifically for the use in product lines. Our work extends prior work in three important aspects: (1) we provide a consistency indicator based on a variability-aware type system, (2) we mine features at a fine level of granularity, and (3) we exploit domain knowledge about the relationship between features when available. With a quantitative study, we demonstrate that variability mining can efficiently support developers in locating features.

M. Kuhlemann, C. Kästner, S. Apel, and G. Saake. An Algebra for Refactoring and Feature-Oriented Programming. Technical Report FIN-2011-06, Magdeburg, Germany: University of Magdeburg, September 2011. [ http, bib ]
S. Erdweg, L. Kats, T. Rendel, C. Kästner, K. Ostermann, and E. Visser. Growing a Language Environment with Editor Libraries. In Proceedings of the 10th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 167--176, New York, NY: ACM Press, October 2011. Acceptance rate: 31 % (18/58). [ doi, .pdf, bib ]

Large software projects consist of code written in a multitude of different (possibly domain-specific) languages, which are often deeply interspersed even in single files. While many proposals exist on how to integrate languages semantically and syntactically, the question of how to support this scenario in integrated development environments (IDEs) remains open: How can standard IDE services, such as syntax highlighting, outlining, or reference resolving, be provided in an extensible and compositional way, such that an open mix of languages is supported in a single file? Based on our library-based syntactic extension language for Java, SugarJ, we propose to make IDEs extensible by organizing editor services in editor libraries. Editor libraries are libraries written in the object language, SugarJ, and hence activated and composed through regular import statements on a file-by-file basis. We have implemented an IDE for editor libraries on top of SugarJ and the Eclipse-based Spoofax language workbench. We have validated editor libraries by evolving this IDE into a fully-fledged and schema-aware XML editor as well as an extensible Latex editor, which we used for writing this paper.

C. Kästner, S. Apel, and K. Ostermann. The Road to Feature Modularity? In Proceedings of the 3rd International Workshop on Feature-Oriented Software Development (FOSD), pages 5:1--5:8, New York, NY: ACM Press, September 2011. [ doi, .pdf, bib ]

Modularity of feature representations has been a long standing goal of feature-oriented software development. While some researchers regard feature modules and corresponding composition mechanisms as a modular solution, other researchers have challenged the notion of feature modularity and pointed out that most feature-oriented implementation mechanisms lack proper interfaces and support neither modular type checking nor separate compilation. We step back and reflect on the feature-modularity discussion. We distinguish two notions of modularity, cohesion without interfaces and information hiding with interfaces, and point out the different expectations that, we believe, are the root of many heated discussions. We discuss whether feature interfaces should be desired and weigh their potential benefits and costs, specifically regarding crosscutting, granularity, feature interactions, and the distinction between closed-world and open-world reasoning. Because existing evidence for and against feature modularity and feature interfaces is shaky and inconclusive, more research is needed, for which we outline possible directions.

J. Feigenspan, M. Papendieck, C. Kästner, M. Frisch, and R. Dachselt. FeatureCommander: Colorful #ifdef World. In Proceedings of the 15th International Software Product Line Conference (SPLC), second volume (Demonstration) (SPLC), pages 48:1--48:2, New York, NY: ACM Press, September 2011. [ .pdf, doi, bib ]
C. Kästner, P. Giarrusso, T. Rendel, S. Erdweg, K. Ostermann, and T. Berger. Variability-Aware Parsing in the Presence of Lexical Macros and Conditional Compilation. In Proceedings of the 26th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 805--824, New York, NY: ACM Press, October 2011. Acceptance rate: 37 % (61/166). [ doi, .pdf, bib ]

In many projects, lexical preprocessors are used to manage different variants of the project (using conditional compilation) and to define compile-time code transformations (using macros). Unfortunately, while being a simply way to implement variability, conditional compilation and lexical macros hinder automatic analysis, even though such analysis would be urgently needed to combat variability-induced complexity. To analyze code with its variability, we need to parse it without preprocessing it. However, current parsing solutions use heuristics, support only a subset of the language, or suffer from exponential explosion. As part of the TypeChef project, we contribute a novel variability-aware parser that can parse unpreprocessed code without heuristics in practicable time. Beyond the obvious task of detecting syntax errors, our parser paves the road for further analysis, such as variability-aware type checking. We implement variabilityaware parsers for Java and GNU C and demonstrate practicability by parsing the product line MobileMedia and the entire X86 architecture of the Linux kernel with 6065 variable features.

S. Erdweg, L. Kats, T. Rendel, C. Kästner, K. Ostermann, and E. Visser. SugarJ: Library-Based Language Extensibility. In Proceedings of the Companion of the 26th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 187--188, New York, NY: ACM Press, 2011. Poster. [ doi, bib ]
S. Erdweg, L. Kats, T. Rendel, C. Kästner, K. Ostermann, L. Kats, and E. Visser. Library-Based Model-Driven Software Development with SugarJ. In Proceedings of the Companion of the 26th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 17--18, New York, NY: ACM Press, 2011. Demonstration paper. [ doi, bib ]
S. Erdweg, T. Rendel, C. Kästner, and K. Ostermann. SugarJ: Library-based Syntactic Language Extensibility. In Proceedings of the 26th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 391--406, New York, NY: ACM Press, October 2011. Acceptance rate: 37 % (61/166). Distinguished Paper Award. [ .pdf, doi, bib ]

Existing approaches to extend a programming language with syntactic sugar often leave a bitter taste, because they cannot be used with the same ease as the main extension mechanism of the programming language—libraries. Sugar libraries are a novel approach for syntactically extending a programming language within the language. A sugar library is like an ordinary library, but can, in addition, export syntactic sugar for using the library. Sugar libraries maintain the composability and scoping properties of ordinary libraries and are hence particularly well-suited for embedding a multitude of domain-specific languages into a host language. They also inherit the self-applicability of libraries, which means that the syntax extension mechanism can be applied in the definition of sugar libraries themselves. To demonstrate the expressiveness and applicability of sugar libraries, we have developed SugarJ, a language on top of Java, SDF and Stratego that supports syntactic extensibility. SugarJ employs a novel incremental parsing mechanism that allows changing the syntax within a source file. We demonstrate SugarJ by five language extensions, including embeddings of XML and closures in Java, all available as sugar libraries. We illustrate the utility of self-applicability by embedding XML Schema, a metalanguage to define XML languages.

S. Apel, J. Liebig, B. Brandl, C. Lengauer, and C. Kästner. Semistructured Merge: Rethinking Merge in Revision Control Systems. In Proceedings of the European Software Engineering Conference and ACM SIGSOFT Symposium on the Foundations of Software Engineering (ESEC/FSE), pages 190--200, New York, NY: ACM Press, September 2011. Acceptance rate: 17 % (34/203). [ .pdf, bib ]

An ongoing problem in revision control systems is how to resolve conflicts in a merge of independently developed revisions. Unstructured revision control systems are purely text-based and solve conflicts based on textual similarity. Structured revision control systems are tailored to specific languages and use language-specific knowledge for conflict resolution. We propose semistructured revision control systems that inherit the strengths of both classes of systems: generality and expressiveness. The idea is to provide structural information of the underlying software artifacts—declaratively, in the form of annotated grammars. This way, a wide variety of languages can be supported and the information provided can assist the automatic resolution of two classes of conflicts: ordering conflicts and semantic conflicts. The former can be resolved independently of the language and the latter can be resolved using specific conflict handlers supplied by the user. We have been developing a tool that supports semistructured merge and conducted an empirical study on 24 software projects developed in Java, C#, and Python comprising 180 merge scenarios. We found that semistructured merge reduces the number of conflicts in 60 % of the sample merge scenarios by, on average, 34 %. Our study reveals that renaming is challenging in that it can significantly increase the number of conflicts during semistructured merge, which we discuss.

N. Siegmund, M. Rosenmüller, M. Kuhlemann, C. Kästner, S. Apel, and G. Saake. SPL Conqueror: Toward Optimization of Non-functional Properties in Software Product Lines. Software Quality Journal (SQJ), Special Issue on Quality Engineering for Software Product Lines, 20(3):487--517, 2011. [ .pdf, http, doi, bib ]

A software product line (SPL) is a family of related programs of a domain. The programs of an SPL are distinguished in terms of features, which are end-uservisible characteristics of programs. Based on a selection of features, stakeholders can derive tailor-made programs that satisfy functional requirements. Besides functional requirements, different application scenarios raise the need for optimizing non-functional properties of a variant. The diversity of application scenarios leads to heterogeneous optimization goals with respect to non-functional properties (e.g., performance vs. footprint vs. energy optimized variants). Hence, an SPL has to satisfy different and sometimes contradicting requirements regarding non-functional properties. Usually, the actually required non-functional properties are not known before product derivation and can vary for each application scenario and customer. Allowing stakeholders to derive optimized variants requires to measure non-functional properties after the SPL is developed. Unfortunately, the high variability provided by SPLs complicates measurement and optimization of non-functional properties due to a large variant space. With SPL Conqueror, we provide a holistic approach to optimize non-functional properties in SPL engineering. We show how non-functional properties can be qualitatively specified and quantitatively measured in the context of SPLs. Furthermore, we discuss the variant-derivation process in SPL Conqueror that reduces the effort of computing an optimal variant. We demonstrate the applicability of our approach by means of nine case studies of a broad range of application domains (e.g., database management and operating systems). Moreover, we show that SPL Conqueror is implementation and language independent by using SPLs that are implemented with different mechanisms, such as conditional compilation and feature-oriented programming.

