Difference between Join and CountDownLatch in Java

Difference between Join and CountDownLatch?

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Difference 1:

Thread join method waits for other thread to finish before further executing current thread.
If t1.join is called in main thread, then main thread will wait at that point until t1 finishes its job.

CountDownLatch on the other end wait for its counter to reach 0 before executing current thread.

ExecutorService service = Executors.newFixedThreadPool(5);
final CountDownLatch latch = new CountDownLatch(3);

for(int i = 0; i < 5; i++) {
    service.submit(new Runnable() {
        public void run() {
            latch.countDown();
        }
    });
}

latch.await();

When latch.countDown is called, associated counter will be decremented and as soon as it reaches 0, main thread which was blocked at line latch.await() proceeds further.


Note:
Thread join method wait for joined thread to finish the execution before the main thread on which join method is called to starts its execution. Whereas in CountDownLatch, latch.await doesn't wait for the thread that calls latch.countDown() to be finished, it proceeds once the counter value reaches 0 and it has no association with the state of the thread that calls countDown().

Difference 2:

We can call join method when we have control over the threads but while using ExecutorService we don't have control over individual threads instead we deal with just submitting the task to framework and it internally manages threads in this situation using CountDownLatch is right approach.

Example above in difference 1 can be used as Reference.


Usage:

Example 1:
CountDownLatch is useful in Multiplayer games, Lets say we have a online chess game that can only be played when two player joins, in this case we will initialize the CountDownLatch to 2 and starts the game only after 2 threads(player) joins(calls countDown()).

Example 2:
Lets say we have some Timer task that we want to start only after all the modules of the application get loaded or when all the services is up.
 

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Enjoy !!!! 

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