Creating a NotifyingBlockingThreadPoolExecutor Blog


    A Thread Pool is a useful tool for performing a collection of tasks in parallel. This becomes more and more relevant as CPUs introduce multi-core architectures that can benefit from parallelizing our programs. Java 5 introduced this framework as part of the new concurrency support, with theThreadPoolExecutor class and other assisting classes. The ThreadPoolExecutor framework is powerful yet flexible enough, allowing user-specific configurations and providing relevant hooks and saturation strategies to deal with a full queue. To best follow this article, you may find it useful to open the ThreadPoolExecutor Java API in a parallel tab.

    The Need for a Blocking Thread Pool

    Recently, my colleague Yaneeve Shekel had the need for a thread pool that would work on several tasks in parallel but would wait to add new tasks until a free thread was there to handle them. This is really not something bizarre: in fact, this need is quite common. Yaneeve needed it to analyze a huge directory with a very long list of files, where there was no point in piling on more and moreFileAnalyzeTask instances without a free thread to handle them. The analyze operation takes some time, while the speed in which we can pile files for analysis is much higher. Thus, not controlling for thread availability for the task would create a huge queue with a possible memory problem, and for no benefit.

    Other cases in which you'd need a thread pool that can wait to add new tasks:

    • Doing some in-memory task on a long list of database records. You would not want to run and turn each record to a task in theThreadPoolExecutor queue while the threads are busy with some long operation on previous records, as doing this would exhaust your memory. The right way to do it is to query the database, run over the result set and create enough tasks for a fixed sized queue, and then wait until there is room in the queue. You can use a cursor to represent the result set, but even if you get back a dynamic result set, the database will not reply with the entire bulk of records; it will send you a limited amount of records and update your result set object while you run over it, forwarding to the next records of your result set, thus only forwarding through the result set. When the queue is ready for more tasks, it reads the next records from the database.

    • Analyzing a long file with "independent lines": each line can be analyzed separately by a different thread. Again, there is no sense in reading the entire file into LineTask objects if there is no available thread to handle them. This scenario is in fact a true need raised in a forum asking for a recommended solution.

    The problem is that ThreadPoolExecutor doesn't give you the required behavior -- blocking when the queue is full -- out of the box. A feature request was even submitted to the Java Bug database (Bug Id 6648211, "Need for blockingThreadPoolExecutor"), but it was put on "very low priority," as the user is supposedly able to quite easily implement this behavior.

    At a first glance it looks odd; you think that aThreadPoolExecutor with a boundedBlockingQueue will give you exactly this behavior. But apparently it does not. In fact, by default it throwsRejectedExecutionException if a task is submitted and the queue is full. This happens becauseThreadPoolExecutor.execute(Runnable) does not call the blocking method BlockingQueue.put(...) when queuing a task, but rather the unblocking Queue.offer(...), with a timeout of 0, which means "try but do not wait.". And if the result is false (offer failed), it calls the saturation policy -- the assigned RejectExecutionHandler for this thread pool -- with the default handler throwing an exception. Though it seems that there is no real logic in this, it is in fact a design decision, allowing the user to react to the fact that a task is rejected rather than just deciding in the framework to wait or block.

    Suggested Solutions

    There are several ways to allow blocking on a full queue:

    • We may implement our own BlockingThreadPoolExecutorand override the execute(...) method, so it will call the BlockingQueue.put(...) instead ofBlockingQueue.offer(...). But this may not be so elegant as we interfere quite brutally in howexecute() works (and we cannot callsuper.execute(...) since we do the queuing).

