Why invoke Thread.currentThread.interrupt() in a catch InterruptException block?

Question

This is done to keep state.

When you catch the InterruptedException and swallow it, you essentially prevent any higher-level methods/thread groups from noticing the interrupt. Which may cause problems.

By calling Thread.currentThread().interrupt(), you set the interrupt flag of the thread, so higher-level interrupt handlers will notice it and can handle it appropriately.

Java Concurrency in Practice discusses this in more detail in Chapter 7.1.3: Responding to Interruption. Its rule is:

Only code that implements a thread's interruption policy may swallow an interruption request. General-purpose task and library code should never swallow interruption requests.

I think this code sample makes things a bit clear. The class which does the job :

public class InterruptedSleepingRunner implements Runnable {
    @Override
    public void run() {
        doAPseudoHeavyWeightJob();
    }

    private void doAPseudoHeavyWeightJob() {
        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            // You are kidding me
            System.out.println(i + " " + i * 2);
            // Let me sleep <evil grin>
            if (Thread.currentThread().isInterrupted()) {
                System.out.println("Thread interrupted\n Exiting...");
                break;
            } else {
                sleepBabySleep();
            }
        }
    }

    protected void sleepBabySleep() {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

The Main class:

public class InterruptedSleepingThreadMain {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new InterruptedSleepingRunner());
        thread.start();
        // Giving 10 seconds to finish the job.
        Thread.sleep(10000);
        // Let me interrupt
        thread.interrupt();
    }
}

Try calling interrupt without setting the status back.

Note:

http://download.oracle.com/javase/7/docs/technotes/guides/concurrency/threadPrimitiveDeprecation.html

How do I stop a thread that waits for long periods (e.g., for input)?

For this technique to work, it's critical that any method that catches an interrupt exception and is not prepared to deal with it immediately reasserts the exception. We say reasserts rather than rethrows, because it is not always possible to rethrow the exception. If the method that catches the InterruptedException is not declared to throw this (checked) exception, then it should "reinterrupt itself" with the following incantation:

Thread.currentThread().interrupt();

This ensures that the Thread will reraise the InterruptedException as soon as it is able.

I would consider it a bad practice or at least a bit risky. Usually higher level methods do not perform blocking operations and they will never see InterruptedException there. If you mask it in every place you perform interruptible operation, you will never get it.

The only rationale for Thread.currentThread.interrupt() and not raising any other exception or signaling interrupt request in any other way (e.g. setting interrupted local variable variable in a thread's main loop) is the situation where you really can't do anything with the exception, like in the finally blocks.

See Péter Török's answer, if you want to better understand implications of the Thread.currentThread.interrupt() call.