Java thread InterruptedException block not executing

Question

My Java class is given below. It's a small exercise to test thread join (wait) and thread sleep (timed wait).

public class BasicThreadTest {

    public static void main(String[] args) {
        testThreadWait();
        System.out.println(Thread.currentThread().getName() + " exiting");
    }

    private static void testThreadWait() {
        Thread thread1 = new Thread(() -> {
            String currentThread = Thread.currentThread().getName();
            System.out.println(String.format("%s execution started", currentThread));
            long waitMillis = 20000L;
            try {
                System.out.println(String.format("%s going for timed wait of %d millis", currentThread, waitMillis));
                Thread.sleep(waitMillis);
            } catch (InterruptedException e) {
                System.out.println(String.format("%s timed wait over after %d millis", currentThread, waitMillis));
            }
            System.out.println(String.format("%s execution ending", currentThread));
        });
        thread1.setName("Thread-1");

        Thread thread2 = new Thread(() -> {
            String currentThread = Thread.currentThread().getName();
            System.out.println(String.format("%s execution started", currentThread));
            try {
                System.out.println(currentThread + " about to wait for " + thread1.getName());
                thread1.join();
            } catch (InterruptedException e) {
                System.out.println(String.format("%s wait over for %s", currentThread, thread1.getName()));
            }
            System.out.println(String.format("%s execution ending", currentThread));
        });
        thread2.setName("Thread-2");

        thread2.start();
        thread1.start();
    }
}

No matter what order I start the two threads in I never get the two InterruptedException blocks executed in either sleep() or join(). Below is a sample output:

Thread-2 execution started
Thread-2 about to wait for Thread-1
main exiting
Thread-1 execution started
Thread-1 going for timed wait of 20000 millis
Thread-1 execution ending
Thread-2 execution ending

Any explanation on why this is happening?

Where do you interrupt the threads? You won't get an `InterruptedException` if you don't actually interrupt them. — Andy Turner, Dec 08 '20 at 10:23
Makes sense, I had assumed that `InterruptedException` would always be thrown once wait or timed wait is over. That is not the case and looks like there has to be an explicit invocation of `interrupt()`. — sshekhar, Dec 08 '20 at 10:30

score 1 · Answer 1 · edited Dec 08 '20 at 10:55

1

wait() does not throw an InterruptedException when the block (or wait) finishes.

If you start a third thread that calls thread1.interrupt() then you will get the InterruptedException.

edited Dec 08 '20 at 10:55

spongebob

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answered Dec 08 '20 at 10:25

Stephan

23
4

score 1 · Accepted Answer · answered Dec 08 '20 at 10:26

You won't get an InterruptedException if you don't actually interrupt the threads.

You could try, for example:

    thread2.start();
    thread1.start();
    
    Thread.sleep(1000);

    thread1.interrupt();
    thread2.interrupt();

Ideone demo

Output:

Thread-2 execution started
Thread-1 execution started
Thread-1 going for timed wait of 20000 millis
Thread-2 about to wait for Thread-1
Thread-1 timed wait over after 20000 millis
Thread-1 execution ending
Thread-2 wait over for Thread-1
Thread-2 execution ending
main exiting

Java thread InterruptedException block not executing

2 Answers2