等等 - 通知工作？

Question

This is the code I have written, to test the working of wait() and notify() . Now I have a bunch of doubts.

class A extends Thread {
  public void run() {
    try {
      wait();
      for (int i = 1; i <= 5; i++) {
        System.out.println(i);
        sleep(500);
      }
    } catch (Exception e) {
    }
    System.out.println("End Of Thread");
  }
}

class ThreadWaitNotify {
  public static void main(String args[]) {
    try {
      A t = new A();
      t.start();
      t.wait();
      t.notify();
      t.join();
      System.out.println("End Of Main");
    } catch (Exception e) {
    }
  }
}

My questions are:

1. When I write t.wait() in main, the main does not execute further, and I am not able to resume it further. How to do that?
2. Secondly I have written wait() in thread also, due to which it only prints "End Of Thread", not the loop? Even if I notify() from main or not...
3. Now if I write notify() in main it does not complete execution. While on commenting that line it finishes execution and prints "End Of Main".

Answer 1

My questions are:

When I write t.wait() in main, the main does not execute further, and I am not able to resume it further. How to do that?

The thread that is running main does not hold the lock on t when it calls t.wait() . As you'll see from the JavaDoc for wait() :

Throws : IllegalMonitorStateException - if the current thread is not the owner of the object's monitor.

So your call to wait() is resulting in the IllegalMonitorStateException being thrown. Your empty catch block simply discards the error, which makes it difficult to debug. You can show the error using ex.printStackTrace() ; or you can re-throw it as an unchecked exception:

    throw new RuntimeException(ex);

To fix the wait() call, you'd need to synchronize on t:

synchronized (t) {
    t.wait();
}

You'll also need to hold the same lock when you call notify().

Secondly I have written wait() in thread also, due to which it only prints "End Of Thread", not the loop? Even if I notify() from main or not...

Same thing is happening here, but in a different thread. The new thread that you create doesn't own the lock on t, so the wait() call throws an Exception. Again, you're discarding the exception, by not handling it correctly.

Now if I write notify() in main it does not complete execution. While on commenting that line it finishes execution and prints "End Of Main".

I assume you mean "commenting out the calls to wait() and notify()". This is expected. The run() method has finished, that is: it caught the IllegalMonitorStateException that was thrown and carried on to the end of the method. The join() method returns when the thread has finished executing. This happens almost immediately, because the wait() call throws an exception immediately.