how to execute a piece of code only after all threads are done

Question

Hi,

I have a logging code which needs to be executed after all threads are executed.

Thread t1 = new MyThread();
Thread t2 = new MyThread();
t1.run();
t2.run();

doLogging();

Is there any way to execute doLogging() only after both threads are done with their processing. Now that doLogging() is called as soon as t1, t2 are started.

Thanks, Shafi

Answer 1

Just join() all threads before your doLogging() call:

t1.join();
t2.join();

// the following line will be executed when both threads are done
doLogging();

Note that the order of join() calls doesn't matter if you want to wait for all of your threads.

Answer 2

If you are running the code from the main thread, bear in mind that it will cause the UI to hang until both threads complete. This may not be the desired effect.

Instead, you could consider a construction where each thread synchronizes over your Logger object and perform calls to it within that construct. Since I don't know what you are specifically trying to accomplish, the other solution is what Joachim suggested, only placing that code within a thread.

Answer 3

In addition to the join() solution there is also something called CountDownLatch in the java.util.concurrent library. It allows you to initialize it to a certain number and then wait until it was hit the specified number of times.

Simple example:

CountDownLatch latch = new CountDownLatch(NUMBER_OF_THREADS);
for(int i=0; i<NUMBER_OF_THREADS;i++)
   new Thread(myCode).start();

latch.await();

The latch must be explicitly hit by the worker threads for this to work though:

latch.countDown()