Thread.sleep vs. Object.wait

In my last 2 posts (here and here), I talked about some simple but overlooked topics in Java threading. I want to wrap up this series by pointing out yet another easily overlooked topic related to threading: the subtle difference between Thread.sleep and Object.wait.

In Java, both Thread.sleep and Object.wait make the current thread wait for a specified amount of time. This is useful when the current thread needs to wait for some other thread before it can proceed. I sometimes see developers use these interchangeably in close proximity in code but this can be problematic as the two methods are quite different.

Consider the example from my previous post. In that example, thread1 acquired a lock, slept for a minute while thread2 was blocked waiting for the same lock. It used TimeUnit.MINUTES.sleep(1) which uses Thread.sleep to make thread1 wait. Let’s change it to use Object.wait and see what happens:

private static void test() {
    final Object lock = new Object();

    Thread thread1 = new Thread(new Runnable() {
        @Override public void run() {
            synchronized (lock) {
                System.out.println("Thread1 acquired lock");
                try {
                    lock.wait(1 * 60 * 1000);
                    //TimeUnit.MINUTES.sleep(1);
                } catch (InterruptedException ignore) {}
            }
        }

    });
    thread1.start();

    Thread thread2 = new Thread(new Runnable() {
        @Override public void run() {
            synchronized (lock) {
                System.out.println("Thread2 acquired lock");
            }
        }
    });
    thread2.start();
}

Once you run this sample, you’ll see the following right away:

Thread1 acquired lock
Thread2 acquired lock

So what happened? When thread1 went to sleep, it released the lock and thread2 acquired it right away. Unlike Thread.sleep (where locks are not released) in Object.wait, locks are released as the thread goes to sleep. In some situations, this might be appropriate, in others, maybe not, so this is an important thing to keep in mind when deciding which sleep method to use.