    • There is the option to create a ThreadPoolExecutorwith the CallerRunsPolicy reject strategy. This strategy, in the case of a full queue, sends the exceeding task to be executed by the thread that called execute() (the producer), thus killing two birds with one stone: the task is handled and the producer is busy in handling the task and not in overloading the queue with additional tasks. There are, however, two flaws in this strategy. First, the task is not handled in the order it was produced; this is usually not so problematic anyhow, as there is no real guarantee on the order of context switch between the worker threads that influences task progress and order. Second, when the producer is working on its task, no one fills the queue. So if one of the worker threads, or more, finish their tasks while the producer is still working, they will become idle. It requires fine configuration tuning of the queue size in order to minimize it, but you can never guarantee to avoid this situation. It would have been nice if there was a way to set theThreadPoolExecutor in a true Leader-Followersmanner (a design pattern in which the producer gets to run the task while a thread from the pool becomes the new producer), but theCallerRunsPolicy strategy does not work like that. (The C++ ACE framework for example, implemented the Leader-Followers pattern. For more details on the Leader-Followers pattern, you can follow this presentation.)

    • One can implement a simple "counting"ThreadPoolExecutor that uses a Semaphoreinitialized to the bound that we want to set, decremented, by calling acquire() at execute(...), and increased back, by calling release() at theafterExecute() hook method, as well as in acatch at the end of execute(...) for the reject scenario. The semaphore is acting in this way as a block on the call to execute(...) and you can in fact use an unbounded BlockingQueue in this case. This solution was suggested by Brian Goetz in a forum reply, and discussed also in his book Java Concurrency in Practice, by Goetz et al., in listing 8.4. Here is how it will look:

      public class BlockingThreadPoolExecutor extends ThreadPoolExecutor { private Semaphore semaphore; public BlockingThreadPoolExecutor(..., int bound, ...) { super(...); this.semaphore = new Semaphore(bound); } @Override public void execute(Runnable task) { boolean acquired = false; do { try { semaphore.acquire(); acquired = true; } catch (InterruptedException e) { // wait forever! } } while(!acquired); try { super.execute(task); } catch(RuntimeException e) { // specifically, handle RejectedExecutionException semaphore.release(); throw e; } catch(Error e) { semaphore.release(); throw e; } } @Override protected void afterExecute(Runnable r, Throwable t) { semaphore.release(); } } 

      This is a nice solution. A nice adaptation may be to usetryAcquire(timeout) as it is always a better practice to allow a timeout on blocking operations. But anyway, I personally don't like self-managing the blocking operation when theThreadPoolExecutor may have its own bounded queue. It doesn't make sense for me. I prefer the following solution that uses the bounded queue blocking and the saturation policy.

    • The fourth solution is to create aThreadPoolExecutor with a bounded queue and our ownRejectExecutionHandler that will block on the queue waiting for it to be ready to take new tasks. We prefer to wait on the queue with a timeout and to notify the user if the timeout occurs, so that we will not wait forever in case of some problem in pulling the tasks from the queue. However, for most reasonable scenarios, the caller will not have to take any action when the queue is full, as the producer thread will just wait on the queue. I prefer this approach is it seems the most simple using the original design of ThreadPoolExecutor.

    Which brings us to this code (see the Resources section to download it):

    public class BlockingThreadPoolExecutor extends ThreadPoolExecutor { public BlockingThreadPoolExecutor( int poolSize, int queueSize, long keepAliveTime, TimeUnit keepAliveTimeUnit, long maxBlockingTime, TimeUnit maxBlockingTimeUnit, Callable<Boolean> blockingTimeCallback) { super( poolSize, // Core size poolSize, // Max size keepAliveTime, keepAliveTimeUnit, new ArrayBlockingQueue<Runnable>( // to avoid redundant threads Math.max(poolSize, queueSize) ), // our own RejectExecutionHandler – see below new BlockThenRunPolicy( maxBlockingTime, maxBlockingTimeUnit, blockingTimeCallback ) ); super.allowCoreThreadTimeOut(true); } @Override public void setRejectedExecutionHandler (RejectedExecutionHandler h) { throw new unsupportedOperationException( "setRejectedExecutionHandler is not allowed on this class."); } // ... } 

    This is our new blocking thread pool. But as you may see, the real thing is still missing and that is our own newRejectExecutionHandler. In the constructor we pass parameters to our super, ThreadPoolExecutor. We use the full version constructor since the most important parameter that we wish to pass to our base class is theRejectExecutionHandler, which is the last parameter. We create a new object of the type BlockThenRunPolicy, our own class (presented in a moment). The name of this saturation policy means exactly what it does: if a task is rejected due to saturation, block on the task submission in the producer thread context, and when there is enough capacity to take the task, accept it. We implement the BlockThenRunPolicy class as a private inner class inside ourBlockingThreadPoolExecutor, as no one else should know it.

