how to stop a thread with thread interrupt method

Question

I am trying to learn thread interrupt and how to make a thread terminate without calling stop.

public class Test implements Runnable{
        static Thread threadTest=null;
        public static void main(String args[]){
          System.out.println("Hello i am main thread");
          Test thread= new Test();
          threadTest= new Thread(thread);
          threadTest.start();   
}

private static void exitThread() {
    threadTest.interrupt();
}

 @Override
 public void run() {
    boolean run = true;
    while (run) {
        try {
            System.out.println("Sleeping");
            Thread.sleep((long) 10000);
            exitThread();
            System.out.println("Processing");

        } catch (InterruptedException e) {

            run = false;
        }
    }

}


}

Output

Hello i am main thread

Sleeping

Processing

Sleeping

I am unable to understand why Sleeping is printed second time and interrupted exception is thrown second time rather than first time.I have checked posts where volatile keyword is used to stop a thread in java.but i am unable to understand how that will be used in this scenario as thread gets stopped with interrupt.

Nathan Hughes · Accepted Answer · 2015-11-02T22:20:13.170

In order to see the thread being interrupted instead of entering the sleep method a second time, change the while loop test in the run method to check the interrupt flag:

@Override
 public void run() {
    while (!Thread.currentThread().isInterrupted()) {
        try {
            System.out.println("Sleeping");
            Thread.sleep((long) 10000);
            exitThread();
            System.out.println("Processing");
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }
}

The thread will sleep, then set its own interrupt flag, then check the flag and terminate. InterruptedException would be thrown by the Thread#sleep method only if the thread was sleeping while the interrupt flag was set.

Your local boolean variable is not needed. If Thread#sleep throws an InterruptedException (which it won't in this example because the thread checks the interrupted flag and leaves the while loop) then the interrupt flag is cleared, restoring it in the catch block allows the while test to see that the thread was interrupted.

In real programs the thread would be interrupted from another thread, there's no reason for a thread to interrupt itself (it can just return instead).

just curious, but is there any difference or side effects if i wrap the while loop with the try-catch and catch any interrupted exception instead? I assume that would allow you to immediately just return from the catch and exit from the while loop — experiment unit 1998X, Feb 09 '23 at 09:34

score 1 · Answer 2 · answered Aug 08 '15 at 15:51

Calling Thread.interrupt() just sets a flag for the thread. It doesn't do anything else. Only blocking methods (those usually declare throws InterruptedException) respond to that flag being set (by throwing). The flag is sticky in that it remains set until its cleared.

So the first call to the sleep method just runs normally (the interrupted flag isn't set yet). After that your code does nothing that acts on the interrupted status, until the second loop iteration where the sleep call detects the interrupted status and throws the exception.

You can use Thread.interrupted() or Thread.isInterrupted() to check the interrupted status at any time (beware that interrupted() also clears the interrupted status if it was set).

score 1 · Answer 3 · answered Sep 04 '15 at 17:37

here you creating another thread Test class but "main" has its own thread , so the new thread you created is interpreted . Here in this code you are interrupting the new created thread Thread-0 not main thread,when you execute this code you are making thread to sleep before it enters the method exitThread() ,so it is displaying the processing, but if you try to put thread sleep after you enter exitthread() you will have your answer Like in this code:

public class Test implements Runnable { public boolean run = true;

@Override
public void run() {
    while (run) {

        try {
            System.out.println("Sleeping...");
            exitThread();
            Thread.sleep(10000);
            System.out.println("Processing...");
        } catch (InterruptedException e) {
            System.out.println("Thread intreputted " + e);
            run = false;
        }
    }
}

private void exitThread() {
    Thread.currentThread().interrupt();
    if (Thread.currentThread().isInterrupted())
        System.out.println(Thread.currentThread().getName()
                + " is intreputted");
    else
        System.out.println("alive");
}

public static void main(String[] args) {
    System.out.println("hi I am current thread------>"
            + Thread.currentThread().getName());
    Test test = new Test();
    Thread thread = new Thread(test);
    thread.start();
}

}

Hope it will be helpfull

how to stop a thread with thread interrupt method

3 Answers3