Handling Shutdown Event

Question

Hi I have an Standalone application in which when an user logs in a abc.lck file gets created when the application is closed it gets deleted.I have used addshutdownhook() to delete the file when power supply is interrupted that is switching off the power supply when my application is running.My problem is the file is not getting deleted when I manually shutdown the system i.e by start-->shutdown and I should prompt the user with a message to save the changes using cofirm dailog box like in MS Word.Can some one help me Thanking u Chaithu

Answer 1

Use deleteOnExit method instead of adding shutdownhook. However, take a look at this sample,

class Shutdown {

  private Thread thread = null; 
  protected boolean flag=false;
  public Shutdown() {
     thread = new Thread("Sample thread") {
     public void run() {
        while (!flag) {
           System.out.println("Sample thread");
          try {
              Thread.currentThread().sleep(1000);
              } catch (InterruptedException ie) {
               break;
              }
        }
        System.out.println("[Sample thread] Stopped");
     }
   };
  thread.start();
  }
public void stopThread() {
    flag=true;
 }
}

class ShutdownThread extends Thread {
    private Shutdown shutdown = null;
    public ShutdownThread(Shutdown shutdown) {
      super();
      this.shutdown = shutdown;
    }
    public void run() {
         System.out.println("Shutdown thread");
         shutdown.stopThread();
         System.out.println("Shutdown completed");
    }
}

public class Main {
  public static void main(String [] args) {
   Shutdown shutdown = new Shutdown();
   try {
     Runtime.getRuntime().addShutdownHook(new ShutdownThread(shutdown));
     System.out.println("[Main thread] Shutdown hook added");
    } catch (Throwable t) {
      System.out.println("[Main thread] Could not add Shutdown hook");
    }

    try {
       Thread.currentThread().sleep(10000);
    } catch (InterruptedException ie) {}
    System.exit(0);
   }
}

Answer 2

The general contract of addShutdown hook is

The Java virtual machine shuts down in response to two kinds of events:

• The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or

• The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.

A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.

In rare circumstances the virtual machine may abort, that is, stop running without shutting down cleanly. This occurs when the virtual machine is terminated externally, for example with the SIGKILL signal on Unix or the TerminateProcess call on Microsoft Windows. The virtual machine may also abort if a native method goes awry by, for example, corrupting internal data structures or attempting to access nonexistent memory. If the virtual machine aborts then no guarantee can be made about whether or not any shutdown hooks will be run.

Hence during shutdown, the Windows machine may call TerminateProcess and hence your shutdown hook might not be invoked.