    // -------------------------------------------------- // Inner private class of BlockingThreadPoolExecutor // A reject policy that waits on the queue // -------------------------------------------------- private static class BlockThenRunPolicy implements RejectedExecutionHandler { private long blockTimeout; private TimeUnit blocTimeoutUnit; private Callable<Boolean> blockTimeoutCallback; // Straight-forward constructor public BlockThenRunPolicy(...){...} // -------------------------------------------------- @Override public void rejectedExecution( Runnable task, ThreadPoolExecutor executor) { BlockingQueue<Runnable> queue = executor.getQueue(); boolean taskSent = false; while (!taskSent) { if (executor.isShutdown()) { throw new RejectedExecutionException( "ThreadPoolExecutor has shutdown while attempting to offer a new task."); } try { // offer the task to the queue, for a blocking-timeout if (queue.offer(task, blockTimeout, blocTimeoutUnit)) { taskSent = true; } else { // task was not accepted - call the user's Callback Boolean result = null; try { result =; } catch(Exception e) { // wrap the Callback exception and re-throw throw new RejectedExecutionException(e); } // check the Callback result if(result == false) { throw new RejectedExecutionException( "User decided to stop waiting for task insertion"); } else { // user decided to keep waiting (may log it) continue; } } } catch (InterruptedException e) { // we need to go back to the offer call... } } // end of while for InterruptedException } // end of method rejectExecution // -------------------------------------------------- } // end of inner private class BlockThenRunPolicy 

    Note that we may get a timeout when waiting on the queue, on the call to queue.offer(...). It is always the right practice to use a timeout-enabled version of a blocking call, rather than any "wait-forever" version. This way it is easier to be aware of and troubleshoot cases of thread starvation and deadlocks. In this case, we do not log the event of getting the timeout, as we do not have a logger at hand. But still, this is a major event, especially if we set a long timeout that we do not expect to happen. This is why we ask the user to provide a callback so we can report the event and let the user decide whether to just log and keep waiting or stop the wait.

    Our solution preserves the default behavior ofThreadPoolExecutor, except for the saturation policy. Since we use inheritance, any setter or getter of the originalThreadPoolExecutor can be used, excluding thesetRejectedExecutionHandler, which we forbid, throwing an exception if called. Prometheus, another open source approach to the blocking thread pool problem, used a wrapper solution as a straightforward approach (with the following API). However, the wrapper solution requires implementing allExecutorService interface methods -- in order to be a common ExecutorService -- resulting with a quite cumbersome solution compared to our more organic extension.

    Almost Done

    We have a BlockingThreadPoolExecutor. But bear with me for a few more moments, as we are about to ask for more.

    Remember our problem. We have a huge directory filled with files and we wanted to block on the queue if it is full. But we need something more. When all files are sent to the queue, the producer thread knows it is done sending all the files, but it still needs to wait for the worker threads to finish. And we do not want to shut down the thread pool and wait for it to finish that way, as we are going to use it in a few moments again. What we need is a way to wait for the final tasks sent to the thread pool to complete.

    To do that we add a "synchronizer" object for the producer to wait on. The producer will wait on a new method we create, which we called await(), but there is an underlying condition inside that waits for a signal, and this is our Synchronizer. The thread pool signals the Synchronizer when it is idle; that is, all worker threads are idle. To have this info we simply count the number of currently working threads. We do not rely on the getActiveCount() method, as its contract and definition are not clear enough; we prefer to simply do it ourselves using an AtomicInteger to make sure that increment and decrement operations are done atomically, without a need to synchronize around ++ or --.

