To understand why this is, we should quickly review how sessions work. During a client's first request to a server, the server sets a special cookie on the client that holds a server-generated unique ID.
By instead implementing and having one "connection" instance variable per servlet, a servlet can easily handle concurrent requests by letting its server manage the servlet instance pool (which doubles as a connection pool). Let's see the simple example of implementing the Single Thread Model interface. The Single Thread Model interface was designed to guarantee that only one thread is executed at a time in a given servlet instance’s service method. This interface is currently deprecated, excerpts from the Java Doc: Deprecated. Such servlets indicate this desire by implementing the 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 Such a life cycle is pointless for a counter or other servlet application that requires central state maintenance.
Note that Single Thread Model 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 Single Thread Model servlets are used.They are not a good choice for acting as the back-end storage in real-world applications, no matter how tempting it might be to try it.Rather, sessions are best thought of as a handy local cache--a place to store information which, if lost, can be recovered or safely ignored.It is important to note that this guarantee only applies to each servlet instance, since the container may choose to pool such objects.
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