How to wait for all threads to finish using ExecutorService

Question

I need to execute some amount of tasks 4 at a time  something like this   ExecutorService taskExecutor   Executors newFixedThreadPool 4   while            taskExecutor execute new MyTask            wait for completion somehow   How can I get notified once all of them are complete  For now I can t think about anything better than setting some global task counter and decrease it at the end of every task  then monitor in infinite loop this counter to become 0  or get a list of Futures and in infinite loop monitor isDone for all of them  What are better solutions not involving infinite loops   Thanks

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Clean way with ExecutorService  List lt Future lt Void gt  gt  results   null   try        List lt Callable lt Void gt  gt  tasks   new ArrayList lt  gt          ExecutorService executorService   Executors newFixedThreadPool 4        results   executorService invokeAll tasks      catch  InterruptedException ex                catch  Exception ex

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You could call waitTillDone   on this Runner class   Runner runner   Runner runner 4      create pool with 4 threads in thread pool  while            runner run new MyTask        here you submit your task     runner waitTillDone       and this blocks until all tasks are finished  or failed    runner shutdown       once you done you can shutdown the runner   You can reuse this class and call waitTillDone   as many times as you want to before calling shutdown    plus your code is extremly simple  Also you don t have to know the number of tasks upfront   To use it just add this gradle maven compile  com github matejtymes javafixes 1 3 1  dependency to your project   More details can be found here   https   github com MatejTymes JavaFixes

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I created the following working example  The idea is to have a way to process a pool of tasks  I am using a queue as example  with many Threads  determined programmatically by the numberOfTasks threshold   and wait until all Threads are completed to continue with some other processing   import java util PriorityQueue  import java util Queue  import java util concurrent CountDownLatch  import java util concurrent ExecutorService  import java util concurrent Executors       Testing CountDownLatch and ExecutorService to manage scenario where    multiple Threads work together to complete tasks from a single    resource provider  so the processing can be faster     public class ThreadCountDown    private CountDownLatch threadsCountdown   null  private static Queue lt Integer gt  tasks   new PriorityQueue lt  gt      public static void main String   args           Create a queue with  Tasks      int numberOfTasks   2000      while numberOfTasks--  gt  0            tasks add numberOfTasks                 Initiate Processing of Tasks     ThreadCountDown main   new ThreadCountDown        main process tasks         Receiving the Tasks to process  and creating multiple Threads   to process in parallel     private void process Queue lt Integer gt  tasks        int numberOfThreads   getNumberOfThreadsRequired tasks size         threadsCountdown   new CountDownLatch numberOfThreads       ExecutorService threadExecutor   Executors newFixedThreadPool numberOfThreads          Initialize each Thread     while numberOfThreads--  gt  0            System out println  Initializing Thread    numberOfThreads           threadExecutor execute new MyThread  Thread   numberOfThreads               try             Shutdown the Executor  so it cannot receive more Threads          threadExecutor shutdown            threadsCountdown await            System out println  ALL THREADS COMPLETED               continue With Some Other Process Here       catch  InterruptedException ex            ex printStackTrace                Determine the number of Threads to create    private int getNumberOfThreadsRequired int size        int threshold   100      int threads   size   threshold      if  size  gt   threads threshold             threads              return threads        Task Provider  All Threads will get their task from here    private synchronized static Integer getTask        return tasks poll          The Threads will get Tasks and process them  while still available    When no more tasks available  the thread will complete and reduce the threadsCountdown    private class MyThread implements Runnable        private String threadName       protected MyThread String threadName            super            this threadName   threadName              Override     public void run             Integer task          try                Check in the Task pool if anything pending to process             while   task   getTask       null                    processTask task                          catch  Exception ex               ex printStackTrace             finally                 Reduce count when no more tasks to process  Eventually all             Threads will end-up here  reducing the count to 0  allowing             the flow to continue after threadsCountdown await                   threadsCountdown countDown                         private void processTask Integer task           try              System out println this threadName   is Working on Task     task            catch  Exception ex               ex printStackTrace                          Hope it helps

