CODEWITHSUNDEEP
javamultithreadingwait
Ariyan
Ariyan

Reputation: 15158

wait until all threads finish their work in java

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) ?

Upvotes: 127

Views: 249173

Answers (17)

Peter Lawrey
Peter Lawrey

Reputation: 533820

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.

Upvotes: 159

Ravindra babu
Ravindra babu

Reputation: 38950

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?

  1. 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 here

  1. ForkJoinPool or newWorkStealingPool() in Executors

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

Source : Various links from docs.oracle.com

Upvotes: 29

Afsar Ali
Afsar Ali

Reputation: 605

I had similar situation , where i had to wait till all child threads complete its execution then only i could get the status result for each of them .. hence i needed to wait till all child thread completed.

below is my code where i did multi-threading using

 public static void main(String[] args) {
        List<RunnerPojo> testList = ExcelObject.getTestStepsList();//.parallelStream().collect(Collectors.toList());
        int threadCount = ConfigFileReader.getInstance().readConfig().getParallelThreadCount();
        System.out.println("Thread count is : =========  " + threadCount); // 5

        ExecutorService threadExecutor = new DriverScript().threadExecutor(testList, threadCount);


        boolean isProcessCompleted = waitUntilCondition(() -> threadExecutor.isTerminated()); // Here i used waitUntil condition 

        if (isProcessCompleted) {
            testList.forEach(x -> {
                System.out.println("Test Name: " + x.getTestCaseId());
                System.out.println("Test Status : " + x.getStatus());
                System.out.println("======= Test Steps ===== ");

                x.getTestStepsList().forEach(y -> {
                    System.out.println("Step Name: " + y.getDescription());
                    System.out.println("Test caseId : " + y.getTestCaseId());
                    System.out.println("Step Status: " + y.getResult());
                    System.out.println("\n ============ ==========");
                });
            });
        }

Below method is for distribution of list with parallel proccessing

// This method will split my list and run in a parallel process with mutliple threads
    private ExecutorService threadExecutor(List<RunnerPojo> testList, int threadSize) {
        ExecutorService exec = Executors.newFixedThreadPool(threadSize);
        testList.forEach(tests -> {
            exec.submit(() -> {
                driverScript(tests);
            });
        });
        exec.shutdown();
        return exec;
    }

This is my wait until method: here you can wait till your condition satisfies within do while loop . in my case i waited for some max timeout . this will keep checking until your threadExecutor.isTerminated() is true with polling period of 5 sec.

    static boolean waitUntilCondition(Supplier<Boolean> function) {
        Double timer = 0.0;
        Double maxTimeOut = 20.0;

        boolean isFound;
        do {
            isFound = function.get();
            if (isFound) {
                break;
            } else {
                try {
                    Thread.sleep(5000); // Sleeping for 5 sec (main thread will sleep for 5 sec)
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                timer++;
                System.out.println("Waiting for condition to be true .. waited .." + timer * 5 + " sec.");
            }
        } while (timer < maxTimeOut + 1.0);

        return isFound;
    }

Upvotes: 0

Yash
Yash

Reputation: 9588

Wait/block the Thread Main until some other threads complete their work.

As @Ravindra babu said it can be achieved in various ways, but showing with examples.

  • java.lang.Thread.join() Since:1.0

    public static void joiningThreads() throws InterruptedException {
    Thread t1 = new Thread( new LatchTask(1, null), "T1" );
    Thread t2 = new Thread( new LatchTask(7, null), "T2" );
    Thread t3 = new Thread( new LatchTask(5, null), "T3" );
    Thread t4 = new Thread( new LatchTask(2, null), "T4" );

    // Start all the threads
    t1.start();
    t2.start();
    t3.start();
    t4.start();

    // Wait till all threads completes
    t1.join();
    t2.join();
    t3.join();
    t4.join();
}

  • java.util.concurrent.CountDownLatch Since:1.5

    • .countDown() « Decrements the count of the latch group.
    • .await() « The await methods block until the current count reaches zero.

    If you created latchGroupCount = 4 then countDown() should be called 4 times to make count 0. So, that await() will release the blocking threads.

    public static void latchThreads() throws InterruptedException {
    int latchGroupCount = 4;
    CountDownLatch latch = new CountDownLatch(latchGroupCount);
    Thread t1 = new Thread( new LatchTask(1, latch), "T1" );
    Thread t2 = new Thread( new LatchTask(7, latch), "T2" );
    Thread t3 = new Thread( new LatchTask(5, latch), "T3" );
    Thread t4 = new Thread( new LatchTask(2, latch), "T4" );

    t1.start();
    t2.start();
    t3.start();
    t4.start();

    //latch.countDown();

    latch.await(); // block until latchGroupCount is 0.
}

Example code of Threaded class LatchTask. To test the approach use joiningThreads(); and latchThreads(); from main method.

class LatchTask extends Thread {
    CountDownLatch latch;
    int iterations = 10;
    public LatchTask(int iterations, CountDownLatch latch) {
        this.iterations = iterations;
        this.latch = latch;
    }

    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " : Started Task...");

        for (int i = 0; i < iterations; i++) {
            System.out.println(threadName + " : " + i);
            MainThread_Wait_TillWorkerThreadsComplete.sleep(1);
        }
        System.out.println(threadName + " : Completed Task");
        // countDown() « Decrements the count of the latch group.
        if(latch != null)
            latch.countDown();
    }
}
  • CyclicBarriers 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. 
    CyclicBarrier barrier = new CyclicBarrier(3);
barrier.await();
    For example refer this Concurrent_ParallelNotifyies class.