A. Khan, C. Kästner, V. Köppen, and G. Saake. Service Variability Patterns. In Proceedings of the ER Workshop on Software Variability Management (Variability@ER), volume 6999 of Lecture Notes in Computer Science, pages 130--140, Berlin/Heidelberg: Springer-Verlag, 2011. [ http, bib ]
C. Kästner. Virtuelle Trennung von Belangen. In Ausgezeichnete Informatikdissertationen 2010, volume D-11 of Lecture Notes in Informatics, pages 121--130, Bonn, Germany: Gesellschaft für Informatik (GI), 2011. Invited paper. [ .pdf, bib ]

Bedingte Kompilierung ist ein einfaches und häufig benutztes Mittel zur Implementierung von Variabilität in Softwareproduktlinien, welches aber aufgrund negativer Auswirkungen auf Codequalität und Wartbarkeit stark kritisiert wird. Wir zeigen wie Werkzeugunterstützung – Sichten, Visualisierung, kontrollierte Annotationen, Produktlinien-Typsystem – die wesentlichen Probleme beheben kann und viele Vorteile einer modularen Entwicklung emuliert. Wir bieten damit eine Alternative zur klassischen Trennung von Belangen mittels Modulen. Statt Quelltext notwendigerweise in Dateien zu separieren erzielen wir eine virtuelle Trennung von Belangen durch entsprechender Werkzeugunterstüzung.

J. Feigenspan, S. Apel, J. Liebig, and C. Kästner. Exploring Software Measures to Assess Program Comprehension. In Proceedings of the 5th International Symposium on Empirical Software Engineering and Measurement (ESEM), pages 1--10, paper 3, Los Alamitos, CA: IEEE Computer Society, September 2011. Acceptance rate: 31 % (33/105). [ .pdf, bib ]

Software measures are often used to assess program comprehension, although their applicability is discussed controversially. Often, their application is based on plausibility arguments, which however is not sufficient to decide whether and how software measures are good predictors for program comprehension. Our goal is to evaluate whether and how software measures and program comprehension correlate. To this end, we carefully designed an experiment. We used four different measures that are often used to judge the quality of source code: complexity, lines of code, concern attributes, and concern operations. We measured how subjects understood two comparable software systems that differ in their implementation, such that one implementation promised considerable benefits in terms of better software measures. We did not observe a difference in program comprehension of our subjects as the software measures suggested it. To explore how software measures and program comprehension could correlate, we used several variants of computing the software measures. This brought them closer to our observed result, however, not as close as to confirm a relationship between software measures and program comprehension. Having failed to establish a relationship, we present our findings as an open issue to the community and initiate a discussion on the role of software measures as comprehensibility predictors.

T. Thüm, C. Kästner, S. Erdweg, and N. Siegmund. Abstract Features in Feature Modeling. In Proceedings of the 15th International Software Product Line Conference (SPLC), pages 191--200, Los Alamitos, CA: IEEE Computer Society, August 2011. Acceptance rate: 29 % (20/69). [ .pdf, bib ]

A software product line is a set of program variants, typically generated from a common code base. Feature models describe variability in product lines by documenting features and their valid combinations. In product-line engineering, we need to reason about variability and program variants for many different tasks. For example, given a feature model, we might want to determine the number of all valid feature combinations or detect specific feature combinations for testing. However, we found that contemporary reasoning approaches can only reason about feature combinations, not about program variants, because they do not take abstract features into account. Abstract features are features used to structure a feature model that, however, do not have any impact at implementation level. Using existing feature-model reasoning mechanisms for product variants leads to incorrect results. We raise awareness of the problem of abstract features for different kinds of analyses on feature models. We argue that, in order to reason about program variants, abstract features should be made explicit in feature models. We present a technique based on propositional formulas to reason about program variants. In practice, our technique can save effort that is caused by considering the same program variant multiple times, for example, in product-line testing.

N. Siegmund, M. Rosenmüller, C. Kästner, P. Giarrusso, S. Apel, and S. Kolesnikov. Scalable Prediction of Non-functional Properties in Software Product Lines. In Proceedings of the 15th International Software Product Line Conference (SPLC), pages 160--169, Los Alamitos, CA: IEEE Computer Society, August 2011. Acceptance rate: 29 % (20/69). Best Paper Award. [ .pdf, bib ]

A software product line (SPL) is a family of related software products, from which users can derive a product that fulfills their needs. Often, users expect a product to have specific non-functional properties, for example, to not exceed a footprint limit or to respond in a given time frame. Unfortunately, it is usually not feasible to generate and measure non-functional properties for each possible product of an SPL in isolation, because an SPL can contain millions of products. Hence, we propose an approach to estimate each product's non-functional properties in advance, based on the product's configuration. To this end, we approximate non-functional properties per features and per feature interaction. We generate and measure a small set of products and approximated non-functional properties by comparing the measurements. Our approach is implementation independent and language independent. We present three different approaches with different trade-offs regarding accuracy and required number of measurements. With nine case studies, we demonstrate that our approach can predict non-functional properties with an accuracy of 2\%.

S. Apel, F. Heidenreich, C. Kästner, and M. Rosenmüller. Third International Workshop on Feature-Oriented Software Development (FOSD 2011). In Proceedings of the 15th International Software Product Line Conference (SPLC), pages 337--338, Los Alamitos, CA: IEEE Computer Society, August 2011. [ .pdf, http, bib ]
K. Ostermann, P. Giarrusso, C. Kästner, and T. Rendel. Revisiting Information Hiding: Reflections on Classical and Nonclassical Modularity. In Proceedings of the 25th European Conference on Object-Oriented Programming (ECOOP), volume 6813 of Lecture Notes in Computer Science, pages 155--178, Berlin/Heidelberg: Springer-Verlag, 2011. Acceptance rate: 26 % (26/100). [ doi, .pdf, epub, bib ]

What is modularity? Which kind of modularity should developers strive for? Despite decades of research on modularity, these basic questions have no definite answer. We submit that the common understanding of modularity, and in particular its notion of information hiding, is deeply rooted in classical logic. We analyze how classical modularity, based on classical logic, fails to address the needs of developers of large software systems, and encourage researchers to explore alternative visions of modularity, based on nonclassical logics, and henceforth called nonclassical modularity.

J. Feigenspan, M. Schulze, M. Papendieck, C. Kästner, R. Dachselt, V. Köppen, and M. Frisch. Using Background Colors to Support Program Comprehension in Software Product Lines. In Proceedings of the 15th International Conference on Evaluation and Assessment in Software Engineering (EASE), pages 66--75, Institution of Engineering and Technology, 2011. Acceptance rate: 40 % (20/50). [ .pdf, bib ]

Background: Software product line engineering provides an effective mechanism to implement variable software. However, the usage of preprocessors, which is typical in industry, is heavily criticized, because it often leads to obfuscated code. Using background colors to support comprehensibility has shown effective, however, scalability to large software product lines (SPLs) is questionable. Aim: Our goal is to implement and evaluate scalable usage of background colors for industrial-sized SPLs. Method: We designed and implemented scalable concepts in a tool called FeatureCommander. To evaluate its effectiveness, we conducted a controlled experiment with a large real-world SPL with over 160,000 lines of code and 340 features. We used a within-subjects design with treatments colors and no colors. We compared correctness and response time of tasks for both treatments. Results: For certain kinds of tasks, background colors improve program comprehension. Furthermore, subjects generally favor background colors. Conclusion: We show that background colors can improve program comprehension in large SPLs. Based on these encouraging results, we will continue our work improving program comprehension in large SPLs.

M. Stengel, J. Feigenspan, M. Frisch, C. Kästner, S. Apel, and R. Dachselt. View Infinity: A Zoomable Interface for Feature-Oriented Software Development. In Proceedings of the 33rd International Conference on Software Engineering (Demonstration Track) (ICSE), pages 1031--1033, New York, NY: ACM Press, 2011. Acceptance rate: 37 % (22/60). [ .pdf, acm, doi, bib ]
M. Pukall, A. Grebhahn, R. Schröter, C. Kästner, W. Cazzola, and S. Götz. JavaAdaptor: Unrestricted Dynamic Software Updates for Java. In Proceedings of the 33rd International Conference on Software Engineering (Demonstration Track) (ICSE), pages 989--991, New York, NY: ACM Press, 2011. Acceptance rate: 37 % (22/60). [ .pdf, acm, doi, bib ]
C. Kästner, S. Apel, T. Thüm, and G. Saake. Type Checking Annotation-Based Product Lines. ACM Transactions on Software Engineering and Methodology (TOSEM), 21(3):Article 14, 2012. [ .pdf, epub, doi, bib ]

Software-product-line engineering is an efficient means to generate a family of program variants for a domain from a single code base. However, because of the potentially high number of possible program variants, it is difficult to test them all and ensure properties like type safety for the entire product line. We present a product-line–aware type system that can type check an entire software product line without generating each variant in isolation. Specifically, we extend the Featherweight Java calculus with feature annotations for product-line development and prove formally that all program variants generated from a well-typed product line are well-typed. Furthermore, we present a solution to the problem of typing mutually exclusive features. We discuss how results from our formalization helped implementing our own product-line tool CIDE for full Java and report of experience with detecting type errors in four existing software-product-line implementations.

J. Liebig, C. Kästner, and S. Apel. Analyzing the Discipline of Preprocessor Annotations in 30 Million Lines of C Code. In Proceedings of the 10th ACM International Conference on Aspect-Oriented Software Development (AOSD), pages 191--202, New York, NY: ACM Press, March 2011. Acceptance rate: 23 % (21/92). [ .pdf, acm, bib ]

The C preprocessor cpp is a widely used tool for implementing variable software. It enables programmers to express variable code of features that may crosscut the entire implementation with conditional compilation. The C preprocessor relies on simple text processing and is independent of the host language (C, C++, Java, and so on). Language independent text processing is powerful and expressive|programmers can make all kinds of annotations in the form of #ifdefs but can render unpreprocessed code difficult to process automatically by tools, such as code aspect refactoring, concern management, and also static analysis and variability-aware type checking. We distinguish between disciplined annotations, which align with the underlying source-code structure, and undisciplined annotations, which do not align with the structure and hence complicate tool development. This distinction raises the question of how frequently programmers use undisciplined annotations and whether it is feasible to change them to disciplined annotations to simplify tool development and to enable programmers to use a wide variety of tools in the first place. By means of an analysis of 40 mediumsized to large-sized C programs, we show empirically that programmers use cpp mostly in a disciplined way: about 85 % of all annotations respect the underlying source-code structure. Furthermore, we analyze the remaining undisciplined annotations, identify patterns, and discuss how to transform them into a disciplined form.

