SingleThreadModel (Controlling Concurrent Access to Shared Resources)

Controlling Concurrent Access to Shared Resources

In a multithreaded server, it is possible for shared resources to be accessed concurrently. Besides scope object attributes, shared resources include in-memory data such as instance or class variables, and external objects such as files, database connections, and network connections. Concurrent access can arise in several situations:

* Multiple Web components accessing objects stored in the Web context
* Multiple Web components accessing objects stored in a session
* Multiple threads within a Web component accessing instance variables. A Web container will typically create a thread to handle each request. If you want to ensure that a servlet instance handles only one request at a time, a servlet can implement the SingleThreadModel interface. If a servlet implements this interface, you are guaranteed that no two threads will execute concurrently in the servlet's service method. A Web container can implement this guarantee by synchronizing access to a single instance of the servlet, or by maintaining a pool of Web component instances and dispatching each new request to a free instance. This interface does not prevent synchronization problems that result from Web components accessing shared resources such as static class variables or external objects.

Source>>http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/Servlets5.html#73895