Finalizing a Servlet

When a servlet container determines that a servlet should be removed from service (for example, when a container wants to reclaim memory resources or when it is being shut down), the container calls the destroy method of the Servlet interface. In this method, you release any resources the servlet is using and save any persistent state. The following destroy method releases the database object created in the init method described in Initializing a Servlet:

public void destroy() {
  bookDB = null;
}

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 how to do the following:

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 in your servlet class a field that counts the number of service methods that are running. The field should have synchronized access methods to increment, decrement, and return its value.

public class ShutdownExample extends HttpServlet {
  private int serviceCounter = 0;
  ...
  // Access methods for serviceCounter
  protected synchronized void enteringServiceMethod() {
    serviceCounter++;
  }
  protected synchronized void leavingServiceMethod() {
    serviceCounter--;
  }
  protected synchronized int numServices() {
    return serviceCounter;
  }
}

The service method should increment the service counter each time the method is entered and should 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 the functionality of the original service method:

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

Notifying Methods to Shut Down

To ensure a clean shutdown, your destroy method should not release 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 notification, another field is required. The field should have the usual access methods:

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

Here is an example of the destroy method using these fields to provide a clean shutdown:

public void destroy() {
  /* 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 service methods to stop. */
  while(numServices() > 0) {
    try {
      Thread.sleep(interval);
    } 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 shutdowns and should interrupt their work, if necessary.

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