How to Kill a Java Thread

Last updated: September 7, 2024

Written by: Eric Goebelbecker

Reviewed by: Michal Aibin

Java Concurrency

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1. Introduction

In this brief article, we’ll cover stopping a Thread in Java – which is not that simple since the Thread.stop() method is deprecated.

As explained in this update from Oracle, stop() can lead to monitored objects being corrupted.

Let’s start with a class that creates and starts a thread. This task won’t end on its own, so we need some way of stopping that thread.

We’ll use an atomic flag for that:

public class ControlSubThread implements Runnable {

    private Thread worker;
    private final AtomicBoolean running = new AtomicBoolean(false);
    private int interval;

    public ControlSubThread(int sleepInterval) {
        interval = sleepInterval;
    }
 
    public void start() {
        worker = new Thread(this);
        worker.start();
    }
 
    public void stop() {
        running.set(false);
    }

    public void run() { 
        running.set(true);
        while (running.get()) {
            try { 
                Thread.sleep(interval); 
            } catch (InterruptedException e){ 
                Thread.currentThread().interrupt();
                System.out.println(
                  "Thread was interrupted, Failed to complete operation");
            }
            // do something here 
         } 
    } 
}

Rather than having a while loop evaluating a constant true, we’re using an AtomicBoolean and now we can start/stop execution by setting it to true/false.

As explained in our introduction to Atomic Variables, using an AtomicBoolean prevents conflicts in setting and checking the variable from different threads.

3. Interrupting a Thread

What happens when sleep() is set to a long interval, or if we’re waiting for a lock that might never be released?

We face the risk of blocking for a long period or never terminating cleanly.

We can create the interrupt() for these situations, let’s add a few methods and a new flag to the class:

public class ControlSubThread implements Runnable {

    private Thread worker;
    private AtomicBoolean running = new AtomicBoolean(false);
    private int interval;

    // ...

    public void interrupt() {
        running.set(false);
        worker.interrupt();
    }

    boolean isRunning() {
        return running.get();
    }

    boolean isStopped() {
        return stopped.get();
    }

    public void run() {
        running.set(true);
        stopped.set(false);
        while (running.get()) {
            try {
                Thread.sleep(interval);
            } catch (InterruptedException e){
                Thread.currentThread().interrupt();
                System.out.println(
                  "Thread was interrupted, Failed to complete operation");
            }
            // do something
        }
        stopped.set(true);
    }
}

We’ve added an interrupt() method that sets our running flag to false and calls the worker thread’s interrupt() method.

If the thread is sleeping when this is called, sleep() will exit with an InterruptedException, as would any other blocking call.

This returns the thread to the loop, and it will exit since running is false.

4. Conclusion

In this quick tutorial, we looked at how to use an atomic variable, optionally combined with a call to interrupt(), to cleanly shut down a thread. This is definitely preferable to calling the deprecated stop() method and risking locking forever and memory corruption.

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