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

Question

I have a logging code which needs to be executed after all Threadss 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 and t2 are started.

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

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()