How to wait for a ThreadPoolExecutor to finish

Question

My Question: How to execute a bunch of threaded objects on a ThreadPoolExecutor and wait for them all to finish before moving on?

I\'m new to Thread

Answer 1

Another approach is to use CompletionService, very useful if you have to attempt any task result:

//run 3 task at time
final int numParallelThreads = 3;

//I used newFixedThreadPool for convenience but if you need you can use ThreadPoolExecutor
ExecutorService executor = Executors.newFixedThreadPool(numParallelThreads);
CompletionService completionService = new ExecutorCompletionService(executor);

int numTaskToStart = 15;

for(int i=0; i (or something you need)
    MyTask mt = new MyTask();

    completionService.submit(mt);
}

executor.shutdown(); //it cannot be queued more task

try {
    for (int t = 0; t < numTaskToStart ; t++) {
        Future f = completionService.take();
        String result = f.get();
        // ... something to do ...
    }
} catch (InterruptedException e) {
    //termination of all started tasks (it returns all not started tasks in queue)
    executor.shutdownNow();
} catch (ExecutionException e) {
    // ... something to catch ...
}