wait until all threads finish their work in java

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I'm writing an application that has 5 threads that get some information from web simultaneously and fill 5 different fields in a buffer class. I need to validate buffer data and store it in a database when all threads finished their job. How can I do this (get alerted when all threads finished their work) ?

The approach I take is to use an ExecutorService to manage pools of threads.

ExecutorService es = Executors.newCachedThreadPool();
for(int i=0;i<5;i++)
    es.execute(new Runnable() { /*  your task */ });
es.shutdown();
boolean finished = es.awaitTermination(1, TimeUnit.MINUTES);
// all tasks have finished or the time has been reached.

ExecutorService, Java+. Java Concurrency Although, it won't wait until all threads stop executing​. This blocks the thread until all tasks complete their execution or the Jobs · Meta Baeldung · The Full Archive · Write for Baeldung · Editors  Although, it won't wait until all threads stop executing. Waiting for existing threads to complete their execution can be achieved by using the awaitTermination() method. This blocks the thread until all tasks complete their execution or the specified timeout is reached:

You can join to the threads. The join blocks until the thread completes.

for (Thread thread : threads) {
    thread.join();
}

Note that join throws an InterruptedException. You'll have to decide what to do if that happens (e.g. try to cancel the other threads to prevent unnecessary work being done).

How to wait for all threads to finish, using ExecutorService?, Community · Java; How to wait for all threads to finish, using. it at the end of every task, then monitor in the infinite loop this counter to become  All threads must wait to do their job, until all threads are running (alive). I use wait/notify to synchronize: when a thread is created it waits until it gets a notify to continue. It works, but not sure it's the best way.

Have a look at various solutions.

  1. join() API has been introduced in early versions of Java. Some good alternatives are available with this concurrent package since the JDK 1.5 release.

  2. ExecutorService#invokeAll()

    Executes the given tasks, returning a list of Futures holding their status and results when everything is completed.

    Refer to this related SE question for code example:

    How to use invokeAll() to let all thread pool do their task?

  3. CountDownLatch

    A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

    A CountDownLatch is initialized with a given count. The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.

    Refer to this question for usage of CountDownLatch

    How to wait for a thread that spawns it's own thread?

  4. ForkJoinPool or newWorkStealingPool() in Executors

  5. Iterate through all Future objects created after submitting to ExecutorService

How to wait for multiple thread to finish run (Threads forum at , Only after all the threads finish running should the foll message appear The thread that must wait for all of them could simply loop through the ArrayList calling join on each Jim ur solution worked.. but duh . you knew it would right . Manish Veerwal wrote: you can use the JAVA APi for that purpose. How to wait for all threads to finish, using ExecutorService? returning a list of Futures holding their status and results when all complete. and wait until

Apart from Thread.join() suggested by others, java 5 introduced the executor framework. There you don't work with Thread objects. Instead, you submit your Callable or Runnable objects to an executor. There's a special executor that is meant to execute multiple tasks and return their results out of order. That's the ExecutorCompletionService:

ExecutorCompletionService executor;
for (..) {
    executor.submit(Executors.callable(yourRunnable));
}

Then you can repeatedly call take() until there are no more Future<?> objects to return, which means all of them are completed.


Another thing that may be relevant, depending on your scenario is CyclicBarrier.

A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point. CyclicBarriers are useful in programs involving a fixed sized party of threads that must occasionally wait for each other. The barrier is called cyclic because it can be re-used after the waiting threads are released.

Thread synchronization wait/notify, This applies above all to JDK, in your case to the java.util.concurrent package. This is exactly the case here – you wish that all your threads wait for each thread waits in latch.await until all the threads finish the work and let  Guarded Blocks in Java. One tool we can use to coordinate actions of multiple threads in Java – is guarded blocks. Such blocks keep a check for a particular condition before resuming the execution. With that in mind, we'll make use of: Object.wait() – to suspend a thread. Object.notify() – to wake a thread up.

Another possibility is the CountDownLatch object, which is useful for simple situations : since you know in advance the number of threads, you initialize it with the relevant count, and pass the reference of the object to each thread. Upon completion of its task, each thread calls CountDownLatch.countDown() which decrements the internal counter. The main thread, after starting all others, should do the CountDownLatch.await() blocking call. It will be released as soon as the internal counter has reached 0.

Pay attention that with this object, an InterruptedException can be thrown as well.

Java concurrency - CountDownLatch Example, CountDownLatch works by having a counter initialized with number of threads, This count is essentially the number of threads, for which latch should wait. BaseHealthChecker.java : This class is a Runnable and parent for all specific can start and development team will finish first… and testing team will also finish. A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes. A CountDownLatch is initialized with a given count. The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately.

Java Language, The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any​  wait()- It tells the calling thread to give up the lock and go to sleep until some other thread enters the same monitor and calls notify(). notify()- It wakes up one single thread that called wait() on the same object. It should be noted that calling notify() does not actually give up a lock on a resource.

Differences between wait() and join() methods in Java , The wait() is used in with notify() and notifyAll() methods, but join() is used in Java to wait until one thread finishes its execution. wait() is mainly used for shared  Java – Wait for thread to finish November 14, 2015 Last updated January 13, 2019 by Catalin There are certain actions such as resource loading that might need to finish before running any other task. If these actions are run in a separate thread, you can wait for thread to finish by using the.join() method.

Thread: join() (Using join() to wait for threads to finish), Thread: join() (Using join() to wait for threads to finish) : Thread « java.lang « Java by API. The java.lang.Object.wait() causes current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object. In other words, this method behaves exactly as if it simply performs the call wait(0). The current thread must own this object's monitor.

Comments
  • Thread.join is a rather low-level very Java idiosynchratic way to solve the issue. Moreover it's problematic because the Thread API is flawed: you cannot know whether the join completed successfuly or not (see Java Concurrency In Practice). Higher level abstraction, like using a CountDownLatch may be preferrable and will look more natural to programmers who aren't "stuck" in the Java-idiosynchratic mindset. Don't argue with me, go argue with Doug Lea ; )
  • possible duplicate of How to wait for a set of threads to complete?
  • @Leonid that is exactly what shutdown() does.
  • while(!es.awaitTermination(1, TimeUnit.MINUTES));
  • @AquariusPower You could just tell it to wait longer, or forever.
  • oh.. I get it; so I added a message in the loop saying it is waiting all threads to finish; thanks!
  • @Amogh An ExecutorService won't shutdown itself unless it has been GCed. If you do es.awaitTermination you will be preventing it from being cleaned up so you will be waiting for another thread to terminate it.
  • Do these threads run in parallel or sequentially to each other?
  • @JamesWebster: Parallel.
  • @James Webster: The statement t.join(); means that the current thread blocks until the thread t terminates. It does not affect thread t.
  • Thanks. =] Studied paralellism at uni, but that was the sole thing I struggled to learn! Thankfully I don't have to use it much now or when I do it's not too complex or there are no shared resources and blocking isn't critical
  • @4r1y4n Whether the code provided is truly parallel depends on what you're trying to do with it, and is related more to aggregating data spread throughout collections using joined threads. You're joining threads, which potentially means "joining" data. Also, parallelism does NOT necessarily mean concurrency. That is dependent on the CPUs. It could very well be that the threads are running in parallel, but the computations are happening in whatever order is determined by the underlying CPU.
  • This is close, but I'd still make a couple of adjustments. executor.submit returns a Future<?>. I would add these futures to a list, and then loop through the list calling get on each future.
  • Also, you can instantiate a constructor using Executors, e.g., Executors.newCachedThreadPool (or similar)
  • This is active waiting, which will cause CPU to be constantly running an empty loop. Very wasteful.