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Trail: Essential Java Classes
Lesson: Accessing System Resources

## Writing a Security Manager

> To write your own security manager, you must create a subclass of the SecurityManager class. Your SecurityManager subclass overrides various methods from SecurityManager to customize the verifications and approvals needed in your Java application.

> This page walks through an example security manager that restricts reading and writing to the file system. To get approval from the security manager, a method that opens a file for reading invokes one of SecurityManager's `checkRead()` methods. Similarly, a method that opens a file for writing invokes one of SecurityManager's `checkWrite()` methods. If the security manager approves the operation then the `checkXXX()` method returns, otherwise `checkXXX()` throws a SecurityException.

> To impose a stricter policy on file system accesses, our example SecurityManager subclass must override SecurityManager's `checkRead()` and `checkWrite()` methods. SecurityManager provides three versions of `checkRead()` and two versions of `checkWrite()`. Each of which should verify whether the application is allowed to open a file for I/O. A policy frequently implemented by browsers is that applets loaded over the network cannot read from or write to the local file system unless the user approves it.

> The policy implemented by our example prompts the user for a password when the application attempts to open a file for reading or for writing. If the password is correct then the access is allowed.

> All security managers must be a subclass of SecurityManager. Thus, our `PasswordSecurityManager` class extends SecurityManager.

> >     class PasswordSecurityManager extends SecurityManager {
> >         . . .
> >     }

> Next, PasswordSecurityManager declares a private instance variable `password` to contain the password that the user must enter in order to allow the restricted file system accesses. The password is set upon construction:

> >     PasswordSecurityManager(String password) {
> >         super();
> >         this.password = password;
> >     }

> The next method in the PasswordSecurityManager class is a private helper method named `accessOK()`. This method prompts the user for a password and verifies it. If the user enters a valid password, the method returns true; otherwise, it returns false.

> >     private boolean accessOK() {
> >         int c;
> >         DataInputStream dis = new DataInputStream(System.in);
> >         String response;

> >         System.out.println("What's the secret password?");
> >         try {
> >             response = dis.readLine();
> >             if (response.equals(password))
> >                 return true;
> >             else
> >                 return false;
> >         } catch (IOException e) {
> >             return false;
> >         }
> >     }

> Finally at the end of the PasswordSecurityManager class are the three overridden `checkRead()` methods and the two overridden `checkWrite()` methods:

> >     public void checkRead(FileDescriptor filedescriptor) {
> >         if (!accessOK())
> >             throw new SecurityException("Not a Chance!");
> >     }
> >     public void checkRead(String filename) {
> >         if (!accessOK())
> >             throw new SecurityException("No Way!");
> >     }
> >     public void checkRead(String filename, Object executionContext) {
> >         if (!accessOK())
> >             throw new SecurityException("Forget It!");
> >     }
> >     public void checkWrite(FileDescriptor filedescriptor) {
> >         if (!accessOK())
> >             throw new SecurityException("Not!");
> >     }
> >     public void checkWrite(String filename) {
> >         if (!accessOK())
> >             throw new SecurityException("Not Even!");
> >     }

> All the `checkXXX()` methods call `accessOK()` to prompt the user for a password. If access is not OK, then `checkXXX()` throws a SecurityException. Otherwise, `checkXXX()` returns normally. Note that SecurityException is a runtime exception, and as such does not need to be declared in the `throws` clause of these methods.

> `checkRead()` and `checkWrite()` are just a few of the many of SecurityManager's `checkXXX()` methods that verify various kinds of operations. You can override or add any number of `checkXXX()` methods to implement your security policy. You do not need to override all of SecurityManager's `checkXXX()` methods, just the ones that you want to customize. However, the default implementation provided by the SecurityManager class for all `checkXXX()` methods throws a SecurityException. In other words, by default the SecurityManager class disallows all operations that are subject to security restrictions. So you may find that you have to override many of SecurityManager's `checkXXX()` methods to get the behavior you want.

> All of SecurityManager's `checkXXX()` methods operate in the same way:

> - If access is allowed, the method returns.
> - If access is not allowed, the method throws a SecurityException.
> Make sure that you implement your overridden `checkXXX()` methods in this manner.

> Well, that's it for our SecurityManager subclass. As you can see implementing a SecurityManager is simple. You just:

> - Create a SecurityManager subclass.
> - Override a few methods.
> The tricky part is determining which methods to override and implementing your security policy. Deciding What SecurityManager Methods to Override will help you figure out which methods you should override depending on what types of operations you'd like to protect. The next page shows you how to install the PasswordSecurityManager class as the on-duty security manager for your Java application.

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