Question

thread.join() will call thread.wait(), but who and when notifies (either with thread.notify() or notifyAll()) the thread.wait()?

As we know, thread join will wait for the thread to be completed, but who calls notify on it?

Answer 1

Edit:

Oh, you are talking about inside of the Thread object itself. Inside of join() we do see a wait(). Something like:

while (isAlive()) {
    wait(0);
}

The notify() for this is handled by the Thread subsystem. When the run() method finishes, the notify() is called on the Thread object. I’m not sure if the code that actually calls notify() can be seen — it seems to be done in native code.

---

No user code needs to call notify() on that Thread object. The Java Thread code handles this internally. Once the thread finishes, the join() call will return.

For example, the following code will execute fine and the join() call will return fine without any wait() or notify() calls.

Thread thread = new Thread(new Runnable() {
   public void run() {
      // no-op, just return immediately
   }
});
thread.start();
thread.join();

It is important to note that this behavior should probably not be relied upon. The notify() call is internal to the thread system. You should use join() if you are waiting for a thread to finish.