Controlling Concurrent Access to Shared Resources

In a multithreaded server, it is possible for shared resources to be accessed concurrently. In addition to 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. In addition, the Servlet 2.4 specification deprecates the SingleThreadModel interface.

When resources can be accessed concurrently, they can be used in an inconsistent fashion. To prevent this, you must control the access using the synchronization techniques described in the Threads lesson in The Java Tutorial, Fourth Edition, by Sharon Zakhour et al. (Addison-Wesley, 2006).

The preceding section showed five scoped attributes shared by more than one servlet: bookDB, cart, currency, hitCounter, and orderCounter. The bookDB attribute is discussed in the next section. The cart, currency, and counters can be set and read by multiple multithreaded servlets. To prevent these objects from being used inconsistently, access is controlled by synchronized methods. For example, here is the Counter class, located at tut-install/javaeetutorial5/examples/web/bookstore1/src/java/com/sun/bookstore1/util/:

public class Counter {
    private int counter;
    public Counter() {
        counter = 0;
    }
    public synchronized int getCounter() {
        return counter;
    }
    public synchronized int setCounter(int c) {
        counter = c;
        return counter;
    }
    public synchronized int incCounter() {
        return(++counter);
    }
}