How to know if other threads have finished?

Question

I have an object with a method named StartDownload(), that starts three threads.

How do I get a notification when each thread has finished executing?

Answer 1

You should really prefer a solution that uses java.util.concurrent. Find and read Josh Bloch and/or Brian Goetz on the topic.

If you are not using java.util.concurrent.* and are taking responsibility for using Threads directly, then you should probably use join() to know when a thread is done. Here is a super simple Callback mechanism. First extend the Runnable interface to have a callback:

public interface CallbackRunnable extends Runnable {
    public void callback();
}

Then make an Executor that will execute your runnable and call you back when it is done.

public class CallbackExecutor implements Executor {

    @Override
    public void execute(final Runnable r) {
        final Thread runner = new Thread(r);
        runner.start();
        if ( r instanceof CallbackRunnable ) {
            // create a thread to perform the callback
            Thread callerbacker = new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        // block until the running thread is done
                        runner.join();
                        ((CallbackRunnable)r).callback();
                    }
                    catch ( InterruptedException e ) {
                        // someone doesn't want us running. ok, maybe we give up.
                    }
                }
            });
            callerbacker.start();
        }
    }

}

The other sort-of obvious thing to add to your CallbackRunnable interface is a means to handle any exceptions, so maybe put a public void uncaughtException(Throwable e); line in there and in your executor, install a Thread.UncaughtExceptionHandler to send you to that interface method.

But doing all that really starts to smell like java.util.concurrent.Callable. You should really look at using java.util.concurrent if your project permits it.