ExecutorService, how to wait for all tasks to finish

What is the simplest way to to wait for all tasks of ExecutorService to finish? My task is primarily computational, so I just want to run a large number of jobs - one on each core. Right now my setup looks like this:

ExecutorService es = Executors.newFixedThreadPool(2);
for (DataTable singleTable : uniquePhrases) {	
    es.execute(new ComputeDTask(singleTable));
}
try{
    es.wait();
} 
catch (InterruptedException e){
    e.printStackTrace();
}

ComputeDTask implements runnable. This appears to execute the tasks correctly, but the code crashes on wait() with IllegalMonitorStateException. This is odd, because I played around with some toy examples and it appeared to work.

uniquePhrases contains several tens of thousands of elements. Should I be using another method? I am looking for something as simple as possible

The simplest approach is to use ExecutorService.invokeAll() which does what you want in a one-liner. In your parlance, you'll need to modify or wrap ComputeDTask to implement Callable<>, which can give you quite a bit more flexibility. Probably in your app there is a meaningful implementation of Callable.call(), but here's a way to wrap it if not using Executors.callable().

ExecutorService es = Executors.newFixedThreadPool(2);
List<Callable<Object>> todo = new ArrayList<Callable<Object>>(singleTable.size());

for (DataTable singleTable: uniquePhrases) { 
    todo.add(Executors.callable(new ComputeDTask(singleTable))); 
}

List<Future<Object>> answers = es.invokeAll(todo);

As others have pointed out, you could use the timeout version of invokeAll() if appropriate. In this example, answers is going to contain a bunch of Futures which will return nulls (see definition of Executors.callable(). Probably what you want to do is a slight refactoring so you can get a useful answer back, or a reference to the underlying ComputeDTask, but I can't tell from your example.

If it isn't clear, note that invokeAll() will not return until all the tasks are completed. (i.e., all the Futures in your answers collection will report .isDone() if asked.) This avoids all the manual shutdown, awaitTermination, etc... and allows you to reuse this ExecutorService neatly for multiple cycles, if desired.

There are a few related questions on SO:

• https://stackoverflow.com/questions/1250643/java-how-to-wait-for-all-threads-to-finish">How to wait for all threads to finish

• https://stackoverflow.com/questions/2314402/return-values-from-java-threads">Return values from java threads

• https://stackoverflow.com/questions/370707/invokeall-is-not-willing-to-acept-a-collectioncallablet">invokeAll() not willing to accept a Collection<Callable<t>>

• https://stackoverflow.com/questions/1360936/do-i-need-to-synchronize-on-the-result-of-an-invokeall-call">Do I need to synchronize?

None of these are strictly on-point for your question, but they do provide a bit of color about how folks think Executor/ExecutorService ought to be used.

If you want to wait for all tasks to complete, use the shutdown method instead of wait. Then follow it with awaitTermination.

Also, you can use Runtime.availableProcessors to get the number of hardware threads so you can initialize your threadpool properly.

If waiting for all tasks in the ExecutorService to finish isn't precisely your goal, but rather waiting until a specific batch of tasks has completed, you can use a CompletionService — specifically, an ExecutorCompletionService.

The idea is to create an ExecutorCompletionService wrapping your Executor, submit some known number of tasks through the CompletionService, then draw that same number of results from the completion queue using either take() (which blocks) or poll() (which does not). Once you've drawn all the expected results corresponding to the tasks you submitted, you know they're all done.

Let me state this one more time, because it's not obvious from the interface: You must know how many things you put into the CompletionService in order to know how many things to try to draw out. This matters especially with the take() method: call it one time too many and it will block your calling thread until some other thread submits another job to the same CompletionService.

There are some examples showing how to use CompletionService in the book Java Concurrency in Practice.

If you want to wait for the executor service to finish executing, call shutdown() and then, awaitTermination(units, unitType), e.g. awaitTermination(1, MINUTE). The ExecutorService does not block on it's own monitor, so you can't use wait etc.

You could wait jobs to finish on a certain interval:

int maxSecondsPerComputeDTask = 20;
try {
    while (!es.awaitTermination(uniquePhrases.size() * maxSecondsPerComputeDTask, TimeUnit.SECONDS)) {
        // consider giving up with a 'break' statement under certain conditions
    }
} catch (InterruptedException e) {
    throw new RuntimeException(e);    
}

Or you could use ExecutorService.submit(Runnable) and collect the Future objects that it returns and call get() on each in turn to wait for them to finish.

ExecutorService es = Executors.newFixedThreadPool(2);
Collection<Future<?>> futures = new LinkedList<<Future<?>>();
for (DataTable singleTable : uniquePhrases) {
    futures.add(es.submit(new ComputeDTask(singleTable)));
}
for (Future<?> future : futures) {
   try {
       future.get();
   } catch (InterruptedException e) {
       throw new RuntimeException(e);
   } catch (ExecutionException e) {
       throw new RuntimeException(e);
   }
}

InterruptedException is extremely important to handle properly. It is what lets you or the users of your library terminate a long process safely.