C. Kästner, P. Giarrusso, and K. Ostermann. Partial Preprocessing C Code for Variability Analysis. In Proceedings of the 5th Int'l Workshop on Variability Modelling of Software-Intensive Systems (VaMoS), pages 137--140, New York, NY: ACM Press, January 2011. Acceptance rate: 55 % (21/38). [ acm, .pdf, bib ]

The C preprocessor is commonly used to implement variability. Given a feature selection, code fragments can be excluded from compilation with #ifdef and similar directives. However, the token-based nature of the C preprocessor makes variability implementation difficult and errorprone. Additionally, variability mechanisms are intertwined with macro definitions, macro expansion, and file inclusion. To determine whether a code fragment is compiled, the entire file must be preprocessed. We present a partial preprocessor that preprocesses file inclusion and macro expansion, but retains variability information for further analysis. We describe the mechanisms of the partial preprocessor, provide a full implementation, and present some initial experimental results. The partial preprocessor is part of a larger endeavor in the TypeChef project to check variability implementations (syntactic correctness, type correctness) in C projects such as the Linux kernel.

S. Apel, D. Batory, K. Czarnecki, F. Heidenreich, C. Kästner, and O. Nierstrasz, editors. Proceedings of the Second International Workshop on Feature-Oriented Software Development (FOSD), October 10, 2010, Eindhoven, The Netherlands. New York, NY: ACM Press, October 2010. [ .pdf, http, bib ]
A. Kenner, C. Kästner, S. Haase, and T. Leich. TypeChef: Toward Type Checking #ifdef Variability in C. In Proceedings of the 2nd International Workshop on Feature-Oriented Software Development (FOSD), pages 25--32, New York, NY: ACM Press, October 2010. Acceptance rate: 55 % (11/20). [ .pdf, acm, bib ]

Software product lines have gained momentum as an approach to generate many variants of a program, each tailored to a specific use case, from a common code base. However, the implementation of product lines raises new challenges, as potentially millions of program variants are developed in parallel. In prior work, we and others have developed product-line–aware type systems to detect type errors in a product line, without generating all variants. With TypeChef, we build a similar type checker for product lines written in C that implements variability with #ifdef directives of the C preprocessor. However, a product-line–aware type system for C is more difficult than expected due to several peculiarities of the preprocessor, including lexical macros and unrestricted use of #ifdef directives. In this paper, we describe the problems faced and our progress to solve them with TypeChef. Although TypeChef is still under development and cannot yet process arbitrary C code, we demonstrate its capabilities so far with a case study: By type checking the open-source web server Boa with potentially 2^110 variants, we found type errors in several variants.

S. Apel, W. Scholz, C. Lengauer, and C. Kästner. Language-Independent Reference Checking in Software Product Lines. In Proceedings of the 2nd International Workshop on Feature-Oriented Software Development (FOSD), pages 64--71, New York, NY: ACM Press, October 2010. Acceptance rate: 55 % (11/20). [ .pdf, acm, bib ]

Feature-Oriented Software Development (FOSD) is a paradigm for the development of software product lines. A challenge in FOSD is to guarantee that all software systems of a software product line are correct. Recent work on type checking product lines can provide a guarantee of type correctness without generating all possible systems. We generalize previous results by abstracting from the specifics of particular programming languages. In a first attempt, we present a reference-checking algorithm that performs key tasks of product-line type checking independently of the target programming language. Experiments with two sample product lines written in Java and C are encouraging and give us confidence that this approach is promising.

S. Apel, S. Kolesnikov, J. Liebig, C. Kästner, M. Kuhlemann, and T. Leich. Access Control in Feature-Oriented Programming. Science of Computer Programming (SCP), Special Issue on Feature-Oriented Software Development, 77(3):174--187, March 2012. [ .pdf, doi, bib ]

In feature-oriented programming (FOP) a programmer decomposes a program in terms of features. Ideally, features are implemented modularly so that they can be developed in isolation. Access control is an important ingredient to attain feature modularity as it provides mechanisms to hide and expose internal details of a module's implementation. But developers of contemporary feature-oriented languages have not considered access control mechanisms so far. The absence of a well-defined access control model for FOP breaks encapsulation of feature code and leads to unexpected program behaviors and inadvertent type errors. We raise awareness of this problem, propose three feature-oriented access modifiers, and present a corresponding access modifier model. We offer an implementation of the model on the basis of a fully-fledged feature-oriented compiler. Finally, by analyzing ten feature-oriented programs, we explore the potential of feature-oriented modifiers in FOP.

S. Apel, W. Scholz, C. Lengauer, and C. Kästner. Dependences and Interactions in Feature-Oriented Design. In Proceedings of the 21st IEEE International Symposium on Software Reliability Engineering (ISSRE), pages 161--170, Los Alamitos, CA: IEEE Computer Society, October 2010. Acceptance rate: 31 % (40/130). [ .pdf, bib ]

Feature-oriented software development (FOSD) aims at the construction, customization, and synthesis of large-scale software systems. We propose a novel software design paradigm, called feature-oriented design, which takes the distinguishing properties of FOSD into account, especially the clean and consistent mapping between features and their implementations as well as the tendency of features to interact inadvertently. We extend the lightweight modeling language Alloy with support for feature-oriented design and call the extension FeatureAlloy. By means of an implementation and four case studies, we demonstrate how feature-oriented design with FeatureAlloy facilitates separation of concerns, variability, and reuse of models of individual features and helps in defining and detecting semantic dependences and interactions between features.

S. Schulze, S. Apel, and C. Kästner. Code Clones in Feature-Oriented Software Product Lines. In Proceedings of the 9th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 103--112, New York, NY: ACM Press, October 2010. Acceptance rate: 31 % (18/59). [ .pdf, acm, bib ]

Some limitations of object-oriented mechanisms are known to cause code clones (e.g., extension using inheritance). Novel programming paradigms such as feature-oriented programming (FOP) aim at alleviating these limitations. However, it is an open issue whether FOP is really able to avoid code clones or whether it even facilitates (FOP-specific) clones. To address this issue, we conduct an empirical analysis on ten feature-oriented software product lines with respect to code cloning. We found that there is a considerable amount of clones in feature-oriented software product lines and that a large fraction of these clones is FOP-specific (i.e., caused by limitations of feature-oriented mechanisms). Based on our results, we initiate a discussion on the reasons for FOP-specific clones and on how to cope with them. We exemplary show how such clones can be removed by the application of refactoring.

C. Kästner. Virtual Separation of Concerns: Toward Preprocessors 2.0. PhD thesis, Magdeburg, Germany: University of Magdeburg, May 2010. Logos Verlag Berlin, isbn 978-3-8325-2527-9. [ doi, http, .pdf, bib ]

Conditional compilation with preprocessors such as cpp is a simple but effective means to implement variability. By annotating code fragments with #ifdef and #endif directives, different program variants with or without these annotated fragments can be created, which can be used (among others) to implement software product lines. Although, such annotation-based approaches are frequently used in practice, researchers often criticize them for their negative effect on code quality and maintainability. In contrast to modularized implementations such as components or aspects, annotation-based implementations typically neglect separation of concerns, can entirely obfuscate the source code, and are prone to introduce subtle errors. Our goal is to rehabilitate annotation-based approaches by showing how tool support can address these problems. With views, we emulate modularity; with a visual representation of annotations, we reduce source code obfuscation and increase program comprehension; and with disciplined annotations and a product-line–aware type system, we prevent or detect syntax and type errors in the entire software product line. At the same time we emphasize unique benefits of annotations, including simplicity, expressiveness, and being language independent. All in all, we provide tool-based separation of concerns without necessarily dividing source code into physically separated modules; we name this approach virtual separation of concerns. We argue that with these improvements over contemporary preprocessors, virtual separation of concerns can compete with modularized implementation mechanisms. Despite our focus on annotation-based approaches, we do intend not give a definite answer on how to implement software product lines. Modular implementations and annotation-based implementations both have their advantages; we even present an integration and migration path between them. Our goal is to rehabilitate preprocessors and show that they are not a lost cause as many researchers think. On the contrary, we argue that – with the presented improvements – annotation-based approaches are a serious alternative for product-line implementation.

J. Feigenspan, C. Kästner, M. Frisch, R. Dachselt, and S. Apel. Visual Support for Understanding Product Lines. In Proceedings of the 18th International Conference on Program Comprehension (ICPC), pages 34--35, Los Alamitos, CA: IEEE Computer Society, 2010. Demonstration paper. [ doi, .pdf, bib ]

The C preprocessor is often used in practice to implement variability in software product lines. Using #ifdef statements provokes problems such as obfuscated source code, yet they will still be used in practice at least in the medium-term future. With CIDE, we demonstrate a tool to improve understanding and maintaining code that contains #ifdef statements by visualizing them with colors and providing different views on the code.

