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 15 · 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

14

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
2

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

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

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