Java Executor partial shutdown

Question

Lets have one classic Executor in application. Many parts of application use this executor for some computations, each computation can be cancelled, for this I can call shutdown() or shutdownNow() on Executor.

But I want to shutdown only part of tasks in Executor. Sadly I can't have access to Future objects, they are private part of computation implementation (actually computation is backed by actor framework jetlang)

I want something like Executor wrapper, which I could pass to computation and which should be backed by real Executor. Something like this:

// main application executor
Executor applicationExecutor = Executors.newCachedThreadPool();

// starting computation
Executor computationExecutor = new ExecutorWrapper(applicationExecutor);
Computation computation = new Computation(computationExecutor);
computation.start();

// cancelling computation
computation.cancel();
// shutting down only computation tasks
computationExecutor.shutdown();

// applicationExecutor remains running and happy

Or any other idea?

Answer 1

For those, who wants good ends: there is final solution, partially based of Ivan Sopov's answer. Luckily jetlang uses for running its tasks only Executor interface (not ExecutorService), so I make wrapper class which supports stopping tasks created only by this wrapper.

static class StoppableExecutor implements Executor {
    final ExecutorService executor;
    final List<Future<?>> futures = Lists.newArrayList();
    boolean stopped;

    public StoppableExecutor(ExecutorService executor) {
        this.executor = executor;
    }

    void stop() {
        this.stopped = true;
        synchronized (futures) {
            for (Iterator<Future<?>> iterator = futures.iterator(); iterator.hasNext();) {
                Future<?> future = iterator.next();
                if (!future.isDone() && !future.isCancelled()) {
                    System.out.println(future.cancel(true));
                }
            }
            futures.clear();
        }
    }

    @Override
    public void execute(Runnable command) {
        if (!stopped) {
            synchronized (futures) {
                Future<?> newFuture = executor.submit(command);
                for (Iterator<Future<?>> iterator = futures.iterator(); iterator.hasNext();) {
                    Future<?> future = iterator.next();
                    if (future.isDone() || future.isCancelled())
                        iterator.remove();
                }
                futures.add(newFuture);
            }
        }
    }
}

Using this is pretty straightforward:

ExecutorService service = Executors.newFixedThreadPool(5);
StoppableExecutor executor = new StoppableExecutor(service);

// doing some actor stuff with executor instance
PoolFiberFactory factory = new PoolFiberFactory(executor);

// stopping tasks only created on executor instance
// executor service is happily running other tasks
executor.stop();

That's all. Works nice.

Answer 2

How about having your Computation be a Runnable (and run using the provided Executor) until a boolean flag is set? Something along the lines of :

public class Computation
{
  boolean volatile stopped;

  public void run(){
    while(!stopped){
    //do magic
  }

  public void cancel)(){stopped=true;}
}

What you are doing is essentially stopping the thread. However, it does not get garbage-collected, but is instead re-used because it is managed by the Executor. Look up "what is the proper way to stop a thread?".

EDIT: please note the code above is quite primitive in the sense it assumes the body of the while loop takes a short amount of time. If it does not, the check will be executed infrequently and you will notice a delay between canceling a task and it actually stopping.

Answer 3

Something like this? You may do partial shutdown:

for (Future<?> future : %ExecutorServiceWrapperInstance%.getFutures()) {
    if (%CONDITION%) {
        future.cancel(true);
    }
}

Here is the code:

package com.sopovs.moradanen;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class ExecutorServiceWrapper implements ExecutorService {

private final ExecutorService realService;
private List<Future<?>> futures = new ArrayList<Future<?>>();

public ExecutorServiceWrapper(ExecutorService realService) {
    this.realService = realService;
}

@Override
public void execute(Runnable command) {
    realService.execute(command);
}

@Override
public void shutdown() {
    realService.shutdown();

}

@Override
public List<Runnable> shutdownNow() {
    return realService.shutdownNow();
}

@Override
public boolean isShutdown() {
    return realService.isShutdown();
}

@Override
public boolean isTerminated() {
    return realService.isTerminated();
}

@Override
public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
    return realService.awaitTermination(timeout, unit);
}

@Override
public <T> Future<T> submit(Callable<T> task) {
    Future<T> future = realService.submit(task);
    synchronized (this) {
        futures.add(future);
    }
    return future;
}

public synchronized List<Future<?>> getFutures() {
    return Collections.unmodifiableList(futures);
}

@Override
public <T> Future<T> submit(Runnable task, T result) {
    Future<T> future = realService.submit(task, result);
    synchronized (this) {
        futures.add(future);
    }
    return future;
}

@Override
public Future<?> submit(Runnable task) {
    Future<?> future = realService.submit(task);
    synchronized (this) {
        futures.add(future);
    }
    return future;
}

@Override
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException {
    List<Future<T>> future = realService.invokeAll(tasks);
    synchronized (this) {
        futures.addAll(future);
    }
    return future;
}

@Override
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
        throws InterruptedException {
    List<Future<T>> future = realService.invokeAll(tasks, timeout, unit);
    synchronized (this) {
        futures.addAll(future);
    }
    return future;
}

@Override
public <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException {
    //don't know what to do here. Maybe this method is not needed by the framework
    //than just throw new NotImplementedException();
    return realService.invokeAny(tasks);
}

@Override
public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
    //don't know what to do here. Maybe this method is not needed by the framework
    //than just throw new NotImplementedException();
    return realService.invokeAny(tasks, timeout, unit);
}
}