S. Apel, C. Lengauer, B. Möller, and C. Kästner. An Algebraic Foundation for Automatic Feature-Based Program Synthesis. Science of Computer Programming (SCP), 75(11):1022--1047, November 2010. [ doi, .pdf, bib ]

Feature-Oriented Software Development (FOSD) provides a multitude of formalisms, methods, languages, and tools for building variable, customizable, and extensible software. Along different lines of research, different notions of a feature have been developed. Although these notions have similar goals, no common basis for evaluation, comparison, and integration exists. We present a feature algebra that captures the key ideas of feature orientation and provides a common ground for current and future research in this field, in which also alternative options can be explored. Furthermore, our algebraic framework is meant to serve as a basis for the upcoming development paradigms automatic feature-based program synthesis and architectural metaprogramming.

S. Apel, C. Kästner, A. Größlinger, and C. Lengauer. Type Safety for Feature-Oriented Product Lines. Automated Software Engineering -- An International Journal (ASE), 17(3):251--300, 2010. [ http, .pdf, doi, bib ]

A feature-oriented product line is a family of programs that share a common set of features. A feature implements a stakeholder's requirement and represents a design decision or configuration option. When added to a program, a feature involves the introduction of new structures, such as classes and methods, and the refinement of existing ones, such as extending methods. A feature-oriented decomposition enables a generator to create an executable program by composing feature code solely on the basis of the feature selection of a user – no other information needed. A key challenge of product line engineering is to guarantee that only well-typed programs are generated. As the number of valid feature combinations grows combinatorially with the number of features, it is not feasible to type check all programs individually. The only feasible approach is to have a type system check the entire code base of the feature-oriented product line. We have developed such a type system on the basis of a formal model of a feature-oriented Java-like language. The type system guaranties type safety for feature-oriented product lines. That is, it ensures that every valid program of a well-typed product line is well-typed. Our formal model including type system is sound and complete.

J. Liebig, S. Apel, C. Lengauer, C. Kästner, and M. Schulze. An Analysis of the Variability in Forty Preprocessor-Based Software Product Lines. In Proceedings of the 32nd International Conference on Software Engineering (ICSE), pages 105--114, New York, NY: ACM Press, May 2010. Acceptance rate: 14 % (52/380). [ .pdf, acm, doi, bib ]

Over 30 years ago, the preprocessor cpp was developed to extend the programming language C by lightweight metaprogramming capabilities. Despite its error-proneness and low abstraction level, the cpp is still widely being used in presentday software projects to implement variable software. However, not much is known about how the cpp is employed to implement variability. To address this issue, we have analyzed forty open-source software projects written in C. Specifically, we answer the following questions: How does program size influence variability? How complex are extensions made via cpp's variability mechanisms? At which level of granularity are extensions applied? What is the general type of extensions? These questions revive earlier discussions on understanding and refactoring of the preprocessor. To answer them, we introduce several metrics measuring the variability, complexity, granularity, and type of extensions. Based on the data obtained, we suggest alternative implementation techniques. The data we have collected can influence other research areas, such as language design and tool support.

S. Apel, J. Liebig, C. Lengauer, C. Kästner, and W. Cook. Semistructured Merge in Revision Control Systems. In Proceedings of the 4th Int'l Workshop on Variability Modelling of Software-Intensive Systems (VaMoS), pages 13--20, Essen, Germany: University of Duisburg-Essen, January 2010. [ .pdf, bib ]

Revision control systems are a major means to manage versions and variants of today's software systems. An ongoing problem in these systems is how to resolve conflicts when merging independently developed revisions. Unstructured revision control systems are purely text-based and solve conflicts based on textual similarity. Structured revision control systems are tailored to specific languages and use language-specific knowledge for conflict resolution. We propose semistructured revision control systems to inherit the strengths of both classes of systems: generality and expressiveness. The idea is to provide structural information of the underlying software artifacts in the form of annotated grammars, which is motivated by recent work on software product lines. This way, a wide variety of languages can be supported and the information provided can assist the resolution of conflicts. We have implemented a preliminary tool and report on our experience with merging Java artifacts. We believe that drawing a connection between revision control systems and product lines has benefits for both fields.

C. Kästner, S. Apel, and G. Saake. Virtuelle Trennung von Belangen (Präprozessor 2.0). In Proceedings of the Software Engineering 2010 -- Fachtagung des GI-Fachbereichs Softwaretechnik (SE), volume P-159 of Lecture Notes in Informatics, pages 165--176, Bonn, Germany: Gesellschaft für Informatik (GI), February 2010. Acceptance rate: 36 % (17/47). [ .pdf, bib ]

Bedingte Kompilierung mit Präprozessoren wie cpp ist ein einfaches, aber wirksames Mittel zur Implementierung von Variabilität in Softwareproduktlinien. Durch das Annotieren von Code-Fragmenten mit #ifdef und #endif können verschiedene Programmvarianten mit oder ohne diesen Fragmenten generiert werden. Obwohl Präprozessoren häufig in der Praxis verwendet werden, werden sie oft für ihre negativen Auswirkungen auf Codequalität und Wartbarkeit kritisiert. Im Gegensatz zu modularen Implementierungen, etwa mit Komponenten oder Aspekte, vernachlässigen Präprozessoren die Trennung von Belangen im Quelltext, sind anfällig für subtile Fehler und verschlechtern die Lesbarkeit des Quellcodes. Wir zeigen, wie einfache Werkzeugunterstützung diese Probleme adressieren und zum Teil beheben bzw. die Vorteile einer modularen Implementierung emulieren kann. Gleichzeitig zeigen wir Vorteile von Präprozessoren wie Einfachheit und Sprachunabhängigkeit auf.

M. Kuhlemann, C. Kästner, and S. Apel. Reducing Code Replication in Delegation-Based Java Programs. In Java Software and Embedded Systems, pages 171--183, Hauppauge, NY: Nova Science Publishers, Inc., 2010. [ http, bib ]
M. Pukall, C. Kästner, S. Götz, W. Cazzola, and G. Saake. Flexible Runtime Program Adaptations in Java -- A Comparison. Technical Report FIN-2009-14, Magdeburg, Germany: University of Magdeburg, November 2009. [ .pdf, http, bib ]
S. Apel, W. Cook, K. Czarnecki, C. Kästner, N. Loughran, and O. Nierstrasz, editors. Proceedings of the First International Workshop on Feature-Oriented Software Development (FOSD), October 6, 2009, Denver, Colorado, USA. New York, NY: ACM Press, October 2009. [ .pdf, http, bib ]
J. Feigenspan, C. Kästner, S. Apel, and T. Leich. How to Compare Program Comprehension in FOSD Empirically -- An Experience Report. In Proceedings of the 1st International Workshop on Feature-Oriented Software Development (FOSD), pages 55--62, New York, NY: ACM Press, October 2009. [ .pdf, doi, bib ]

There are many different implementation approaches to realize the vision of feature oriented software development, ranging from simple preprocessors, over feature-oriented programming, to sophisticated aspect-oriented mechanisms. Their impact on readability and maintainability (or program comprehension in general) has caused a debate among researchers, but sound empirical results are missing. We report experience from our endeavor to conduct experiments to measure the influence of different implementation mechanisms on program comprehension. We describe how to design such experiments and report from possibilities and pitfalls we encountered. Finally, we present some early results of our first experiment on comparing CPP with CIDE.

S. Apel, J. Liebig, C. Kästner, M. Kuhlemann, and T. Leich. An Orthogonal Access Modifier Model for Feature-Oriented Programming. In Proceedings of the 1st International Workshop on Feature-Oriented Software Development (FOSD), pages 27--34, New York, NY: ACM Press, October 2009. [ .pdf, doi, bib ]

In feature-oriented programming (FOP), a programmer decomposes a program in terms of features. Ideally, features are implemented modularly so that they can be developed in isolation. Access control is an important ingredient to attain feature modularity as it provides mechanisms to hide and expose internal details of a module's implementation. But developers of contemporary feature-oriented languages did not consider access control mechanisms so far. The absence of a well-defined access control model for FOP breaks the encapsulation of feature code and leads to unexpected and undefined program behaviors as well as inadvertent type errors, as we will demonstrate. The reason for these problems is that common object-oriented modifiers, typically provided by the base language, are not expressive enough for FOP and interact in subtle ways with feature-oriented language mechanisms. We raise awareness of this problem, propose three feature-oriented modifiers for access control, and present an orthogonal access modifier model.

C. Kästner, and S. Apel. Virtual Separation of Concerns -- A Second Chance for Preprocessors. Journal of Object Technology (JOT), 8(6):59--78, September 2009. Refereed Column. [ .pdf, http, bib ]

Conditional compilation with preprocessors like cpp is a simple but effective means to implement variability. By annotating code fragments with #ifdef and #endif directives, different program variants with or without these fragments can be created, which can be used (among others) to implement software product lines. Although, preprocessors are frequently used in practice, they are often criticized for their negative effect on code quality and maintainability. In contrast to modularized implementations, for example using components or aspects, preprocessors neglect separation of concerns, are prone to introduce subtle errors, can entirely obfuscate the source code, and limit reuse. Our aim is to rehabilitate the preprocessor by showing how simple tool support can address these problems and emulate some benefits of modularized implementations. At the same time we emphasize unique benefits of preprocessors, like simplicity and language independence. Although we do not have a definitive answer on how to implement variability, we want highlight opportunities to improve preprocessors and encourage research toward novel preprocessor-based approaches.

C. Kästner, S. Apel, and M. Kuhlemann. A Model of Refactoring Physically and Virtually Separated Features. In Proceedings of the 8th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 157--166, New York, NY: ACM Press, October 2009. Acceptance rate: 31 % (19/62). [ doi, acm, .pdf, bib ]

Physical separation with class refinements and method refinements à la AHEAD and virtual separation using annotations à la #ifdef or CIDE are two competing groups of implementation approaches for software product lines with complementary advantages. Although both groups have been mainly discussed in isolation, we strive for an integration to leverage the respective advantages. In this paper, we provide the basis for such an integration by providing a model that supports both, physical and virtual separation, and by describing refactorings in both directions. We prove the refactorings complete, such that every virtually separated product line can be automatically transformed into a physically separated one (replacing annotations by refinements) and vice versa. To demonstrate the feasibility of our approach, we have implemented the refactorings in our tool CIDE and conducted four case studies.

