Java - synchronizing multiple threads on time-step basis

Question

Suppose I have a Master that keeps a list of SlaveThread objects. On each time step, I want the Master to run the SlaveThreads in parallel, however, at the end of the time step, I want the SlaveThreads to to wait for each other to complete the current time step before advancing forward. Also, I don't want to reinstantiate the SlaveThreads on each time step. I have 2 possible solutions, and I don't know how to make either of them work:

1) The run() method in SlaveThread is in a while(true) loop. After the execution of a single loop in the SlaveThread, I'll have SlaveThread notify the Master(which I don't know how to do), and the Master does something like

try{
    for (int i = 0; i < numSlaveThreads; i++) {
        while (!slaveThreads[i].getCompletedThisIter()) {
        wait()
        }
      }
  System.out.println("Joined");

}

before advancing to the next time step. How would I do this? How can I have a single SlaveThread notify just the master?

2) The run() in Slave is not in while(true) loop, then I have to call start() on it on every iteration. But the thread state of the Slave at this point will be terminated. How can I call start() on it again without reinstantiating it?

Answer 1

That is exactly what barriers are for, you can realize this with a CyclicBarrier or a CountDownLatch. These are synchronizers used to delay the progress of threads until the desired state is reached, in your case the threads have completed their computation.

Here it depends on the details how you want to realize:

Latches are for waiting for events; barriers are for waiting for other threads.

For the CyclicBarrier that would done in the following fashion:

// whereby count is the number of your slave threads
this.barrier = new CyclicBarrier(count); 

Then in the Runnable definition of your slaves you will insert at the end of the computation: barrier.await()

public class Slaves implements Runnable {

   // ...

   @Override
   public void run() {

      while(condition) {

         // computation
         // ...

         try {
            // do not proceed, until all [count] threads
            // have reached this position
            barrier.await();
         } catch (InterruptedException ex) {
            return;
         } catch (BrokenBarrierException ex) {
            return;
         }
      }
   }
}

Your slave threads will not proceed, until all your threads have finished the computation. This way you do not need to realize signaling between another master thread.

If, however, you have some code you want to execute after all threads have reached that position (master signaling) you can pass an additional Runnable to the CyclicBarrier constructor, which will be executed after all threads have arrived at the barrier.

this.barrier = new CyclicBarrier(count,
   new Runnable() {
      @Override
      public void run() {
         // signal your master thread, update values, etc.
      }
    }
 );