In what cases does Future.get() throw ExecutionException or InterruptedException

Question

My code snippet:

ExecutorService executor = Executors.newSingleThreadExecutor();
try {
    Task t = new Task(response,inputToPass,pTypes,unit.getInstance(),methodName,unit.getUnitKey());
    Future<SCCallOutResponse> fut = executor.submit(t);
    response = fut.get(unit.getTimeOut(),TimeUnit.MILLISECONDS);
} catch (TimeoutException e) {
    // if the task is still running, a TimeOutException will occur while fut.get()
    cat.error("Unit " + unit.getUnitKey() + " Timed Out");
    response.setVote(SCCallOutConsts.TIMEOUT);
} catch (InterruptedException e) {
    cat.error(e);
} catch (ExecutionException e) {
    cat.error(e);
} finally {
    executor.shutdown();
}

How should i handle the InterruptedException and ExecutionException in the code?

And in what cases, are these exceptions thrown?

Solution 1

ExecutionException and InterruptedException are two very different things.

ExecutionException wraps whatever exception the thread being executed threw, so if your thread was, for instance, doing some kind of IO that caused an IOException to get thrown, that would get wrapped in an ExecutionException and rethrown.

An InterruptedException is not a sign of anything having gone wrong. It is there to give you a way to let your threads know when it's time to stop so that they can finish up their current work and exit gracefully. Say I want my application to stop running, but I don't want my threads to drop what they're doing in the middle of something (which is what would happen if I made them daemon threads). So when the application is being shutdown, my code calls the interrupt method on these threads, which sets the interrupt flag on them, and the next time those threads are waiting or sleeping they check the interrupt flag and throw an InterruptedException, which I can use to bail out of whatever infinite-loop processing/sleeping logic the threads are engaged in. (And if the thread doesn't wait or sleep, it can just check the interrupt flag periodically.) So it is an instance of an exception being used to change the logical flow. The only reason you would log it at all is in an example program to show you what's happening, or if you're debugging a problem where interrupt logic is not working correctly.

Solution 2

InterruptedException will be thrown if interrupt is called on the waiting thread before the computation has completed.

ExecutionException will be thrown if the computation involved (Task in this case) throws an exception itself.

How you want to handle this will entirely depend on your application.

EDIT: Here's a demonstration of being interrupted:

import java.util.concurrent.*;

public class Test
{
    public static void main(String[] args) throws Exception
    {
        ExecutorService executor = Executors.newFixedThreadPool(2);
        Future<String> future = executor.submit(new SlowCallable());
        executor.submit(new Interruptor(Thread.currentThread()));
        try
        {
            System.out.println(future.get());
        }
        catch (InterruptedException e)
        {
            System.out.println("I was interrupted");
        }
    }

    private static class Interruptor implements Callable<String>
    {
        private final Thread threadToInterrupt;

        Interruptor(Thread threadToInterrupt)
        {
            this.threadToInterrupt = threadToInterrupt;
        }

        public String call() throws Exception
        {
            Thread.sleep(2000);
            threadToInterrupt.interrupt();
            return "interrupted other thread";
        }
    }

    private static class SlowCallable implements Callable<String>
    {
        public String call() throws Exception
        {
            Thread.sleep(5000);
            return "finished";
        }
    }
}

Solution 3

Sample code to return three types of Exceptions.

import java.util.concurrent.*;
import java.util.*;

public class ExceptionDemo{
    public static void main(String args[]){
        int poolSize=1;
        int maxPoolSize=1;
        int queueSize=30;
        long aliveTive=60;
        ArrayBlockingQueue<Runnable> queue= new ArrayBlockingQueue<Runnable>(queueSize);
        ThreadPoolExecutor executor= new ThreadPoolExecutor(poolSize,maxPoolSize,aliveTive,
                        TimeUnit.MILLISECONDS,queue);
        List<Future> futures = new ArrayList<Future>();
        for ( int i=0; i < 5; i++){
            futures.add(executor.submit(new RunnableEx()));
        }
        for ( Iterator it = futures.iterator(); it.hasNext();){
            try {
                Future f = (Future)it.next();
                f.get(4000,TimeUnit.MILLISECONDS);
            }catch(TimeoutException terr){
                System.out.println("Timeout exception");
                terr.printStackTrace();
            }
            catch(InterruptedException ierr){
                System.out.println("Interrupted exception:");
                ierr.printStackTrace();
            }catch(ExecutionException err){
                System.out.println("Exeuction exception:");
                err.printStackTrace();
                Thread.currentThread().interrupt();
            }
        }
        executor.shutdown();
    }
}

class RunnableEx implements Runnable{
    public void run() {
        // code in here
        System.out.println("Thread name:"+Thread.currentThread().getName());
        try{
            Random r = new Random();
            if (r.nextInt(2) == 1){
                Thread.sleep(2000);
            }else{
                Thread.sleep(4000);
            }
            System.out.println("eee:"+1/0);
        }catch(InterruptedException irr){
            irr.printStackTrace();
        }
    }
}

output:

Thread name:pool-1-thread-1
Timeout exception
Thread name:pool-1-thread-1
java.util.concurrent.TimeoutException
        at java.util.concurrent.FutureTask.get(FutureTask.java:201)
        at ExceptionDemo.main(ExceptionDemo.java:20)
Thread name:pool-1-thread-1
Exeuction exception:
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
        at java.util.concurrent.FutureTask.report(FutureTask.java:122)
        at java.util.concurrent.FutureTask.get(FutureTask.java:202)
        at ExceptionDemo.main(ExceptionDemo.java:20)
Caused by: java.lang.ArithmeticException: / by zero
        at RunnableEx.run(ExceptionDemo.java:49)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:744)
Interrupted exception:
java.lang.InterruptedException
        at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:400)
        at java.util.concurrent.FutureTask.get(FutureTask.java:199)
        at ExceptionDemo.main(ExceptionDemo.java:20)
Timeout exception
java.util.concurrent.TimeoutException
        at java.util.concurrent.FutureTask.get(FutureTask.java:201)
Thread name:pool-1-thread-1
        at ExceptionDemo.main(ExceptionDemo.java:20)
Thread name:pool-1-thread-1
Timeout exception
java.util.concurrent.TimeoutException
        at java.util.concurrent.FutureTask.get(FutureTask.java:201)
        at ExceptionDemo.main(ExceptionDemo.java:20)

TimeoutException : Exception thrown when a blocking operation times out.

In above example, some tasks are taking more time (due to 4 seconds sleep) and blocking operation of get() on Future

Either increase the time-out or optimize Runnable task.

ExecutionException: Exception thrown when attempting to retrieve the result of a task that aborted by throwing an exception => The computation threw an exception

In above example, this Exception is simulated through ArithmeticException: / by zero

Generally, you should catch it fix the root cause if it is trivial as quoted in the example.

InterruptedException: Thrown when a thread is waiting, sleeping, or otherwise occupied, and the thread is interrupted, either before or during the activity.

In above example, this Exception is simulated by interrupting current Thread during ExecutionException.

Generally, you should catch it don't act on it.

Comments

[java_geek]

if i call Thread.interrupt(), the interrupt flag is set to true. But InterruptedException is not thrown

[Saurabh]

It does not have to be an unchecked exception. Callable.call() can throw anything.

[Jon Skeet]

@java_geek: You need to interrupt the waiting thread - see my edited post for an example.

[G 1]

Is there a way to prevent ExecutionException from throwing, when there is an exception in the processing of a task. Even if you caught the Exception throws in the processing and handles it, it still wraps the exception to ExecutionException and throws it in the get.

[Ravindra babu]

@downvoter, re-check question and answer. Java official documentation reasons are quoted here.

[prime]

Is there a relationship between InterruptedException and ExecutionException , just to know which one to catch first. In the SO question he catches the InterruptedException first and then the ExecutionException. Will the behavior will be the same if we swap the order ?

[prime]

Is there a relationship between InterruptedException and ExecutionException , just to know which one to catch first. In the SO question he catches the InterruptedException first and then the ExecutionException. Will the behavior will be the same if we swap the order ?

[prime]

Is there a relationship between InterruptedException and ExecutionException , just to know which one to catch first. In the SO question he catches the InterruptedException first and then the ExecutionException. Will the behavior will be the same if we swap the order ?

[Jon Skeet]

@prime: Both just derive from Exception, so swapping the order will make no difference.

[prime]

@JonSkeet thank you for confirming. Are there any easy way to check two exceptions are related or not. Or can we just do it by looking at whether they derive from same or not.

[Jon Skeet]

@prime: Exactly the latter - just check the documentation. Or switch them round and see whether the code still compiles...

[prime]

@JonSkeet Ah Compile errors. Forgot about that :) Thank you.

[Ravindra babu]

InterruuptedException should be caught first

[prime]

Any idea why ? Can you give me an example where the order matters

[Nathan Hughes]

@prime: see stackoverflow.com/a/10964899/217324. Both of the exceptions extend Exception, neither is more specific than the other. Order doesn't matter in this case.

[Ravindra babu]

For above use case, any order works since they are unrelated in hierarchy. But if you catch parent exception first and child exception later, compile will throw "exception has already been caught"