M. Kuhlemann, D. Batory, and C. Kästner. Safe Composition of Non-Monotonic Features. In Proceedings of the 8th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 177--185, New York, NY: ACM Press, October 2009. Acceptance rate: 31 % (19/62). [ acm, doi, bib ]

Programs can be composed from features. We want to verify automatically that all legal combinations of features can be composed safely without errors. Prior work on this problem assumed that features add code monotonically. We generalize prior work to enable features to both add and remove code, describe our analyses and implementation, and review case studies. We observe that more expressive features can increase the complexity of developed programs rapidly – up to the point where automated concepts as presented in this paper are not a helpful tool but a necessity for verification.

S. Apel, and C. Kästner. An Overview of Feature-Oriented Software Development. Journal of Object Technology (JOT), 8(5):49--84, July/August 2009. Refereed Column. [ .pdf, http, bib ]

Feature-oriented software development (FOSD) is a paradigm for the construction, customization, and synthesis of large-scale software systems. In this survey, we give an overview and a personal perspective on the roots of FOSD, connections to other software development paradigms, and recent developments in this field. Our aim is to point to connections between different lines of research and to identify open issues.

S. Apel, C. Kästner, A. Größlinger, and C. Lengauer. Type-Safe Feature-Oriented Product Lines. Technical Report MIP-0909, Passau, Germany: Department of Informatics and Mathematics, University of Passau, June 2009. [ .pdf, http, bib ]
C. Kästner, S. Apel, and M. Kuhlemann. LJ^AR: A Model of Refactoring Physically and Virtually Separated Features. Technical Report FIN-2009-08, Magdeburg, Germany: University of Magdeburg, May 2009. [ .pdf, bib ]
C. Kästner, S. Apel, S. ur Rahman, M. Rosenmüller, D. Batory, and G. Saake. On the Impact of the Optional Feature Problem: Analysis and Case Studies. In Proceedings of the 13rd International Software Product Line Conference (SPLC), pages 181--190, Pittsburgh, PA: SEI, August 2009. Acceptance rate: 36 % (30/83). [ .pdf, bib ]

A software product-line is a family of related programs that are distinguished in terms of features. A feature implements a stakeholders' requirement. Different program variants specified by distinct feature selections are produced from a common code base. The optional feature problem describes a common mismatch between variability intended in the domain and dependencies in the implementation. When this occurs, some variants that are valid in the domain cannot be produced due to implementation issues. There are many different solutions to the optional feature problem, but they all suffer from drawbacks such as reduced variability, increased development effort, reduced efficiency, or reduced source code quality. In this paper, we examine the impact of the optional feature problem in two case studies in the domain of embedded database systems, and we survey different state-of-the-art solutions and their trade-offs. Our intension is to raise awareness of the problem, to guide developers in selecting an appropriate solution for their product-line project, and to identify opportunities for future research.

C. Kästner, S. Apel, and G. Saake. Sichere Produktlinien: Herausforderungen für Syntax- und Typ-Prüfungen. In Proceedings of the 26. Workshop der GI-Fachgruppe Programmiersprachen und Rechenkonzepte (), pages 37--38, Kiel, Germany: University of Kiel, May 2009. [ http, bib ]
F. Steimann, T. Pawlitzki, S. Apel, and C. Kästner. Types and Modularity for Implicit Invocation with Implicit Announcement. ACM Transactions on Software Engineering and Methodology (TOSEM), 20(1):Article 1; 43 pages, June 2010. [ acm, .pdf, doi, bib ]

Through implicit invocation, procedures are called without explicitly referencing them. Implicit announcement adds to this implicitness by not only keeping implicit which procedures are called, but also where or when – under implicit invocation with implicit announcement, the call site contains no signs of that, or what it calls. Recently, aspect-oriented programming has popularized implicit invocation with implicit announcement as a possibility to separate concerns that lead to interwoven code if conventional programming techniques are used. However, as has been noted elsewhere, as currently implemented it establishes strong implicit dependencies between components, hampering independent software development and evolution. To address this problem, we present a type-based modularization of implicit invocation with implicit announcement that is inspired by how interfaces and exceptions are realized in JAVA. By extending an existing compiler and by rewriting several programs to make use of our proposed language constructs, we found that the imposed declaration clutter tends to be moderate; in particular, we found that for general applications of implicit invocation with implicit announcement, fears that programs utilizing our form of modularization become unreasonably verbose are unjustified.

S. Apel, F. Janda, S. Trujillo, and C. Kästner. Model Superimposition in Software Product Lines. In Proceedings of the 2nd International Conference on Model Transformation (ICMT), volume 5563 of Lecture Notes in Computer Science, pages 4--19, Berlin/Heidelberg: Springer-Verlag, June 2009. Acceptance rate: 21 % (14/67). [ http, doi, .pdf, bib ]

In software product line engineering, feature composition generates software tailored to specific requirements from a common set of artifacts. Superimposition is a popular technique to merge code pieces belonging to different features. The advent of model-driven development raises the question of how to support the variability of software product lines in modeling techniques. We propose to use superimposition as a model composition technique in order to support variability. We analyze the feasibility of superimposition as a model composition technique, offer a corresponding tool for model composition, and discuss our experiences with three case studies (including one industrial study) using this tool.

S. Apel, C. Kästner, A. Größlinger, and C. Lengauer. Feature (De)composition in Functional Programming. In Proceedings of the 8th International Conference on Software Composition (SC) (SC), volume 5634 of Lecture Notes in Computer Science, pages 9--26, Berlin/Heidelberg: Springer-Verlag, July 2009. Acceptance rate: 33 % (10/30). [ http, doi, .pdf, bib ]

The separation of concerns is a fundamental principle in software engineering. Crosscutting concerns are concerns that do not align with hierarchical and block decomposition supported by mainstream programming languages. In the past, crosscutting concerns have been studied mainly in the context of object orientation. Feature orientation is a novel programming paradigm that supports the (de)composition of crosscutting concerns in a system with a hierarchical block structure. By means of two case studies we explore the problem of crosscutting concerns in functional programming and propose two solutions based on feature orientation.

S. Boxleitner, S. Apel, and C. Kästner. Language-Independent Quantification and Weaving for Feature Composition. In Proceedings of the 8th International Conference on Software Composition (SC) (SC), volume 5634 of Lecture Notes in Computer Science, pages 45--54, Berlin/Heidelberg: Springer-Verlag, July 2009. Acceptance rate: 33 % (10/30). Short Paper. [ http, doi, .pdf, bib ]

Based on a general model of feature composition, we present a composition language that enables programmers by means of quantification and weaving to formulate extensions to programs written in different languages. We explore the design space of composition languages that rely on quantification and weaving and discuss our choices. We outline a tool that extends an existing infrastructure for feature composition and discuss results of three initial case studies.

C. Kästner, S. Apel, S. Trujillo, M. Kuhlemann, and D. Batory. Guaranteeing Syntactic Correctness for all Product Line Variants: A Language-Independent Approach. In Proceedings of the 47th International Conference Objects, Models, Components, Patterns (TOOLS EUROPE), volume 33 of Lecture Notes in Business Information Processing, pages 175--194, Berlin/Heidelberg: Springer-Verlag, June 2009. Acceptance rate: 28 % (19/67). [ .pdf, http, doi, bib ]

A software product line (SPL) is a family of related program variants in a well-defined domain, generated from a set of features. A fundamental difference from classical application development is that engineers develop not a single program but a whole family with hundreds to millions of variants. This makes it infeasible to separately check every distinct variant for errors. Still engineers want guarantees on the entire SPL. A further challenge is that an SPL may contain artifacts in different languages (code, documentation, models, etc.) that should be checked. In this paper, we present CIDE, an SPL development tool that guarantees syntactic correctness for all variants of an SPL. We show how CIDE's underlying mechanism abstracts from textual representation and we generalize it to arbitrary languages. Furthermore, we automate the generation of safe plug-ins for additional languages from annotated grammars. To demonstrate the language-independent capabilities, we applied CIDE to a series of case studies with artifacts written in Java, C++, C, Haskell, ANTLR, HTML, and XML.

C. Kästner, T. Thüm, G. Saake, J. Feigenspan, T. Leich, F. Wielgorz, and S. Apel. FeatureIDE: Tool Framework for Feature-Oriented Software Development. In Proceedings of the 31st International Conference on Software Engineering (ICSE), pages 611--614, Los Alamitos, CA: IEEE Computer Society, May 2009. Acceptance rate: 33 % (24/72). Formal Demonstration paper. [ .pdf, bib ]

Tools support is crucial for the acceptance of a new programming language. However, providing such tool support is a huge investment that can usually not be provided for a research language. With FeatureIDE, we have built an IDE for AHEAD that integrates all phases of featureoriented software development. To reuse this investment for other tools and languages, we refactored FeatureIDE into an open source framework that encapsulates the common ideas of feature-oriented software development and that can be reused and extended beyond AHEAD. Among others, we implemented extensions for FeatureC++ and FeatureHouse, but in general, FeatureIDE is open for everybody to showcase new research results and make them usable to a wide audience of students, researchers, and practitioners.

