TimerTask keeps running

I have a question about the behaviour of Timer class in Java. This is the code: http://pastebin.com/mqcL9b1n

public class Main {

    public static void main(String[] args) {
        Main m = new Main();
        m.foo();
        m = null;
    }

    public void foo() {
        Timer t =开发者_开发知识库 new Timer();
        t.schedule(new SysPrint(), 200);
    }

}

class SysPrint extends TimerTask {

    public void run() {
        System.out.println("Yes!");
    }
}

What happens is that if you run that program, it will print "Yes!" and it's not gonna do anything else (the program doesn't end).

The Java documentation says: After the last live reference to a Timer object goes away and all outstanding tasks have completed execution, the timer's task execution thread terminates gracefully (and becomes subject to garbage collection).

As I see this thing, the "last live reference" to the Timer object is gone after the 'foo()' functions ends. And the only task scheduled was the "Yes!" task that was executed, so I guess that after the process printed "Yes!", the Timer object should end and the process should terminate.

What happened here?

Java is not exiting because your thread running the Timer is still kicking around. You have to mark that thread as being a daemon thread before Java will exit. You probably don't have access to the thread itself so unless Timer has a method to mark it so you'll have a hard time doing that. You'll need to manually stop it in a finally clause.

try {
   timer = new Timer();
   timer.schedule( new SysPrint(), 200 );
} finally {
   timer.cancel();
}

I believe the code below should do the trick.

public class Main {
  public static void main(String[] args) {
    Main m = new Main();
    m.foo();
    m = null;
  }

  public void foo() {
    Timer t = new Timer();
    t.schedule(new SysPrint(), 200);
  }
}

class SysPrint extends TimerTask {
  SysPrint(Timer timer) {
    this.timer = timer;
  }

  public void run() {
    System.out.println("Yes!");
    timer.cancel();
  }

  private Timer timer;
}

When you create a Timer object. A TimerThread is created. And it the internal thread to run your task. You can view the method run() of TimerThread. You can see it has a while loop.

 private void mainLoop() {
    while (true) {....

The TimerThread not set to a daemon, so the main method execute completely, the jvm not exists.

That why your program is always running and don't stop.