Ensures that servlets handle
only one request at a time. This interface has no methods.
If a servlet implements this interface, you are guaranteed
that no two threads will execute concurrently in the
servlet's service method. The servlet container
can make this guarantee by synchronizing access to a single
instance of the servlet, or by maintaining a pool of servlet
instances and dispatching each new request to a free servlet.
This interface does not prevent
synchronization problems that result from servlets accessing shared
resources such as static class variables or classes outside
the scope of the servlet.