Just use

latch = new CountDownLatch(noThreads)

In each thread

latch.countDown();

and as barrier

latch.await();

Root cause for IllegalMonitorStateException:

> Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.

From your code, you have just called wait() on ExecutorService without owning the lock.

Below code will fix IllegalMonitorStateException

try 
{
    synchronized(es){
        es.wait(); // Add some condition before you call wait()
    }
} 

Follow one of below approaches to wait for completion of all tasks, which have been submitted to ExecutorService.

1. Iterate through all Future tasks from submit on ExecutorService and check the status with blocking call get() on Future object

2. Using invokeAll on ExecutorService

3. Using CountDownLatch

4. Using ForkJoinPool or newWorkStealingPool of Executors(since java 8)

5. Shutdown the pool as recommended in oracle documentation page

    void shutdownAndAwaitTermination(ExecutorService pool) {
       pool.shutdown(); // Disable new tasks from being submitted
       try {
       // Wait a while for existing tasks to terminate
       if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
           pool.shutdownNow(); // Cancel currently executing tasks
           // Wait a while for tasks to respond to being cancelled
           if (!pool.awaitTermination(60, TimeUnit.SECONDS))
           System.err.println("Pool did not terminate");
       }
    } catch (InterruptedException ie) {
         // (Re-)Cancel if current thread also interrupted
         pool.shutdownNow();
         // Preserve interrupt status
         Thread.currentThread().interrupt();
    }
   

If you want to gracefully wait for all tasks for completion when you are using option 5 instead of options 1 to 4, change

    if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {

to

a while(condition) which checks for every 1 minute.

You can use ExecutorService.invokeAll method, It will execute all task and wait till all threads finished their task.

Here is complete javadoc

You can also user overloaded version of this method to specify the timeout.

Here is sample code with ExecutorService.invokeAll

public class Test {
	public static void main(String[] args) throws InterruptedException, ExecutionException {
		ExecutorService service = Executors.newFixedThreadPool(3);
		List<Callable<String>> taskList = new ArrayList<>();
		taskList.add(new Task1());
		taskList.add(new Task2());
		List<Future<String>> results = service.invokeAll(taskList);
		for (Future<String> f : results) {
			System.out.println(f.get());
		}
	}

}

class Task1 implements Callable<String> {
	@Override
	public String call() throws Exception {
		try {
			Thread.sleep(2000);
			return "Task 1 done";
		} catch (Exception e) {
			e.printStackTrace();
			return " error in task1";
		}
	}
}

class Task2 implements Callable<String> {
	@Override
	public String call() throws Exception {
		try {
			Thread.sleep(3000);
			return "Task 2 done";
		} catch (Exception e) {
			e.printStackTrace();
			return " error in task2";
		}
	}
}

I also have the situation that I have a set of documents to be crawled. I start with an initial "seed" document which should be processed, that document contains links to other documents which should also be processed, and so on.

In my main program, I just want to write something like the following, where Crawler controls a bunch of threads.

Crawler c = new Crawler();
c.schedule(seedDocument); 
c.waitUntilCompletion()

The same situation would happen if I wanted to navigate a tree; i would pop in the root node, the processor for each node would add children to the queue as necessary, and a bunch of threads would process all the nodes in the tree, until there were no more.

I couldn't find anything in the JVM which I thought was a bit surprising. So I wrote a class ThreadPool which one can either use directly or subclass to add methods suitable for the domain, e.g. schedule(Document). Hope it helps!

ThreadPool Javadoc | Maven

Add all threads in collection and submit it using invokeAll. If you can use invokeAll method of ExecutorService, JVM won’t proceed to next line until all threads are complete.

Here there is a good example: invokeAll via ExecutorService

Submit your tasks into the Runner and then wait calling the method waitTillDone() like this:

Runner runner = Runner.runner(2);

for (DataTable singleTable : uniquePhrases) {

    runner.run(new ComputeDTask(singleTable));
}

// blocks until all tasks are finished (or failed)
runner.waitTillDone();

runner.shutdown();

To use it add this gradle/maven dependency: 'com.github.matejtymes:javafixes:1.0'

For more details look here: https://github.com/MatejTymes/JavaFixes or here: http://matejtymes.blogspot.com/2016/04/executor-that-notifies-you-when-task.html

There are several approaches.

You can call first ExecutorService.shutdown and then ExecutorService.awaitTermination which returns:

> true if this executor terminated and false if the timeout elapsed > before termination

So:

> There is a function called awaitTermination But a timeout has to be > provided in it. Which is not a guarantee that when this returns all > the tasks would have been finished. Is there a way to achieve this ?

