Question

    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                TimeUnit.SECONDS.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    });

    thread.start();
    try {
        thread.wait();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println("end");
}

this sentence will throw :

    Exception in thread "main" java.lang.IllegalMonitorStateException 
at java.lang.Object.wait(Native Method)

but use “thread.join()” to replace “thread.wait(0)’ doesn’t throw any exception.

The puzzle is
I query the thread.join() source code: it will go to :

while(isAlive) 
wait(0);

it means they both trigger wait(0). but why the result is so different?

Answer 1

Take another look at the source code and you will find the join is done while holding the monitor ( while synchronized ). If you want to use “thread.wait(0)” you need to wrap it in a synchronized block or method.

Have a look at: http://docs.oracle.com/javase/tutorial/essential/concurrency/guardmeth.html

    Object o = new Object();
    synchronized (o) {
        o.wait(timeInMS);
    }

and so in your code try

    synchronized (thread) {
        thread.wait(0);
    }