    Here we use the beforeExecute() andafterExecute() hook methods, but must take care of tasks that failed at the execute point, before assuming position in the queue, in which case decreasing the counter must be done. OurSynchronizer class manages the blocking wait on theawait() method, by waiting on a Conditionthat is signaled only when there are no tasks in the queue.

    The resulting code is this:

    public class NotifyingThreadPoolExecutor extends ThreadPoolExecutor { private AtomicInteger tasksInProcess = new AtomicInteger(); // using our own private inner class, see below private Synchronizer synchronizer = new Synchronizer(); @Override public void execute(Runnable task) { // count a new task in process tasksInProcess.incrementAndGet(); try { super.execute(task); } catch(RuntimeException e) { // specifically, handle RejectedExecutionException tasksInProcess.decrementAndGet(); throw e; } catch(Error e) { tasksInProcess.decrementAndGet(); throw e; } } @Override protected void afterExecute(Runnable r, Throwable t) { super.afterExecute(r, t); // synchronizing on the pool (and all its threads) // we need the synchronization to avoid more than one signal // if two or more threads decrement almost together and come // to the if with 0 tasks together synchronized(this) { tasksInProcess.decrementAndGet(); if (tasksInProcess.intValue() == 0) { synchronizer.signalAll(); } } } public void await() throws InterruptedException { synchronizer.await(); } // (there is also an await with timeout, see the full source code) } 

    We need now to provide the Synchronizer class that does the actual locking and synchronization work. We prefer to implement the Synchronizer class as a private inner class inside our NotifyingThreadPoolExecutor, as no one else should know it.

    //-------------------------------------------------------------- // Inner private class of NotifyingThreadPoolExecutor // for signaling when queue is idle //-------------------------------------------------------------- private class Synchronizer { private final Lock lock = new ReentrantLock(); private final Condition done = lock.newCondition(); private boolean isDone = false; // called from the containing class NotifyingThreadPoolExecutor private void signalAll() { lock.lock(); // MUST lock! try { isDone = true; done.signalAll(); } finally { lock.unlock(); // unlock even in case of an exception } } public void await() throws InterruptedException { lock.lock(); // MUST lock! try { while (!isDone) { // avoid signaling on 'spuriously' wake-up done.await(); } } finally { isDone = false; // for next call to await lock.unlock(); // unlock even in case of an exception } } // (there is also an await with timeout, see the full source code) } // end of private inner class Synchronizer //-------------------------------------------------------------- 

    As we needed both the notifying and the blocking features together, we combined them both to aNotifyingBlockingThreadPoolExecutor, whose code and an example of use can be found in the example source code.


    Occasionally there is a need to accomplish something that is not supported out of the box in a framework at hand. The first thought is usually: "it ought to be there, I must have missed something!" Then, after a while, after we search and investigate, we realize that the thing is indeed missing. At this point we are close to convincing ourselves that there is a reason for not having this ability, and we probably don't really need it. ("There must be a reason for not having it there. Who are we to argue?!") But the bravest of us would not compromise. Good frameworks are built to be extended, and so be it.

    In this article we presented the need we faced for a blocking thread pool. This need is not a whim; other people, as can be seen in the resources list, already raised this need. It is a bit surprising that the Java API does not provide this ability, but as seen, there are couple of good extensions that can be implemented to support this need. While implementing this feature, we went through the difference betweenoffer() and put() on aBlockingQueue, the Rejection Policy of the thread pool framework, the beforeExecute() andafterExecute() hook methods, and some other relatedjava.concurrent players, such as Lock,Signals, and AtomicInteger.

    The implementation presented here, together with the sample code provided, may serve both as a good solution for the blocking thread pool need, as well as a reference for other relatedThreadPoolExecutor extensions.


    I would like to thank Yaneeve Shekel for bringing this problem to me and working with me on parts of the code presented here.