To demonstrate the first case, when a Thread is blocked in Object.wait() method, we’ll use Thread implementation where it obtains object’s lock and waits on it for notification from other Thread:

static class Waiter extends Thread {
    private Object lock = new Object();

    @Override
    public void run() {
        try {
            System.out.println("Waiter running...");
            synchronized (lock) {
                lock.wait();
            }
        } catch (InterruptedException e) {
            System.out.println("Waiter thread interrupted!");
            e.printStackTrace();
        }
    }
}

Well, this example is contrived, but is enough for our case – the thread is blocked.

Note: the correct way to use Object.wait() is to call in inside a loop, which checks for end condition. The reason is that JVM can spuriously wake up a Thread – we’ll write a separate post about this.

Now let’s interrupt the Waiter, when it is waiting in lock.wait() method:

public static void main(String[] args) throws Exception {
    Thread waiter = new Waiter();
    waiter.start();

    // give the waiter some time to lock itself
    TimeUnit.SECONDS.sleep(1);

    waiter.interrupt();
}

As can be seen in logs, the thread has been unblocked:

Waiter running...
Waiter thread interrupted!
java.lang.InterruptedException
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:502)
        at com.farenda.java.lang.ThreadInterrupted$Waiter.run(ThreadInterrupted.java:14)

The same happens when a Thread is waiting for another Thread to die using its join() method. This is because the Thread.join() is implemented as a loop that internally calls Object.wait() as can be observe in the exception stacktrace below:

static class Cleaner extends Thread {
    private final Thread other;

    public Cleaner(Thread other) {
        this.other = other;
    }

    @Override
    public void run() {
        try {
            System.out.println("Cleaner running...");
            // Wait for the other thread's death:
            other.join();
            System.out.println("Not waiting...");
        } catch (InterruptedException e) {
            System.out.println("Cleaner thread interrupted!");
            e.printStackTrace();
        }
    }
}

Again let’s run the code and then interrupt the thread:

public static void main(String[] args) throws Exception {
    Thread cleaner = new Cleaner(Thread.currentThread());
    cleaner.start();

    // give some time to call join()
    TimeUnit.MILLISECONDS.sleep(100);

    cleaner.interrupt();

    TimeUnit.MILLISECONDS.sleep(50);
}

As you can see, the message “Not waiting…” is never printed. What’s more interesting, the InterruptedException has been thrown from Object.wait() method:

Cleaner running...
Cleaner thread interrupted!
java.lang.InterruptedException
        at java.lang.Object.wait(Native Method)
        at java.lang.Thread.join(Thread.java:1249)
        at java.lang.Thread.join(Thread.java:1323)
        at com.farenda.java.lang.ThreadInterrupted$Cleaner.run(ThreadInterrupted.java:34)

The last example of java.lang.InterruptedException is from interrupted Thread.sleep() – java.util.concurrent.TimeUnit is just a wrapper around Thread.sleep() that allows to express time in human readable way:

static class Sleeper extends Thread {
    @Override
    public void run() {
        try {
            System.out.println("Sleeper running...");
            TimeUnit.SECONDS.sleep(10);
        } catch (InterruptedException e) {
            System.out.println("Sleeper thread interrupted!");
            e.printStackTrace();
        }
    }
}

Again, simple code using Thread.interrupt() to interrupt a thread waiting on blocking Thread.sleep() call:

public static void main(String[] args) throws Exception {
    Thread sleeper = new Sleeper();
    sleeper.start();

    // give it some time to call sleep()
    TimeUnit.MILLISECONDS.sleep(100);

    sleeper.interrupt();
}

Note that unlike Thread.join() this one doesn’t call Object.wait() underneath:

Sleeper running...
Sleeper thread interrupted!
java.lang.InterruptedException: sleep interrupted
        at java.lang.Thread.sleep(Native Method)
        at java.lang.Thread.sleep(Thread.java:340)
        at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
        at com.farenda.java.lang.ThreadInterrupted$Sleeper.run(ThreadInterrupted.java:48)

• How to use Thread.join()
• Check out Java Concurrency Tutorial for more cool things!