Synchronizing on a Thread Object

Question

From Niko's java blog

These classes are defined in the same file. What is the output? (1 correct answer)

class Job extends Thread {
    private Integer number = 0;
    public void run() {
        for (int i = 1; i < 1000000; i++) {
            number++;
        }
    }
    public Integer getNumber() {
        return number;
    }
}
public class Test {
    public static void main(String[] args) 
    throws InterruptedException {
        Job thread = new Job();
        thread.start();
        synchronized (thread) {
            thread.wait();
        }
        System.out.println(thread.getNumber());
    }
}

• It prints 0.
• It prints 999999.
• The output is not guaranteed to be any of the above.

The output is 999999. I understand that when a Thread completes its run() method, it terminates and with it, all the locks the Thread had get released. However, in this exercise, it's used the Thread object as a lock, should not it be considered as a normal object? I mean the lock it's not owned by the Thread thread but by the main Thread.

Answer 1

This code relies on an implementation detail that was not documented before Java 7, and now is (in the documentation of the join() method), but with the following words:

As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.

So I don't know where this question was asked, but it really tests if you know the hidden corner-cases of threading, that you should never use in any sane program.