Terminate a thread in java with a little problem

Question

The code below is that I try to stop a thread I created without using stop(). However, The thread seems like it would executed once more after it be stopped. I wonder why this happened? Plus: If I change the iteration from 100 to 10, it works without any problems. Can anyone help me with this problem? Thanks in advance!

  public class TerminateThread implements Runnable{
    private boolean flag = true;
    public TerminateThread(String name) {
        this.name = name;
    }
    private String name;
    public void run() {
        int i = 0;
        while(flag) {
            System.out.println(name+"--->"+i++);
        }

    }
    public void Terminate() {
        this.flag = false;
    }

    public static void main(String[] args) {
        TerminateThread tt = new TerminateThread("Thread");
        new Thread(tt).start();
        for (int i = 0; i <= 100; i++) {
            if (i == 88) {
                tt.Terminate();
                System.out.println("Thread terminated!!!");
            }
            System.out.println("main--->" + i);
        }

    }

}

The outcome is :

score 5 · Accepted Answer · answered Feb 28 '20 at 03:08

5

Well the thing is, your code doesn't terminate the thread, it simply makes it finish its work at the next iteration.

What could be happening is the flag variable isn't being instantly updated to all threads when changed, and you are setting it with the main thread. To remedy this, declare flag as volatile.

private volatile boolean flag = true;

More Info: What is the volatile keyword useful for

answered Feb 28 '20 at 03:08

CausingUnderflowsEverywhere

1,908
13
33

Still, it could set the terminated flag between the check and the output (which is very likely, if you look what happens on `System.out.println()`) – Johannes Kuhn Feb 28 '20 at 03:14
Yup, but it looks like OP is showing it happens much later than the set flag call, so it must be the problem of the value not being updated across threads. But I did intend to point that out with the first sentence. – CausingUnderflowsEverywhere Feb 28 '20 at 03:16
Locks in java are not fair. It appears that the main thread grabs the lock for the output stream before after releasing it before the second thread grabs it for a while. – Johannes Kuhn Feb 28 '20 at 03:18
Wouldn't the second thread be waiting on the monitor, and execute next as when the main thread's sout call finishes it releases the lock and calls notify() ? – CausingUnderflowsEverywhere Feb 28 '20 at 03:21
You should post an answer if you feel it'll help some people. – CausingUnderflowsEverywhere Feb 28 '20 at 03:22
@JohannesKuhn why do you have the same profile description as I do? "SOreadytohelp" eg stack overflow ready to help haha. [so off topic] – CausingUnderflowsEverywhere Feb 28 '20 at 03:23

Terminate a thread in java with a little problem

1 Answers1