M. Rosenmüller, C. Kästner, N. Siegmund, S. Sunkle, S. Apel, T. Leich, and G. Saake. SQL à la Carte -- Toward Tailor-made Data Management. In Proceedings of the 13. GI-Fachtagung Datenbanksysteme für Business, Technologie und Web (BTW), volume P-144 of Lecture Notes in Informatics, pages 117--136, Bonn, Germany: Gesellschaft für Informatik (GI), March 2009. [ .pdf, http, bib ]

The size of the structured query language (SQL) continuously increases. Extensions of SQL for special domains like stream processing or sensor networks come with own extensions, more or less unrelated to the standard. In general, underlying DBMS support only a subset of SQL plus vendor specific extensions. In this paper, we analyze application domains where special SQL dialects are needed or are already in use. We show how SQL can be decomposed to create an extensible family of SQL dialects. Concrete dialects, e.g., a dialect for web databases, can be generated from such a family by choosing SQL features à la carte. A family of SQL dialects simplifies analysis of the standard when deriving a concrete dialect, makes it easy to understand parts of the standard, and eases extension for new application domains. It is also the starting point for developing tailor-made data management solutions that support only a subset of SQL. We outline how such customizable DBMS can be developed and what benefits, e.g., improved maintainability and performance, we can expect from this.

N. Siegmund, C. Kästner, M. Rosenmüller, F. Heidenreich, S. Apel, and G. Saake. Bridging the Gap between Variability in Client Application and Database Schema. In Proceedings of the 13. GI-Fachtagung Datenbanksysteme für Business, Technologie und Web (BTW), volume P-144 of Lecture Notes in Informatics, pages 297--306, Bonn, Germany: Gesellschaft für Informatik (GI), March 2009. [ .pdf, http, bib ]

Database schemas are used to describe the logical design of a database. Diverse groups of users have different perspectives on the schema which leads to different local schemas. Research has focused on view integration to generate a global, consistent schema out of different local schemas or views. However, this approach seems to be too constrained when the generated global view should be variable and only a certain subset is needed. Variable schemas are needed in software product lines in which products are tailored to the needs of stakeholders. We claim that traditional modeling techniques are not sufficient for expressing a variable database schema. We show that software product line methodologies, when applied to the database schemas, overcome existing limitations and allow the generation of tailor-made database schemas.

S. Apel, C. Kästner, and C. Lengauer. Vergleich und Integration von Komposition und Annotation zur Implementierung von Produktlinien. In Proceedings of the Software Engineering 2009 -- Fachtagung des GI-Fachbereichs Softwaretechnik (SE), volume P-143 of Lecture Notes in Informatics, pages 101--112, Bonn, Germany: Gesellschaft für Informatik (GI), March 2009. [ .pdf, http, bib ]

Es gibt eine Vielzahl sehr unterschiedlicher Techniken, Sprachen und Werkzeuge zur Entwicklung von Softwareproduktlinien. Trotzdem liegen gemeinsame Mechanismen zu Grunde, die eine Klassifikation in Kompositions- und Annotationsansatz erlauben. Während der Kompositionsansatz in der Forschung große Beachtung findet, kommt im industriellen Umfeld hauptsächlich der Annotationsansatz zur Anwendung. Wir analysieren und vergleichen beide Ansätze anhand von drei repräsentativen Vertretern und identifizieren anhand von sechs Kriterien individuelle Stärken und Schwächen. Wir stellen fest, dass die jeweiligen Stärken und Schwächen komplementär sind. Aus diesem Grund schlagen wir die Integration des Kompositions- und Annotationsansatzes vor, um so die Vorteile beider zu vereinen, dem Entwickler eine breiteres Spektrum an Implementierungsmechanismen zu Verfügung zu stellen und die Einführung von Produktlinientechnologie in bestehende Softwareprojekte zu erleichtern.

S. Apel, C. Kästner, A. Größlinger, and C. Lengauer. On Feature Orientation and Functional Programming. Technical Report MIP-0806, Passau, Germany: Department of Informatics and Mathematics, University of Passau, November 2008. [ .pdf, bib ]
T. Thüm, D. Batory, and C. Kästner. Reasoning about Edits to Feature Models. In Proceedings of the 31st International Conference on Software Engineering (ICSE), pages 254--264, Los Alamitos, CA: IEEE Computer Society, May 2009. Acceptance rate: 12 % (50/405). [ .pdf, bib ]

Features express the variabilities and commonalities among programs in a software product line (SPL). A feature model defines the valid combinations of features, where each combination corresponds to a program in an SPL. SPLs and their feature models evolve over time. We classify the evolution of a feature model via modifications as refactorings, specializations, generalizations, or arbitrary edits. We present an algorithm to reason about feature model edits to help designers determine how the program membership of an SPL has changed. Our algorithm takes two feature models as input (before and after edit versions), where the set of features in both models are not necessarily the same, and it automatically computes the change classification. Our algorithm is able to give examples of added or deleted products and efficiently classifies edits to even large models that have thousands of features.

S. Apel, C. Kästner, and C. Lengauer. FeatureHouse: Language-Independent, Automated Software Composition. In Proceedings of the 31st International Conference on Software Engineering (ICSE), pages 221--231, Los Alamitos, CA: IEEE Computer Society, May 2009. Acceptance rate: 12 % (50/405). [ .pdf, bib ]

Superimposition is a composition technique that has been applied successfully in many areas of software development. Although superimposition is a general-purpose concept, it has been (re)invented and implemented individually for various kinds of software artifacts. We unify languages and tools that rely on superimposition by using the language-independent model of feature structure trees (FSTs). On the basis of the FST model, we propose a general approach to the composition of software artifacts written in different languages, Furthermore, we offer a supporting framework and tool chain, called FEATUREHOUSE. We use attribute grammars to automate the integration of additional languages, in particular, we have integrated Java, C#, C, Haskell, JavaCC, and XML. Several case studies demonstrate the practicality and scalability of our approach and reveal insights into the properties a language must have in order to be ready for superimposition.

C. Kästner, and S. Apel. Integrating Compositional and Annotative Approaches for Product Line Engineering. In Proceedings of the GPCE Workshop on Modularization, Composition and Generative Techniques for Product Line Engineering (McGPLE), pages 35--40, Passau, Germany: Department of Informatics and Mathematics, University of Passau, October 2008. [ .pdf, bib ]

Software product lines can be implemented with many different approaches. However, there are common underlying mechanisms which allow a classification into compositional and annotative approaches. While research focuses mainly on composition approaches like aspect- or feature-oriented programming because those support feature traceability and modularity, in practice annotative approaches like preprocessors are common as they are easier to adopt. In this paper, we compare both groups of approaches and find complementary strengths. We propose an integration of compositional and annotative approaches to combine advantages, increase flexibility for the developer, and ease adoption.

M. Rosenmüller, N. Siegmund, S. ur Rahman, and C. Kästner. Modeling Dependent Software Product Lines. In Proceedings of the GPCE Workshop on Modularization, Composition and Generative Techniques for Product Line Engineering (McGPLE), pages 13--18, Passau, Germany: Department of Informatics and Mathematics, University of Passau, October 2008. [ .pdf, bib ]

Software product line development is a mature technique to implement similar programs tailored to serve the needs of multiple users while providing a high degree of reuse. This approach also scales for larger product lines that use smaller product lines to fulfill special tasks. In such compositions of SPLs, the interacting product lines depend on each other and programs generated from these product lines have to be correctly configured to ensure correct communication between them. Constraints between product lines can be used to allow only valid combinations of generated programs. This, however, is not sufficient if multiple instances of one product line are involved. In this paper we present an approach that uses UML and OO concepts to model compositions of SPLs. The model extends the approach of constraints between SPLs to constraints between instances of SPLs and integrates SPL specialization. Based on this model we apply a feature-oriented approach to simplify the configuration of complete SPL compositions.

N. Siegmund, M. Rosenmüller, M. Kuhlemann, C. Kästner, and G. Saake. Measuring Non-functional Properties in Software Product Lines for Product Derivation. In Proceedings of the 15th Asia-Pacific Software Engineering Conference (APSEC), pages 187--194, Los Alamitos, CA: IEEE Computer Society, December 2008. Acceptance rate: 30 % (66/221). [ .pdf, bib ]

Software product lines (SPLs) enable stakeholders to derive different software products for a domain while providing a high degree of reuse of their code units. Software products are derived in a configuration process by combining different code units. This configuration process becomes complex if SPLs contain hundreds of features. In many cases, a stakeholder is not only interested in functional but also in resulting non-functional properties of a desired product. Because SPLs can be used in different application scenarios alternative implementations of already existing functionality are developed to meet special nonfunctional requirements, like restricted binary size and performance guarantees. To enable these complex configurations we discuss and present techniques to measure nonfunctional properties of software modules and use these values to compute SPL configurations optimized to the users needs.

M. Pukall, C. Kästner, and G. Saake. Towards Unanticipated Runtime Adaptation of Java Applications. In Proceedings of the 15th Asia-Pacific Software Engineering Conference (APSEC), pages 85--92, Los Alamitos, CA: IEEE Computer Society, December 2008. Acceptance rate: 30 % (66/221). [ .pdf, bib ]

Modifying an application usually means to stop the application, apply the changes, and start the application again. That means, the application is not available for at least a short time period. This is not acceptable for highly available applications. One reasonable approach which faces the problem of unavailability is to change highly available applications at runtime. To allow extensive runtime adaptation the application must be enabled for unanticipated changes even of already executed program parts. This is due to the fact that it is not predictable what changes become necessary and when they have to be applied. Since Java is commonly used for developing highly available applications, we discuss its shortcomings and opportunities regarding unanticipated runtime adaptation. We present an approach based on Java HotSwap and object wrapping which overcomes the identified shortcomings and evaluate it in a case study.

C. Kästner, S. Trujillo, and S. Apel. Visualizing Software Product Line Variabilities in Source Code. In Proceedings of the 2nd International SPLC Workshop on Visualisation in Software Product Line Engineering (ViSPLE), pages 303--313, September 2008. [ .pdf, bib ]

Implementing software product lines is a challenging task. Depending on the implementation technique the code that realizes a feature is often scattered across multiple code units. This way it becomes difficult to trace features in source code which hinders maintenance and evolution. While previous effort on visualization technologies in software product lines has focused mainly on the feature model, we suggest tool support for feature traceability in the code base. With our tool CIDE, we propose an approach based on filters and views on source code in order to visualize and trace features in source code.

