multithreading - How to wait for completion of multiple tasks in Java?

Question

What is the proper way to implement concurrency in Java applications? I know about Threads and stuff, of course, I have been programming for Java for 10 years now, but haven't had too much experience with concurrency.

For example, I have to asynchronously load a few resources, and only after all have been loaded, can I proceed and do more work. Needless to say, there is no order how they will finish. How do I do this?

In JavaScript, I like using the jQuery.deferred infrastructure, to say

$.when(deferred1,deferred2,deferred3...)
 .done(
   function(){//here everything is done
    ...
   });

But what do I do in Java?

Answer 1

You can achieve it in multiple ways.

1.ExecutorService invokeAll() API

Executes the given tasks, returning a list of Futures holding their status and results when all complete.

2.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.

3.ForkJoinPool or newWorkStealingPool() in Executors is other way

Have a look at related SE questions:

How to wait for a thread that spawns it's own thread?

Executors: How to synchronously wait until all tasks have finished if tasks are created recursively?