http://www.jboss.org/jboss-servlet-api_4.0_spec: The Java Servlet 4.0 API classes (JBoss by Red Hat) 
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package javax.servlet;


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.

Note that SingleThreadModel does not solve all thread safety
issues.  For example, session attributes and static variables can
still be accessed by multiple requests on multiple threads
at the same time, even when SingleThreadModel servlets are used.
It is recommended that a developer take other means to resolve
those issues instead of implementing this interface, such as
avoiding the usage of an instance variable or synchronizing
the block of the code accessing those resources.
This interface is deprecated in Servlet API version 2.4.

Author:	Various
Deprecated:	As of Java Servlet API 2.4, with no direct
replacement.
/**
 * Ensures that servlets handle
 * only one request at a time. This interface has no methods.
 *
 * <p>If a servlet implements this interface, you are <i>guaranteed</i>
 * that no two threads will execute concurrently in the
 * servlet's <code>service</code> 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.
 *
 * <p>Note that SingleThreadModel does not solve all thread safety
 * issues.  For example, session attributes and static variables can
 * still be accessed by multiple requests on multiple threads
 * at the same time, even when SingleThreadModel servlets are used.
 * It is recommended that a developer take other means to resolve
 * those issues instead of implementing this interface, such as
 * avoiding the usage of an instance variable or synchronizing
 * the block of the code accessing those resources.
 * This interface is deprecated in Servlet API version 2.4.
 *
 *
 * @author	Various
 *
 * @deprecated	As of Java Servlet API 2.4, with no direct
 *	replacement.
 */
@Deprecated
public interface SingleThreadModel {
}