Java Servlet Programming

Single-Thread Model

Although it is standard to have one servlet instance per registered servlet name, it is possible for a servlet to elect instead to have a pool of instances created for each of its names, all sharing the duty of handling requests. Such servlets indicate this desire by implementing the javax.servlet.SingleThreadModel interface. This is an empty, tag interface that defines no methods or variables and serves only to flag the servlet as wanting the alternate life cycle.

A server that loads a SingleThreadModel servlet must guarantee, according to the Servlet API documentation, “that no two threads will execute concurrently the service method of that servlet.” To accomplish this, each thread uses a free servlet instance from the pool, as shown in Figure 3.4. Thus, any servlet implementing SingleThreadModel can be considered thread safe and isn’t required to synchronize access to its instance variables.

Figure 3-4. The Single Thread Model

Such a life cycle is pointless for a counter or other servlet application that requires central state maintenance. The life cycle can be useful, however, in avoiding synchronization while still performing efficient request handling.

For example, a servlet that connects to a database sometimes needs to perform several database commands atomically as part of a single transaction. Normally, this would require the servlet to synchronize ...