  • Executer framework: we can use ExecutorService to create a thread pool, and tracks the progress of the asynchronous tasks with Future.

    • submit(Runnable), submit(Callable) which return Future Object. By using future.get() function we can block the main thread till the working threads completes its work.

    • invokeAll(...) - returns a list of Future objects via which you can obtain the results of the executions of each Callable.

Find example of using Interfaces Runnable, Callable with Executor framework.

@See also

Upvotes: 18

Alon G
Alon G

Reputation: 3383

I created a small helper method to wait for a few Threads to finish: 

public static void waitForThreadsToFinish(Thread... threads) {
        try {
            for (Thread thread : threads) {
                thread.join();
            }
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

Upvotes: 3

Eric
Eric

Reputation: 24984

The existing answers said could join() each thread.

But there are several ways to get the thread array / list:

  • Add the Thread into a list on creation.
  • Use ThreadGroup to manage the threads.

Following code will use the ThreadGruop approach. It create a group first, then when create each thread specify the group in constructor, later could get the thread array via ThreadGroup.enumerate()

Code

SyncBlockLearn.java

import org.testng.Assert;
import org.testng.annotations.Test;

/**
 * synchronized block - learn,
 *
 * @author eric
 * @date Apr 20, 2015 1:37:11 PM
 */
public class SyncBlockLearn {
    private static final int TD_COUNT = 5; // thread count
    private static final int ROUND_PER_THREAD = 100; // round for each thread,
    private static final long INC_DELAY = 10; // delay of each increase,

    // sync block test,
    @Test
    public void syncBlockTest() throws InterruptedException {
        Counter ct = new Counter();
        ThreadGroup tg = new ThreadGroup("runner");

        for (int i = 0; i < TD_COUNT; i++) {
            new Thread(tg, ct, "t-" + i).start();
        }

        Thread[] tArr = new Thread[TD_COUNT];
        tg.enumerate(tArr); // get threads,

        // wait all runner to finish,
        for (Thread t : tArr) {
            t.join();
        }

        System.out.printf("\nfinal count: %d\n", ct.getCount());
        Assert.assertEquals(ct.getCount(), TD_COUNT * ROUND_PER_THREAD);
    }

    static class Counter implements Runnable {
        private final Object lkOn = new Object(); // the object to lock on,
        private int count = 0;

        @Override
        public void run() {
            System.out.printf("[%s] begin\n", Thread.currentThread().getName());

            for (int i = 0; i < ROUND_PER_THREAD; i++) {
                synchronized (lkOn) {
                    System.out.printf("[%s] [%d] inc to: %d\n", Thread.currentThread().getName(), i, ++count);
                }
                try {
                    Thread.sleep(INC_DELAY); // wait a while,
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            System.out.printf("[%s] end\n", Thread.currentThread().getName());
        }

        public int getCount() {
            return count;
        }
    }
}

The main thread will wait for all threads in the group to finish.

Upvotes: 0

18446744073709551615
18446744073709551615

Reputation: 16872

Although not relevant to OP's problem, if you are interested in synchronization (more precisely, a rendez-vous) with exactly one thread, you may use an Exchanger

In my case, I needed to pause the parent thread until the child thread did something, e.g. completed its initialization. A CountDownLatch also works well.

Upvotes: 2

esaj
esaj

Reputation: 16035

Store the Thread-objects into some collection (like a List or a Set), then loop through the collection once the threads are started and call join() on the Threads.

Upvotes: 4

Narendra Yadala
Narendra Yadala

Reputation: 9664

You can use Threadf#join method for this purpose.

Upvotes: 2

interDist
interDist

Reputation: 557

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.

Upvotes: 11

Mark Byers
Mark Byers

Reputation: 839044

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).

Upvotes: 65

Bozho
Bozho

Reputation: 597362

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.

Upvotes: 13

Madis Pukkonen
Madis Pukkonen

Reputation: 1315

I had a similar problem and ended up using Java 8 parallelStream.

requestList.parallelStream().forEach(req -> makeRequest(req));

It's super simple and readable. Behind the scenes it is using default JVM’s fork join pool which means that it will wait for all the threads to finish before continuing. For my case it was a neat solution, because it was the only parallelStream in my application. If you have more than one parallelStream running simultaneously, please read the link below.

More information about parallel streams here.

Upvotes: 1

Maggy
Maggy

Reputation: 1

Use this in your main thread: while(!executor.isTerminated()); Put this line of code after starting all the threads from executor service. This will only start the main thread after all the threads started by executors are finished. Make sure to call executor.shutdown(); before the above loop.

Upvotes: -1

Jeyaraj.J
Jeyaraj.J

Reputation: 11

try this, will work.

  Thread[] threads = new Thread[10];

  List<Thread> allThreads = new ArrayList<Thread>();

  for(Thread thread : threads){

        if(null != thread){

              if(thread.isAlive()){

                    allThreads.add(thread);

              }

        }

  }

  while(!allThreads.isEmpty()){

        Iterator<Thread> ite = allThreads.iterator();

        while(ite.hasNext()){

              Thread thread = ite.next();

              if(!thread.isAlive()){

                   ite.remove();
              }

        }

   }

Upvotes: 1

user974465
user974465

Reputation:

An executor service can be used to manage multiple threads including status and completion. See http://programmingexamples.wikidot.com/executorservice

Upvotes: 1

aioobe
aioobe

Reputation: 421220

You do

for (Thread t : new Thread[] { th1, th2, th3, th4, th5 })
    t.join()

After this for loop, you can be sure all threads have finished their jobs.

Upvotes: 9

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