S. Apel, C. Kästner, and C. Lengauer. Feature Featherweight Java: A Calculus for Feature-Oriented Programming and Stepwise Refinement. In Proceedings of the 7th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 101--112, New York, NY: ACM Press, August 2008. Acceptance rate: 29 % (16/55). [ doi, acm, .pdf, bib ]

Feature-oriented programming (FOP) is a paradigm that incorporates programming language technology, program generation techniques, and stepwise refinement. In their GPCE'07 paper, Thaker et al. suggest the development of a type system for FOP to guarantee safe feature composition, i.e, to guarantee the absence of type errors during feature composition. We present such a type system along with a calculus for a simple feature-oriented, Java-like language, called Feature Featherweight Java (FFJ). Furthermore, we explore four extensions of FFJ and how they affect type soundness.

S. Apel, C. Kästner, and D. Batory. Program Refactoring using Functional Aspects. In Proceedings of the 7th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 161--170, New York, NY: ACM Press, August 2008. Acceptance rate: 29 % (16/55). [ doi, .pdf, acm, bib ]

A functional aspect is an aspect that has the semantics of a transformation; it is a function that maps a program to an advised program. Functional aspects are composed by function composition. In this paper, we explore functional aspects in the context of aspect-oriented refactoring. We show that refactoring legacy applications using functional aspects is just as flexible as traditional aspects in that (a) the order in which aspects are refactored does not matter, and (b) the number of potential aspect interactions is decreased. We analyze several aspect-oriented programs of different sizes to support our claims.

C. Kim, C. Kästner, and D. Batory. On the Modularity of Feature Interactions. In Proceedings of the 7th ACM International Conference on Generative Programming and Component Engineering (GPCE), pages 23--34, New York, NY: ACM Press, August 2008. Acceptance rate: 29 % (16/55). [ doi, acm, .pdf, bib ]

Feature modules are the building blocks of programs in software product lines (SPLs). A foundational assumption of feature-based program synthesis is that features are composed in a predefined order. Recent work on virtual separation of concerns reveals a new model of feature interactions that shows that feature modules can be quantized as compositions of smaller modules called derivatives. We present this model and examine some of its unintuitive consequences, namely, that (1) a given program can be reconstructed by composing features in any order, and (2) the contents of a feature module (as expressed as a composition of derivatives) is determined automatically by a feature order. We show that different orders allow one to “adjust” the contents of a feature module to isolate and study the impact of interactions that a feature has with other features. Using derivatives, we show the utility of generalizing safe composition (SC), a basic analysis of SPLs that verifies program type-safety, to prove that every legal composition of derivatives (and thus any composition order of features) produces a typesafe program, which is a much stronger SC property.

C. Kästner, and S. Apel. Type-checking Software Product Lines -- A Formal Approach. In Proceedings of the 23rd IEEE/ACM International Conference on Automated Software Engineering (ASE), pages 258--267, Los Alamitos, CA: IEEE Computer Society, September 2008. Acceptance rate: 11 % (30/280). [ doi, .pdf, bib ]

A software product line (SPL) is an efficient means to generate a family of program variants for a domain from a single code base. However, because of the potentially high number of possible program variants, it is difficult to test all variants and ensure properties like type-safety for the entire SPL. While first steps to type-check an entire SPL have been taken, they are informal and incomplete. In this paper, we extend the Featherweight Java (FJ) calculus with feature annotations to be used for SPLs. By extending FJ's type system, we guarantee that – given a well-typed SPL – all possible program variants are welltyped as well. We show how results from this formalization reflect and help implementing our own language-independent SPL tool CIDE.

S. Apel, C. Lengauer, B. Möller, and C. Kästner. An Algebra for Features and Feature Composition. In Proceedings of the 12th International Conference on Algebraic Methodology and Software Technology (AMAST), volume 5140 of Lecture Notes in Computer Science, pages 36--50, Berlin/Heidelberg: Springer-Verlag, July 2008. Acceptance rate: 47 % (27/58). [ .pdf, doi, bib ]

Feature-Oriented Software Development (FOSD) provides a multitude of formalisms, methods, languages, and tools for building variable, customizable, and extensible software. Along different lines of research, different notions of a feature have been developed. Although these notions have similar goals, no common basis for evaluation, comparison, and integration exists. We present a feature algebra that captures the key ideas of feature orientation and provides a common ground for current and future research in this field, in which also alternative options can be explored.

S. Apel, C. Kästner, and C. Lengauer. An Overview of Feature Featherweight Java. Technical Report MIP-0802, Passau, Germany: Department of Informatics and Mathematics, University of Passau, April 2008. [ .pdf, bib ]
C. Kästner, S. Apel, S. Trujillo, M. Kuhlemann, and D. Batory. Language-Independent Safe Decomposition of Legacy Applications into Features. Technical Report FIN-2008-02, Magdeburg, Germany: University of Magdeburg, March 2008. [ .pdf, bib ]
S. Apel, C. Kästner, and C. Lengauer. Research Challenges in the Tension Between Features and Services. In Proceedings of the ICSE Workshop on Systems Development in SOA Environments (SDSOA), pages 53--58, New York, NY: ACM Press, May 2008. [ doi, .pdf, bib ]

We present a feature-based approach, known from software product lines, to the development of service-oriented architectures. We discuss five benefits of such an approach: improvements in modularity, variability, uniformity, specifiability, and typeability. Subsequently, we review preliminary experiences and results, and propose an agenda for further research in this direction.

C. Kästner, S. Apel, and M. Kuhlemann. Granularity in Software Product Lines. In Proceedings of the 30th International Conference on Software Engineering (ICSE), pages 311--320, New York, NY: ACM Press, May 2008. Acceptance rate: 15 % (56/371). [ bib, acm, .pdf, doi, epub ]

Building software product lines (SPLs) with features is a challenging task. Many SPL implementations support features with coarse granularity - e.g., the ability to add and wrap entire methods. However, fine-grained extensions, like adding a statement in the middle of a method, either require intricate workarounds or obfuscate the base code with annotations. Though many SPLs can and have been implemented with the coarse granularity of existing approaches, fine-grained extensions are essential when extracting features from legacy applications. Furthermore, also some existing SPLs could benefit from fine-grained extensions to reduce code replication or improve readability. In this paper, we analyze the effects of feature granularity in SPLs and present a tool, called Colored IDE (CIDE), that allows features to implement coarse-grained and fine-grained extensions in a concise way. In two case studies, we show how CIDE simplifies SPL development compared to traditional approaches.

N. Siegmund, M. Kuhlemann, M. Rosenmüller, C. Kästner, and G. Saake. Integrated Product Line Model for Semi-Automated Product Derivation Using Non-Functional Properties. In Proceedings of the 2nd Int'l Workshop on Variability Modelling of Software-Intensive Systems (VaMoS), pages 25--23, Essen, Germany: University of Duisburg-Essen, January 2008. [ http, .pdf, bib ]

Software product lines (SPL) allow to generate tailormade software by manually configuring reusable core assets. However, SPLs with hundreds of features and millions of possible products require an appropriate support for semi-automated product derivation. This derivation has to be based on non-functional properties that are related to core assets and domain features. Both elements are part of different models connected via complex mappings. We propose a model that integrates features and core assets in order to allow semi-automated product derivation.

M. Kuhlemann, and C. Kästner. Reducing the Complexity of AspectJ Mechanisms for Recurring Extensions. In Proceedings of the GPCE Workshop on Aspect-Oriented Product Line Engineering (AOPLE), pages 14--19, 2007. [ .pdf, bib ]

Aspect-Oriented Programming (AOP) aims at modularizing crosscutting concerns. AspectJ is a popular AOP language extension for Java that includes numerous sophisticated mechanisms for implementing crosscutting concerns modularly in one aspect. The language allows to express complex extensions, but at the same time the complexity of some of those mechanisms hamper the writing of simple and recurring extensions, as they are often needed especially in software product lines. In this paper we propose an AspectJ extension that introduces a simplified syntax for simple and recurring extensions. We show that our syntax proposal improves evolvability and modularity in AspectJ programs by avoiding those mechanisms that may harm evolution and modularity if misused. We show that the syntax is applicable for up to 74\% of all pointcut and advice mechanisms by analysing three AspectJ case studies.

S. Apel, C. Kästner, M. Kuhlemann, and T. Leich. Pointcuts, Advice, Refinements, and Collaborations: Similarities, Differences, and Synergies. Innovations in Systems and Software Engineering -- A NASA Journal (ISSE), 3(3-4):281--289, December 2007. [ http, .pdf, bib ]

Aspect-oriented programming (AOP) is a novel programming paradigm that aims at modularizing complex software. It embraces several mechanisms including (1) pointcuts and advice as well as (2) refinements and collaborations. Though all these mechanisms deal with crosscutting concerns, i.e., a special class of design and implementation problems that challenge traditional programming paradigms, they do so in different ways. In this article we explore their relationship and their impact on software modularity. This helps researchers and practitioners to understand their differences and guides to use the right mechanism for the right problem.

S. Trujillo, C. Kästner, and S. Apel. Product Lines that supply other Product Lines: A Service-Oriented Approach. In Proceedings of the SPLC Workshop on Service-Oriented Architectures and Product Lines (SOAPL), pages 69--76, Pittsburgh, PA: SEI, September 2007. [  bib ]

Software product line is a paradigm to develop a family of software products with the goal of reuse. In this paper, we focus on a scenario in which different products from different product lines are combined together in a third product line to yield more elaborate products, i.e., a product line consumes products from third product line suppliers. The issue is not how different products can be produced separately, but how they can be combined together. We propose a service-oriented architecture where product lines are regarded as services, yielding a service-oriented product line. This paper illustrates the approach with an example for a service-oriented architecture of a web portal product line supplied by portlet product lines.

