How to stop a java thread gracefully?

Question

I wrote a thread, it is taking too much time to execute and it seems it is not completely done. I want to stop the thread gracefully. Any help ?

Answer 1

The good way to do it is to have the run() of the Thread guarded by a boolean variable and set it to true from the outside when you want to stop it, something like:

class MyThread extends Thread
{
  volatile boolean finished = false;

  public void stopMe()
  {
    finished = true;
  }

  public void run()
  {
    while (!finished)
    {
      //do dirty work
    }
  }
}

Once upon a time a stop() method existed but as the documentation states

This method is inherently unsafe. Stopping a thread with Thread.stop causes it to unlock all of the monitors that it has locked (as a natural consequence of the unchecked ThreadDeath exception propagating up the stack). If any of the objects previously protected by these monitors were in an inconsistent state, the damaged objects become visible to other threads, potentially resulting in arbitrary behavior.

That's why you should have a guard..

Answer 2

The bad part about using a flag to stop your thread is that if the thread is waiting or sleeping then you have to wait for it to finish waiting/sleeping. If you call the interrupt method on the thread then that will cause the wait or sleep call to be exited with an InterruptedException.

(A second bad part about the flag approach is that most nontrivial code is going to be utilizing libraries like java.util.concurrent, where the classes are specifically designed to use interruption to cancel. Trying to use the hand rolled flag in a task passed into an Executor is going to be awkward.)

Calling interrupt() also sets an interrupted property that you can use as a flag to check whether to quit (in the event that the thread is not waiting or sleeping).

You can write the thread's run method so that the InterruptedException is caught outside whatever looping logic the thread is doing, or you can catch the exception within the loop and close to the call throwing the exception, setting the interrupt flag inside the catch block for the InterruptedException so that the thread doesn't lose track of the fact that it was interrupted. The interrupted thread can still keep control and finish processing on its own terms.

Say I want to write a worker thread that does work in increments, where there's a sleep in the middle for some reason, and I don't want quitting the sleep to make processing quit without doing the remaining work for that increment, I only want it to quit if it is in-between increments:

class MyThread extends Thread
{
    public void run()
    {
        while (!Thread.currentThread().isInterrupted())
        {
            doFirstPartOfIncrement();
            try {
                Thread.sleep(10000L);
            } catch (InterruptedException e) {
                // restore interrupt flag
                Thread.currentThread().interrupt();
            }
            doSecondPartOfIncrement();
        }
    }
}

Here is an answer to a similar question, including example code.

Answer 3

You should not kill Thread from other one. It's considered as fairly bad habit. However, there are many ways. You can use return statement from thread's run method. Or you can check if thread has already been interrupted and then it will cancel it's work. F.e. :

while (!isInterrupted()) { 
  // doStuff
}

Answer 4

Make a volatile boolean stop somewhere. Then in the code that runs in the thread, regularly do

 if (stop) // end gracefully by breaking out of loop or whatever

To stop the thread, set stop to true.

I think you must do it manually this way. After all, only the code running in the thread has any idea what is and isn't graceful.

Answer 5

You need to send a stop-message to the Thread and the Thread itself needs to take action if the message has been received. This is pretty easy, if the long-running action is inside loop:

public class StoppableThread extends Thread {

   private volatile boolean stop = false;

   public void stopGracefully() {
     stop = true;
   }

   public void run() {
     boolean finished = false;
     while (!stop && !finished) {
       // long running action - finished will be true once work is done
     }
   }
}

Answer 6

For a thread to stop itself, no one seems to have mentioned (mis)using exception:

abstract class SelfStoppingThread extends Thread {

    @Override
    public final void run() {
        try {
            doRun();
        } catch (final Stop stop) {
            //optional logging
        }
    }

    abstract void doRun();

    protected final void stopSelf() {
        throw new Stop();
    }

    private static final class Stop extends RuntimeException {};

}

A subclass just need to override doRun() normally as you would with a Thread, and call stopSelf() whenever it feels like it wants to stop. IMO it feels cleaner than using a flag in a while loop.