Waiting on multiple threads to complete in Java

java multithreading synchronization
A Jackson picture A Jackson · Sep 1, 2009 · Viewed 63.2k times · Source

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

Jon Skeet picture Jon Skeet · Sep 1, 2009

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...)

Related questions

How do synchronized static methods work in Java and can I use it for loading Hibernate entities?

If I have a util class with static methods that will call Hibernate functions to accomplish basic data access. I am wondering if making the method synchronized is the right approach to ensure thread-safety. I want this to prevent access …

Read More
C# version of java's synchronized keyword?

Does c# have its own version of the java "synchronized" keyword? I.e. in java it can be specified either to a function, an object or a block of code, like so: public synchronized void doImportantStuff() { // dangerous code goes here. } …

Read More
How do I make my ArrayList Thread-Safe? Another approach to problem in Java?

I have an ArrayList that I want to use to hold RaceCar objects that extend the Thread class as soon as they are finished executing. A class, called Race, handles this ArrayList using a callback method that the RaceCar object …

Read More