You just have to call awaitTermination in a loop.

Using awaitTermination

A full example with this implementation:

public class WaitForAllToEnd {

    public static void main(String[] args) throws InterruptedException {
        final int total_threads = 4;
        ExecutorService executor = Executors.newFixedThreadPool(total_threads);
        for(int i = 0; i < total_threads; i++){
            executor.execute(parallelWork(100 + i * 100));
        }

        int count = 0;

        // This is the relevant part
        // Chose the delay most appropriate for your use case
        executor.shutdown();
        while (!executor.awaitTermination(100, TimeUnit.MILLISECONDS)) {
            System.out.println("Waiting "+ count);
            count++;
        }
    }

    private static Runnable parallelWork(long sleepMillis) {
        return () -> {
            try {
                Thread.sleep(sleepMillis);
            } catch (InterruptedException e) {
                // Do Something
            }
            System.out.println("I am Thread : " + Thread.currentThread().getId());
        };
    }
}

Using CountDownLatch

Another option is to create a CountDownLatch with a count equals to the number of parallel tasks. Each thread calls countDownLatch.countDown();, while the main thread calls countDownLatch.await();.

A full example with this implementation:

public class WaitForAllToEnd {

    public static void main(String[] args) throws InterruptedException {
        final int total_threads = 4;
        CountDownLatch countDownLatch = new CountDownLatch(total_threads);
        ExecutorService executor = Executors.newFixedThreadPool(total_threads);
        for(int i = 0; i < total_threads; i++){
            executor.execute(parallelWork(100 + i * 100, countDownLatch));
        }
        countDownLatch.await();
        System.out.println("Exit");
        executor.shutdown();
    }

    private static Runnable parallelWork(long sleepMillis, CountDownLatch countDownLatch) {
        return () -> {
            try {
                Thread.sleep(sleepMillis);
            } catch (InterruptedException e) {
                // Do Something
            }
            System.out.println("I am Thread : " + Thread.currentThread().getId());
            countDownLatch.countDown();
        };
    }
}

Using Cyclic Barrier

Another approach is to use a Cyclic Barrier

public class WaitForAllToEnd {

    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        final int total_threads = 4;
        CyclicBarrier barrier = new CyclicBarrier(total_threads+ 1);
        ExecutorService executor = Executors.newFixedThreadPool(total_threads);
        for(int i = 0; i < total_threads; i++){
            executor.execute(parallelWork(100 + i * 100, barrier));
        }
        barrier.await();
        System.out.println("Exit");
        executor.shutdown();
    }

    private static Runnable parallelWork(long sleepMillis, CyclicBarrier barrier) {
        return () -> {
            try {
                Thread.sleep(sleepMillis);
            } catch (InterruptedException e) {
                // Do Something
            }
            System.out.println("I am Thread : " + Thread.currentThread().getId());
            try {
                barrier.await();
            } catch (InterruptedException | BrokenBarrierException e) {
              // Do something
            }
        };
    }
}

There are other approaches but those would require changes to your initial requirements, namely:

> How to wait for all tasks to be completed when they are submitted > using ExecutorService.execute() .

I will just wait for the executor to terminate with a specified timeout that you think it is suitable for the tasks to complete.

 try {  
         //do stuff here 
         exe.execute(thread);
    } finally {
        exe.shutdown();
    }
    boolean result = exe.awaitTermination(4, TimeUnit.HOURS);
    if (!result)

    {
        LOGGER.error("It took more than 4 hour for the executor to stop, this shouldn't be the normal behaviour.");
    }

Sounds like you need ForkJoinPool and use the global pool to execute tasks.

public static void main(String[] args) {
    // the default `commonPool` should be sufficient for many cases.
    ForkJoinPool pool = ForkJoinPool.commonPool(); 
    // The root of your task that may spawn other tasks. 
    // Make sure it submits the additional tasks to the same executor that it is in.
    Runnable rootTask = new YourTask(pool); 
    pool.execute(rootTask);
    pool.awaitQuiescence(...);
    // that's it.
}

The beauty is in pool.awaitQuiescence where the method will block utilize the caller's thread to execute its tasks and then return when it is really empty.

how about this?

Object lock = new Object();
CountDownLatch cdl = new CountDownLatch(threadNum);
for (int i = 0; i < threadNum; i++) {
	executorService.execute(() -> {

		synchronized (lock) {
			cdl.countDown();
			try {
				lock.wait();
			} catch (InterruptedException e) {
				Thread.currentThread().interrupt();
			}
		}
	});
}
cdl.await();
synchronized (lock) {
	lock.notifyAll();
}

if you do not add new tasks to ExecutorService , this may wait for all current tasks completed

A simple alternative to this is to use threads along with join. Refer : Joining Threads