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Trail: Servlets
Lesson: Interacting with Clients

## Threading Issues

> HTTP servlets are typically capable of serving multiple clients concurrently. If the methods in your servlet that do work for clients access a shared resource, then you can handle the concurrency by creating a servlet that handles only one client request at a time. (You could also synchronize access to the resource, a topic that is covered in this tutorial's lesson on threads.)

> To have your servlet handle only one client at a time, have your servlet implement the `SingleThreadModel` interface in addition to extending the `HttpServlet` class.

> Implementing the `SingleThreadModel` interface does not involve writing any extra methods. You merely declare that the servlet implements the interface, and the server makes sure that your servlet runs only one `service` method at a time.

> For example, the ReceiptServlet accepts a user's name and credit card number, and thanks the user for their order. If this servlet actually updated a database, for example one that kept track of inventory, then the database connection might be a shared resource. The servlet could either synchronize access to that resource, or it could implement the `SingleThreadModel` interface. If the servlet implemented the interface, the only change in the code from the previous section is the one line shown in bold:

>         public class ReceiptServlet extends HttpServlet
>                                     implements SingleThreadModel {

>             public void doPost(HttpServletRequest request,
>                                HttpServletResponse response)
>     	    throws ServletException, IOException {
>                     ...
>             }
>             ...
>         }

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