interrupt() not working as expected (how does interrupt work?)

Question

I want to interrupt a thread, but invoking interrupt() doesn't seem to work. Below is the sample code:

public class BasicThreadrRunner {
    public static void main(String[] args) {
        Thread t1 = new Thread(new Basic(), "thread1");
        t1.start();
        Thread t3 = new Thread(new Basic(), "thread3");
        Thread t4 = new Thread(new Basic(), "thread4");
        t3.start();
        t1.interrupt();
        t4.start();
    }
}
class Basic implements Runnable{
    public void run(){
        while(true) {
            System.out.println(Thread.currentThread().getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                System.err.println("thread: " + Thread.currentThread().getName());
                //e.printStackTrace();
            }
        }
    }
}

but the output looks like thread 1 is still running. Can anyone explain this as well as how interrupt() works? Thanks!

score 18 · Accepted Answer · answered Nov 08 '11 at 12:20

The thread is still running simply because you catch InterruptedException and keep running. interrupt() primarily sets a flag in the Thread object, which you can check with isInterrupted(). It also causes some methods -- sleep(), join Object.wait(), in particular -- to return immediately by throwing an InterruptedException. It also causes some I/O operations to immediately terminate. If you're seeing the printouts from your catch block, then you can see that interrupt() is working.

mre · Answer 2 · 2014-11-11T15:00:55.737

17

As others have said, you catch the interrupt, but do nothing with it. What you need to do is propagate the interrupt using logic such as,

while(!Thread.currentThread().isInterrupted()){
    try{
        // do stuff
    }catch(InterruptedException e){
        Thread.currentThread().interrupt(); // propagate interrupt
    }
}

Using looping logic, such as while(true) is just lazy coding. Instead, poll the thread's interrupted flag in order to determine termination via interruption.

edited Nov 11 '14 at 15:00

answered Nov 08 '11 at 13:05

mre

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Or you could move the try/catch outside the loop. ;) – Peter Lawrey Nov 08 '11 at 13:35
3

Yes, but that was already mentioned by @MByD and it keeps that bad looping logic intact. :D – mre Nov 08 '11 at 13:39
+1 for mentioning an alternative approach to while ( true ) p.s. Thank you! – Everyone Jan 09 '12 at 11:59
@mre InterruptedException is a checked exception, if //do stuff is doing with Thread.sleep it will work...what about otherwise? Do I need to have Thread.sleep mandatory? – Narendra Jaggi May 17 '20 at 16:43

linker · Answer 3 · 2022-12-29T18:16:42.520

++1, in addition to other answers. I believe the misconception about this was that it seemed the try/catch block finished its job after the Thread.sleep(1000); call i.e. try to sleep for 1000ms, catch anything that might interrupt my sleep attempt.

What is happening actually is that the try/catch block is still very much active while sleeping i.e. try to sleep for 1000ms, catch anything that might interrupt during my sleep

Hence the reason why the exception is being caught immediately (and afterwards) since the thread barely just started its sleep.

interrupt() not working as expected (how does interrupt work?)

3 Answers3

Linked

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