How to timeout a thread

Question

Indeed rather use ExecutorService instead of Timer, here's an SSCCE:

package com.stackoverflow.q2275443;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class Test {
    public static void main(String[] args) throws Exception {
        ExecutorService executor = Executors.newSingleThreadExecutor();
        Future<String> future = executor.submit(new Task());

        try {
            System.out.println("Started..");
            System.out.println(future.get(3, TimeUnit.SECONDS));
            System.out.println("Finished!");
        } catch (TimeoutException e) {
            future.cancel(true);
            System.out.println("Terminated!");
        }

        executor.shutdownNow();
    }
}

class Task implements Callable<String> {
    @Override
    public String call() throws Exception {
        Thread.sleep(4000); // Just to demo a long running task of 4 seconds.
        return "Ready!";
    }
}

Play a bit with the timeout argument in Future#get() method, e.g. increase it to 5 and you'll see that the thread finishes. You can intercept the timeout in the catch (TimeoutException e) block.

Update: to clarify a conceptual misunderstanding, the sleep() is not required. It is just used for SSCCE/demonstration purposes. Just do your long running task right there in place of sleep(). Inside your long running task, you should be checking if the thread is not interrupted as follows:

while (!Thread.interrupted()) {
    // Do your long running task here.
}

There isn't a 100% reliable way to do this for any old task. The task has to be written with this ability in mind.

Core Java libraries like ExecutorService cancel asynchronous tasks with interrupt() calls on the worker thread. So, for example, if the task contains some sort of loop, you should be checking its interrupt status on each iteration. If the task is doing I/O operations, they should be interruptible too—and setting that up can be tricky. In any case, keep in mind that code has to actively check for interrupts; setting an interrupt doesn't necessarily do anything.

Of course, if your task is some simple loop, you can just check the current time at each iteration and give up when a specified timeout has elapsed. A worker thread isn't needed in that case.

Consider using an instance of ExecutorService. Both invokeAll() and invokeAny() methods are available with a timeout parameter.

The current thread will block until the method completes (not sure if this is desirable) either because the task(s) completed normally or the timeout was reached. You can inspect the returned Future(s) to determine what happened.

Assuming the thread code is out of your control:

From the Java documentation mentioned above:

What if a thread doesn't respond to Thread.interrupt?

In some cases, you can use application specific tricks. For example, if a thread is waiting on a known socket, you can close the socket to cause the thread to return immediately. Unfortunately, there really isn't any technique that works in general. It should be noted that in all situations where a waiting thread doesn't respond to Thread.interrupt, it wouldn't respond to Thread.stop either. Such cases include deliberate denial-of-service attacks, and I/O operations for which thread.stop and thread.interrupt do not work properly.

Bottom Line:

Make sure all threads can be interrupted, or else you need specific knowledge of the thread - like having a flag to set. Maybe you can require that the task be given to you along with the code needed to stop it - define an interface with a stop() method. You can also warn when you failed to stop a task.

BalusC said:

Update: to clarify a conceptual misunderstanding, the sleep() is not required. It is just used for SSCCE/demonstration purposes. Just do your long running task right there in place of sleep().

But if you replace Thread.sleep(4000); with for (int i = 0; i < 5E8; i++) {} then it doesn't compile, because the empty loop doesn't throw an InterruptedException.

And for the thread to be interruptible, it needs to throw an InterruptedException.

This seems like a serious problem to me. I can't see how to adapt this answer to work with a general long-running task.

Edited to add: I reasked this as a new question: [ interrupting a thread after fixed time, does it have to throw InterruptedException? ]

I think you should take a look at proper concurrency handling mechanisms (threads running into infinite loops doesn't sound good per se, btw). Make sure you read a little about the "killing" or "stopping" Threads topic.

What you are describing,sound very much like a "rendezvous", so you may want to take a look at the CyclicBarrier.

There may be other constructs (like using CountDownLatch for example) that can resolve your problem (one thread waiting with a timeout for the latch, the other should count down the latch if it has done it's work, which would release your first thread either after a timeout or when the latch countdown is invoked).

I usually recommend two books in this area: Concurrent Programming in Java and Java Concurrency in Practice.

I created a helper class just for this some time ago. Works great:

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
/**
 * TimeOut class - used for stopping a thread that is taking too long
 * @author Peter Goransson
 *
 */
public class TimeOut {

    Thread interrupter;
    Thread target;
    long timeout;
    boolean success;
    boolean forceStop;

    CyclicBarrier barrier;

    /**
     * 
     * @param target The Runnable target to be executed
     * @param timeout The time in milliseconds before target will be interrupted or stopped
     * @param forceStop If true, will Thread.stop() this target instead of just interrupt() 
     */
    public TimeOut(Runnable target, long timeout, boolean forceStop) {      
        this.timeout = timeout;
        this.forceStop = forceStop;

        this.target = new Thread(target);       
        this.interrupter = new Thread(new Interrupter());

        barrier = new CyclicBarrier(2); // There will always be just 2 threads waiting on this barrier
    }

    public boolean execute() throws InterruptedException {  

        // Start target and interrupter
        target.start();
        interrupter.start();

        // Wait for target to finish or be interrupted by interrupter
        target.join();  

        interrupter.interrupt(); // stop the interrupter    
        try {
            barrier.await(); // Need to wait on this barrier to make sure status is set
        } catch (BrokenBarrierException e) {
            // Something horrible happened, assume we failed
            success = false;
        } 

        return success; // status is set in the Interrupter inner class
    }

    private class Interrupter implements Runnable {

        Interrupter() {}

        public void run() {
            try {
                Thread.sleep(timeout); // Wait for timeout period and then kill this target
                if (forceStop) {
                  target.stop(); // Need to use stop instead of interrupt since we're trying to kill this thread
                }
                else {
                    target.interrupt(); // Gracefully interrupt the waiting thread
                }
                System.out.println("done");             
                success = false;
            } catch (InterruptedException e) {
                success = true;
            }


            try {
                barrier.await(); // Need to wait on this barrier
            } catch (InterruptedException e) {
                // If the Child and Interrupter finish at the exact same millisecond we'll get here
                // In this weird case assume it failed
                success = false;                
            } 
            catch (BrokenBarrierException e) {
                // Something horrible happened, assume we failed
                success = false;
            }

        }

    }
}

It is called like this:

long timeout = 10000; // number of milliseconds before timeout
TimeOut t = new TimeOut(new PhotoProcessor(filePath, params), timeout, true);
try {                       
  boolean sucess = t.execute(); // Will return false if this times out
  if (!sucess) {
    // This thread timed out
  }
  else {
    // This thread ran completely and did not timeout
  }
} catch (InterruptedException e) {}