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Just my two cents  To overcome the requirement of CountDownLatch to know the number of tasks beforehand  you could do it the old fashion way by using a simple Semaphore   ExecutorService taskExecutor   Executors newFixedThreadPool 4   int numberOfTasks 0  Semaphore s new Semaphore 0   while            taskExecutor execute new MyTask         numberOfTasks       try       s aquire numberOfTasks         In your task just call s release   as you would latch countDown

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ExecutorService invokeAll   does it for you   ExecutorService taskExecutor   Executors newFixedThreadPool 4   List lt Callable lt   gt  gt  tasks     your tasks    invokeAll   returns when all tasks are complete List lt Future lt   gt  gt  futures   taskExecutor invokeAll tasks

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You could use this code   public class MyTask implements Runnable        private CountDownLatch countDownLatch       public MyTask CountDownLatch countDownLatch            this countDownLatch   countDownLatch              Override     public void run              try                  Do somethings                              this countDownLatch countDown     important            catch  InterruptedException ex                  Thread currentThread   interrupt                         CountDownLatch countDownLatch   new CountDownLatch NUMBER OF TASKS   ExecutorService taskExecutor   Executors newFixedThreadPool 4   for  int i   0  i  lt  NUMBER OF TASKS  i          taskExecutor execute new MyTask countDownLatch      countDownLatch await    System out println  Finish tasks

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You can use Lists of Futures  as well   List lt Future gt  futures   new ArrayList lt Future gt        now add to it  futures add executorInstance submit new Callable lt Void gt        public Void call   throws IOException           do something     return null             then when you want to join on all of them  its essentially the equivalent of joining on each   with the added benefit that it re-raises exceptions from child threads to the main    for Future f  this futures    f get        Basically the trick is to call  get   on each Future one at a time  instead of infinite looping calling isDone   on  all or each    So you re guaranteed to  move on  through and past this block as soon as the last thread finishes   The caveat is that since the  get   call re-raises exceptions  if one of the threads dies  you would raise from this possibly before the other threads have finished to completion  to avoid this  you could add a catch ExecutionException around the get call    The other caveat is it keeps a reference to all threads so if they have thread local variables they won t get collected till after you get past this block  though you might be able to get around this  if it became a problem  by removing Future s off the ArrayList    If you wanted to know which Future  finishes first  you could use some something like https   stackoverflow com a 31885029 32453

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I ve just written a sample program that solves your problem  There was no concise implementation given  so I ll add one  While you can use executor shutdown   and executor awaitTermination    it is not the best practice as the time taken by different threads would be unpredictable   ExecutorService es   Executors newCachedThreadPool        List lt Callable lt Integer gt  gt  tasks   new ArrayList lt  gt          for  int j   1  j  lt   10  j              tasks add new Callable lt Integer gt                    Override             public Integer call   throws Exception                   int sum   0                  System out println  Starting Thread                             Thread currentThread   getId                      for  int i   0  i  lt  1000000  i                          sum    i                                     System out println  Stopping Thread                             Thread currentThread   getId                     return sum                                        try           List lt Future lt Integer gt  gt  futures   es invokeAll tasks           int flag   0           for  Future lt Integer gt  f   futures                Integer res   f get                System out println  Sum      res               if   f isDone                     flag   1                     if  flag    0              System out println  SUCCESS            else             System out println  FAILED           catch  InterruptedException   ExecutionException e            e printStackTrace

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In Java8 you can do it with CompletableFuture   ExecutorService es   Executors newFixedThreadPool 4   List lt Runnable gt  tasks   getTasks    CompletableFuture lt   gt    futures   tasks stream                                   map task - gt  CompletableFuture runAsync task  es                                   toArray CompletableFuture    new   CompletableFuture allOf futures  join        es shutdown

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You could wrap your tasks in another runnable  that will send notifications   taskExecutor execute new Runnable       public void run         taskStartedNotification        new MyTask   run        taskFinishedNotification

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The CyclicBarrier class in Java 5 and later is designed for this sort of thing

