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Handling Service Threads at Servlet Termination

All of a servlet's service methods should be complete when a servlet is removed. The server tries to ensure this by calling the destroy method only after all service requests have returned, or after a server-specific grace period, whichever comes first. If your servlet has operations that take a long time to run (that is, operations that may run longer than the server's grace period), the operations could still be running when destroy is called. You must make sure that any threads still handling client requests complete; the remainder of this section describes a technique for doing this.

If your servlet has potentially long-running service requests, use the techniques in this lesson to:

• Keep track of how many threads are currently running the service method.
   
• Provide a clean shutdown by having the destroy method notify long-running threads of the shutdown and wait for them to complete
   
• Have the long-running methods poll periodically to check for shutdown and, if necessary, stop working, clean up and return.
   

Tracking Service Requests

To track service requests, include a field in your servlet class that counts the number of service methods that are running. The field should have access methods to increment, decrement, and return its value. Because multiple threads will be accessing the field, and the destroy method will wait for the field to reach zero, field accesses should be synchronized. The object in the private field called lock is the object that we synchronize on. For example:

public ShutdownExample extends HttpServlet {
    private int serviceCounter = 0;
    private Object lock = new Object();
    ...
    //Access methods for serviceCounter
    protected void enteringServiceMethod() {
        synchronized(lock) {
	    serviceCounter++;
        }
    }
    protected void leavingServiceMethod() {
        synchronized(lock) {
            serviceCounter--;
            if (serviceCounter == 0 && isShuttingDown())
                notifyAll();
        }
    }
    protected int numServices() {
        synchronized(lock) {
	    return serviceCounter;
        }
    }
}

The service method should increment the service counter each time the method is entered and decrement the counter each time the method returns. This is one of the few times that your HttpServlet subclass should override the service method. The new method should call super.service to preserve all the original HttpServlet.service method's functionality.

protected void service(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
{
    enteringServiceMethod();
    try {
        super.service(req, resp);
    } finally {
        leavingServiceMethod();
    }
}

Providing a Clean Shutdown

To provide a clean shutdown, your destroy method should not destroy any shared resources until all the service requests have completed. One part of doing this is to check the service counter. Another part is to notify the long-running methods that it is time to shut down. For this, another field is required along with the usual access methods. For example:

public ShutdownExample extends HttpServlet {
    private boolean shuttingDown;
    ...
    //Access methods for shuttingDown
    protected void setShuttingDown(boolean flag) {
	shuttingDown = flag;
    }
    protected boolean isShuttingDown() {
	return shuttingDown;
    }
}

An example of the destroy method using these fields to provide a clean shutdown is shown below:

public void destroy() {

    synchronized(lock) {
        /* Check to see whether there are still service methods running,
         * and if there are, tell them to stop. */
        if (numServices() > 0) {
            setShuttingDown(true);
        }

        /* Wait for the all of the service methods to stop.  */
        while(numServices() > 0) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
    }
}

Creating Polite Long-running Methods

The final step in providing a clean shutdown is to make any long-running methods behave politely. Methods that might run for a long time should check the value of the field that notifies them of shut downs, and interrupt their work if neceesary. For example:

public void doPost(...) {
    ...
    for(i = 0; ((i < lotsOfStuffToDo) && !isShuttingDown()); i++) {
	try {
	    partOfLongRunningOperation(i);
	} catch (InterruptedException e) {
        }
    }
}

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