When does Java's Thread.sleep throw InterruptedException?

Question

You should generally NOT ignore the exception. Take a look at the following paper:

Don't swallow interrupts

Sometimes throwing InterruptedException is not an option, such as when a task defined by Runnable calls an interruptible method. In this case, you can't rethrow InterruptedException, but you also do not want to do nothing. When a blocking method detects interruption and throws InterruptedException, it clears the interrupted status. If you catch InterruptedException but cannot rethrow it, you should preserve evidence that the interruption occurred so that code higher up on the call stack can learn of the interruption and respond to it if it wants to. This task is accomplished by calling interrupt() to "reinterrupt" the current thread, as shown in Listing 3. At the very least, whenever you catch InterruptedException and don't rethrow it, reinterrupt the current thread before returning.

public class TaskRunner implements Runnable {
    private BlockingQueue<Task> queue;

    public TaskRunner(BlockingQueue<Task> queue) { 
        this.queue = queue; 
    }

    public void run() { 
        try {
             while (true) {
                 Task task = queue.take(10, TimeUnit.SECONDS);
                 task.execute();
             }
         }
         catch (InterruptedException e) { 
             // Restore the interrupted status
             Thread.currentThread().interrupt();
         }
    }
}

• From Don't swallow interrupts

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See the entire paper here:

http://www.ibm.com/developerworks/java/library/j-jtp05236/index.html?ca=drs-

If an InterruptedException is thrown it means that something wants to interrupt (usually terminate) that thread. This is triggered by a call to the threads interrupt() method. The wait method detects that and throws an InterruptedException so the catch code can handle the request for termination immediately and does not have to wait till the specified time is up.

If you use it in a single-threaded app (and also in some multi-threaded apps), that exception will never be triggered. Ignoring it by having an empty catch clause I would not recommend. The throwing of the InterruptedException clears the interrupted state of the thread, so if not handled properly that info gets lost. Therefore I would propose to run:

} catch (InterruptedException e) {
  Thread.currentThread().interrupt();
  // code for stopping current task so thread stops
}

Which sets that state again. After that, finish execution. This would be correct behaviour, even tough never used.

What might be better is to add this:

} catch (InterruptedException e) {
  throw new RuntimeException("Unexpected interrupt", e);
}

...statement to the catch block. That basically means that it must never happen. So if the code is re-used in an environment where it might happen it will complain about it.

The Java Specialists newsletter (which I can unreservedly recommend) had an interesting article on this, and how to handle the InterruptedException. It's well worth reading and digesting.

Methods like sleep() and wait() of class Thread might throw an InterruptedException. This will happen if some other thread wanted to interrupt the thread that is waiting or sleeping.

A solid and easy way to handle it in single threaded code would be to catch it and retrow it in a RuntimeException, to avoid the need to declare it for every method.