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ExecutorService WORKER THREAD POOL      Executors newFixedThreadPool 10   CountDownLatch latch   new CountDownLatch 2   for  int i   0  i  lt  2  i          WORKER THREAD POOL submit    - gt            try                  doSomething                latch countDown              catch  InterruptedException e                Thread currentThread   interrupt                            wait for the latch to be decremented by the two remaining threads latch await      If doSomething   throw some other exceptions  the latch countDown   seems will not execute  so what should I do

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A bit late to the game but for the sake of completion    Instead of  waiting  for all tasks to finish  you can think in terms of the Hollywood principle   quot don t call me  I ll call you quot  - when I m finished  I think the resulting code is more elegant    Guava offers some interesting tools to accomplish this  An example  Wrap an ExecutorService into a ListeningExecutorService  ListeningExecutorService service   MoreExecutors listeningDecorator Executors newFixedThreadPool 10     Submit a collection of callables for execution    for  Callable lt Integer gt  callable   callables      ListenableFuture lt Integer gt  lf   service submit callable        listenableFutures is a collection   listenableFutures add lf       Now the essential part  ListenableFuture lt List lt Integer gt  gt  lf   Futures successfulAsList listenableFutures    Attach a callback to the ListenableFuture  that you can use to be notified when all futures complete  Futures addCallback lf  new FutureCallback lt List lt Integer gt  gt            Override     public void onSuccess List lt Integer gt  result               do something with all the results             Override     public void onFailure Throwable t               log failure            This also offers the advantage that you can collect all the results in one place once the processing is finished    More information here

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Follow one of below approaches     Iterate through all Future tasks  returned from submit on ExecutorService and check the status with blocking call get   on Future object as suggested by Kiran Use invokeAll   on ExecutorService CountDownLatch ForkJoinPool or Executors html newWorkStealingPool Use shutdown  awaitTermination  shutdownNow APIs of ThreadPoolExecutor in proper sequence    Related SE questions   How is CountDownLatch used in Java Multithreading   How to properly shutdown java ExecutorService

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Basically on an ExecutorService you call shutdown   and then awaitTermination     ExecutorService taskExecutor   Executors newFixedThreadPool 4   while          taskExecutor execute new MyTask       taskExecutor shutdown    try     taskExecutor awaitTermination Long MAX VALUE  TimeUnit NANOSECONDS     catch  InterruptedException e

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You could use your own subclass of ExecutorCompletionService to wrap taskExecutor  and your own implementation of BlockingQueue to get informed when each task completes and perform whatever callback or other action you desire when the number of completed tasks reaches your desired goal

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This is my solution  based in  AdamSkywalker  tip  and it works  package frss main   import java util ArrayList  import java util List  import java util concurrent CompletableFuture  import java util concurrent ExecutorService  import java util concurrent Executors   public class TestHilos        void procesar             ExecutorService es   Executors newFixedThreadPool 4           List lt Runnable gt  tasks   getTasks            CompletableFuture lt   gt    futures   tasks stream   map task - gt  CompletableFuture runAsync task  es   toArray CompletableFuture    new           CompletableFuture allOf futures  join            es shutdown             System out println  FIN DEL PROCESO DE HILOS               private List lt Runnable gt  getTasks             List lt Runnable gt  tasks   new ArrayList lt Runnable gt              Hilo01 task1   new Hilo01            tasks add task1            Hilo02 task2   new Hilo02            tasks add task2           return tasks             private class Hilo01 extends Thread             Override         public void run                 System out println  HILO 1                          private class Hilo02 extends Thread             Override         public void run                 try                   sleep 2000                             catch  InterruptedException e                    e printStackTrace                              System out println  HILO 2                           public static void main String   args            TestHilos test   new TestHilos            test procesar

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There is a method in executor getActiveCount   - that gives the count of active threads    After spanning the thread  we can check if the activeCount   value is 0  Once the value is zero  it is meant that there are no active threads currently running which means task is finished   while  true        if  executor getActiveCount      0          ur own piece of code     break

