Thread waiting another thread

Question

I need an explanation here.

 public static void main(String[] args) {
    FirstThread obj = new FirstThread();
    for (int i = 1; i <= 10; i++) {
      new WaiterThread(obj).start();
    }
    obj.start();
  }

public class FirstThread extends Thread {
  @Override
  public void run() {
    // Do something
  }
}


public class WaiterThread extends Thread {
  Object obj;

  WaiterThread(Object obj) {
    this.obj = obj;
  }

  @Override
  public void run() {
    synchronized (obj) {
           obj.wait();
    }
  }
}

10 threads are created for WaiterThread and are waiting for a single FirstThread object. After FirstThread terminates, all WaiterThread s resumed without obj.notify() or obj.notifyAll() being called anywhere.

Does that mean WaiterThread s stopped waiting for FirstThread because it gets terminated?

Answer 1

As per the documentation of the Thread class, a dying thread calls notifyAll on the instance which represents it.

Furthermore, quoting the same documentation:

It is recommended that applications not use wait, notify, or notifyAll on Thread instances.

Of course, the same recommendation applies to instances of Thread subclasses, which is what your code is doing.

Answer 2

This is a side-effect of the fact that when a thread terminates, it invokes this.notifyAll() (as documented in the javadoc of Thread.join()). The same javadoc also makes the following recommendation:

It is recommended that applications not use wait, notify, or notifyAll on Thread instances