S. Apel, C. Lengauer, D. Batory, B. Möller, and C. Kästner. An Algebra for Feature-Oriented Software Development. Technical Report MIP-0706, Passau, Germany: Department of Informatics and Mathematics, University of Passau, July 2007. [ .pdf, bib ]
C. Kästner, M. Kuhlemann, and D. Batory. Automating Feature-Oriented Refactoring of Legacy Applications. In Proceedings of the ECOOP Workshop on Refactoring Tools (WRT), pages 62--63, Berlin, Germany: TU Berlin, July 2007. [ .pdf, bib ]

Creating a software product line from a legacy application is a difficult task. We propose a tool that helps automating tedious tasks of refactoring legacy applications into features and frees the developer from the burden of performing laborious routine implementations.

C. Kästner. CIDE: Decomposing Legacy Applications into Features. In Proceedings of the 11st International Software Product Line Conference, second volume (Demonstration) (SPLC), pages 149--150, 2007. [ .pdf, bib ]

Taking an extractive approach to decompose a legacy application into features is difficult and laborious with current approaches and tools. We present a prototype of a tooldriven approach that largely hides the complexity of the task.

C. Kästner, S. Apel, and D. Batory. A Case Study Implementing Features Using AspectJ. In Proceedings of the 11st International Software Product Line Conference (SPLC), pages 223--232, Los Alamitos, CA: IEEE Computer Society, September 2007. Acceptance rate: 35 % (28/80). [ .pdf, bib ]

Software product lines aim to create highly configurable programs from a set of features. Common belief and recent studies suggest that aspects are well-suited for implementing features. We evaluate the suitability of AspectJ with respect to this task by a case study that refactors the embedded database system Berkeley DB into 38 features. Contrary to our initial expectations, the results were not encouraging. As the number of aspects in a feature grows, there is a noticeable decrease in code readability and maintainability. Most of the unique and powerful features of AspectJ were not needed. We document where AspectJ is unsuitable for implementing features of refactored legacy applications and explain why.

S. Apel, C. Kästner, T. Leich, and G. Saake. Aspect Refinement - Unifying AOP and Stepwise Refinement. Journal of Object Technology (JOT), Special Issue on TOOLS EUROPE 2007, 6(9):13--33, October 2007. [ .pdf, http, bib ]

Stepwise refinement (SWR) is fundamental to software engineering. As aspectoriented programming (AOP) is gaining momentum in software development, aspects should be considered in the light of SWR. In this paper, we elaborate the notion of aspect refinement that unifies AOP and SWR at the architectural level. To reflect this unification to the programming language level, we present an implementation technique for refining aspects based on mixin composition along with a set of language mechanisms for refining all kinds of structural elements of aspects in a uniform way (methods, pointcuts, advice). To underpin our proposal, we contribute a fully functional compiler on top of AspectJ, present a non-trivial, medium-sized case study, and derive a set of programming guidelines.

S. Apel, C. Kästner, and S. Trujillo. On the Necessity of Empirical Studies in the Assessment of Modularization Mechanisms for Crosscutting Concerns. In Proceedings of the ICSE Workshop on Assessment of Contemporary Modularization Techniques (ACoM), Los Alamitos, CA: IEEE Computer Society, May 2007. [ .pdf, bib ]

Collaborations are a frequently occurring class of crosscutting concerns. Prior work has argued that collaborations are better implemented using Collaboration Languages (CLs) rather than AspectJ-like Languages (ALs). The main argument is that aspects flatten the objectoriented structure of a collaboration, and introduce more complexity rather than benefits – in other words, CLs and ALs differ with regard to program comprehension. To explore the effects of CL and AL modularization mechanisms on program comprehension, we propose to conduct a series of experiments. We present ideas on how to arrange such experiments that should serve as a starting point and foster a discussion with other researchers.

C. Kästner. Aspect-Oriented Refactoring of Berkeley DB. Diplomarbeit, Magdeburg, Germany: University of Magdeburg, March 2007. [ .pdf, bib ]
S. Apel, C. Kästner, M. Kuhlemann, and T. Leich. Modularität von Softwarebausteinen: Aspekte versus Merkmale. iX Magazin für Professionelle Informationstechnik (iX), (10):116--122, October 2006. [ http, bib ]

Schon seit einigen Jahren macht die aspektorientierte Programmierung von sich reden. Daneben zieht in jüngster Zeit die merkmalsorientierte Programmierung die Aufmerksamkeit auf sich. Beide verfolgen mit der Verbesserung der Modularität von Softwarebausteinen ähnliche Ziele, realisieren dies aber auf unterschiedliche Art und Weise - jeweils mit Vor- und Nachteilen.}

S. Apel, C. Kästner, T. Leich, and G. Saake. Aspect Refinement. Technical Report FIN-2006-10, Magdeburg, Germany: University of Magdeburg, August 2006. [ .pdf, bib ]
C. Kästner, S. Apel, and G. Saake. Implementing Bounded Aspect Quantification in AspectJ. In Proceedings of the 4th Workshop on Reflection, AOP and Meta-Data for Software Evolution (RAM-SE), pages 111--122, Magdeburg, Germany: University of Magdeburg, July 2006. [ .pdf, bib ]

The integration of aspects into the methodology of stepwise software development and evolution is still an open issue. This paper focuses on the global quantification mechanism of nowadays aspect-oriented languages that contradicts basic principles of this methodology. One potential solution to this problem is to bound the potentially global effects of aspects to a set of local development steps. We discuss several alternatives to implement such bounded aspect quantification in AspectJ. Afterwards, we describe a concrete approach that relies on meta-data and pointcut restructuring in order to control the quantification of aspects. Finally, we discuss open issues and further work.

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Supervised Theses

Jens Meinicke. VarexJ: A Variability-Aware Interpreter for Java Applications. Master's Thesis, University of Magdeburg, Germany, December 2014. [ bib ]
Jonas Pusch. Variability-Aware Interpretation. Bachelor's Thesis, University of Marburg, Germany, November 2012. [ bib | .pdf]
Markus Kreutzer. Statische Analyse von Produktlinien. Bachelor's Thesis, University of Marburg, Germany, April 2012. [ bib ]
Steffen Haase. A Program Slicing Approach to Feature Identification in Legacy C Code. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, February 2012. [ bib | .pdf]
Constanze Adler. Optional Composition – A Solution to the Optional Feature Problem? Master's Thesis, University of Magdeburg, Germany, December 2010. [ bib ]
Matthias Ritter. Softwareschutz auf Quellcode-Ebene durch Techniken der Softwareproduktlinienentwicklung. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, September 2010. [ bib ]
Andy Kenner. Statische Referenzanalyse in C-Präprozessor-konfigurierten Anwendungen. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, August 2010. Results published as a workshop paper at FOSD 2010bib | .pdf]
Alexander Dreiling. Feature Mining: Semiautomatische Transition von (Alt-)Systemen zu Software-Produktlinien. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, July 2010. A journal paper about the results is currently under reviewbib | .pdf]
Christian Becker. Entwicklung eines nativen Compilers für Feature-orientierte Programmierung. Master's Thesis, University of Magdeburg, Germany, June 2010. [ bib | .pdf]
Thomas Thüm. A Machine-Checked Proof for a Product-Line-Aware Type System. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, January 2010. Best-thesis award of the Denert Foundation for Software Engineering. Results published as part of a journal paper in ACM Transactions on Software Engineering and Methodology (TOSEM), 2011bib | .pdf]
Andreas Schulze. Systematische Analyse von Feature-Interaktionen in Softwareproduktlinien. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, November 2009. [ bib | .pdf]
Dirk Aporius. Verringerung des redundanten Softwareentwicklungsaufwandes für Portable Systeme. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, October 2009. [ bib ]
Janet Feigenspan. Empirical Comparison of FOSD Approaches Regarding Program Comprehension – A Feasibility Study. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, August 2009. Best-thesis award by Metop Research Center and Research Award by IHK Magdeburg. The results were published as part of a journal paper in Empirical Software Engineering, 2012.bib | .pdf]
Malte Rosenthal. Alternative Features in Colored Featherweight Java. Master's Thesis (Diplomarbeit), University of Passau, Germany, July 2009. [ bib | .pdf]
Chau Le Minh. Evaluation feature-basierter service-orientierter Architekturen am Beispiel eines Domotic-Szenarios. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, June 2009. [ bib | .pdf]
Stefan Kegel. Streamed verification of a data stream management benchmark. Bachelor's Thesis (Studienarbeit), University of Magdeburg, Germany, April 2009. [ bib | .pdf]
Janet Feigenspan. Requirements and design for a language-independent IDE framework to support feature-oriented programming. Bachelor's Thesis (Studienarbeit), University of Magdeburg, Germany, February 2009. [ bib | .pdf]
Christian Hübner. Unterstützung der Requirementsanalyse von Navigationssoftware auf Grundlage feature-basierter Domänen-Modelle. Master's Thesis (Diplomarbeit), University of Magdeburg, Germany, December 2008. [ bib ]
Axel Hoffmann. Nachvollziehbare Bewirtschaftung gewachsener Datenbestände großer Unternehmen für das Controlling. Bachelor's Thesis (Studienarbeit), University of Magdeburg, Germany, August 2008. [ bib ]
Thomas Thüm. Reasoning about Feature Model Edits. Bachelor's Thesis (Studienarbeit), University of Magdeburg, Germany, June 2008. Results published as conference paper at the International Conference on Software Engineering (ICSE), 2009.bib | .pdf]