Java ExecutorService: awaitTermination of all recursively created tasks

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This really is an ideal candidate for a Phaser. Java 7 is coming out with this new class. Its a flexible CountdonwLatch/CyclicBarrier. You can get a stable version at JSR 166 Interest Site.

The way it is a more flexible CountdownLatch/CyclicBarrier is because it is able to not only support an unknown number of parties (threads) but its also reusable (thats where the phase part comes in)

For each task you submit you would register, when that task is completed you arrive. This can be done recursively.

Phaser phaser = new Phaser();
ExecutorService e = //

Runnable recursiveRunnable = new Runnable(){
   public void run(){
      //do work recursively if you have to

      if(shouldBeRecursive){
           phaser.register();
           e.submit(recursiveRunnable);
      }

      phaser.arrive();
   }
}

public void doWork(){
   int phase = phaser.getPhase();

   phaser.register();
   e.submit(recursiveRunnable);

   phaser.awaitAdvance(phase);
}

Edit: Thanks @depthofreality for pointing out the race condition in my previous example. I am updating it so that executing thread only awaits advance of the current phase as it blocks for the recursive function to complete.

The phase number won’t trip until the number of arrives == registers. Since prior to each recursive call invokes register a phase increment will happen when all invocations are complete.

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