Waiting on multiple threads to complete in Java

Question

During the course of my program execution, a number of threads are started. The amount of threads varies depending on user defined settings, but they are all executing the same method with different variables.

In some situations, a clean up is required mid execution, part of this is stopping all the threads, I don't want them to stop immediately though, I just set a variable that they check for that terminates them. The problem is that it can be up to 1/2 second before the thread stops. However, I need to be sure that all threads have stopped before the clean up can continues. The cleanup is executed from another thread so technically I need this thread to wait for the other threads to finish.

I have thought of several ways of doing this, but they all seem to be overly complex. I was hoping there would be some method that can wait for a group of threads to complete. Does anything like this exist?

Thanks.

Answer 1

Just join them one by one:

for (Thread thread : threads) {   thread.join(); } 

(You'll need to do something with InterruptedException, and you may well want to provide a time-out in case things go wrong, but that's the basic idea...)

Answer 2

If you are using java 1.5 or higher, you can try CyclicBarrier. You can pass the cleanup operation as its constructor parameter, and just call barrier.await() on all threads when there is a need for cleanup.