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if you use more thread ExecutionServices SEQUENTIALLY and want to wait EACH EXECUTIONSERVICE to be finished  The best way is like below   ExecutorService executer1   Executors newFixedThreadPool THREAD SIZE1   for   lt loop gt        executer1 execute new Runnable                  Override             public void run                                                      executer1 shutdown     try     executer1 awaitTermination Long MAX VALUE  TimeUnit NANOSECONDS       ExecutorService executer2   Executors newFixedThreadPool THREAD SIZE2      for  true          executer2 execute new Runnable                  Override             public void run                                                             executer2 shutdown      catch  Exception e

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So I post my answer from linked question here  incase someone want a simpler way to do this  ExecutorService executor   Executors newFixedThreadPool 10   CompletableFuture   futures   new CompletableFuture 10   int i   0  while             futures i       CompletableFuture runAsync runner  executor      CompletableFuture allOf futures  join       THis will wait until all future ready

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Java 8 - We can use stream API to process stream  Please see snippet below  final List lt Runnable gt  tasks          or any other functional interface tasks stream   parallel   forEach Runnable  run     Uses default pool    alternatively to specify parallelism  new ForkJoinPool 15  submit               - gt  tasks stream   parallel   forEach Runnable  run         get

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Just to provide more alternatives here different to use latch barriers  You can also get the partial results until all of them finish using CompletionService   From Java Concurrency in practice   If you have a batch of computations to submit to an Executor and you want to retrieve their results as they become available  you could retain the Future associated with each task and repeatedly poll for completion by calling get with a timeout of zero  This is possible  but tedious  Fortunately there is a better way  a completion service    Here the implementation  public class TaskSubmiter       private final ExecutorService executor      TaskSubmiter ExecutorService executor    this executor   executor        void doSomethingLarge AnySourceClass source            final List lt InterestedResult gt  info   doPartialAsyncProcess source           CompletionService lt PartialResult gt  completionService   new ExecutorCompletionService lt PartialResult gt  executor           for  final InterestedResult interestedResultItem   info              completionService submit new Callable lt PartialResult gt                      public PartialResult call                         return InterestedResult doAnOperationToGetPartialResult                                       try           for  int t   0  n   info size    t  lt  n  t                  Future lt PartialResult gt  f   completionService take                PartialResult PartialResult   f get                processThisSegment PartialResult                                    catch  InterruptedException e                Thread currentThread   interrupt                       catch  ExecutionException e                throw somethinghrowable e getCause

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you should use executorService shutdown   and executorService awaitTermination method   An example as follows    public class ScheduledThreadPoolExample        public static void main String   args  throws InterruptedException           ScheduledExecutorService executorService   Executors newScheduledThreadPool 5           executorService scheduleAtFixedRate    - gt  System out println  process task                     0  1  TimeUnit SECONDS            TimeUnit SECONDS sleep 10           executorService shutdown            executorService awaitTermination 1  TimeUnit DAYS

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Use a CountDownLatch   CountDownLatch latch   new CountDownLatch totalNumberOfTasks   ExecutorService taskExecutor   Executors newFixedThreadPool 4   while          taskExecutor execute new MyTask        try     latch await      catch  InterruptedException E          handle     and within your task  enclose in try   finally   latch countDown

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here is two options   just bit confuse which one is best to go   Option 1   ExecutorService es   Executors newFixedThreadPool 4   List lt Runnable gt  tasks   getTasks    CompletableFuture lt   gt    futures   tasks stream                                   map task - gt  CompletableFuture runAsync task  es                                   toArray CompletableFuture    new   CompletableFuture allOf futures  join        es shutdown      Option 2   ExecutorService es   Executors newFixedThreadPool 4   List lt  Future lt   gt  gt  futures   new ArrayList lt  gt     for Runnable task   taskList        futures add es submit task       for Future lt   gt  future   futures        try           future get         catch Exception e              do logging and nothing else         es shutdown      Here putting future get    in try catch is good idea right

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This might help  Log i LOG TAG   shutting down executor       executor shutdown    while  true                    try                       Log i LOG TAG   Waiting for executor to terminate                           if  executor isTerminated                            break                      if  executor awaitTermination 5000  TimeUnit MILLISECONDS                             break                                          catch  InterruptedException ignored

[java] How to wait for all threads to finish, using ExecutorService?

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