wait until all threads finish their work in java

Question

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

Answer 1

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.

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• 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.

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@See also

• Find out thread is still alive?