How does thread.join() work conceptually?

Question

From the docs:

The join method allows one thread to wait for the completion of another. If t is a Thread object whose thread is currently executing,

t.join();

causes the current thread to pause execution until t's thread terminates

What I can't get my head around is that this is a method on a thread that's different to one it's being called from. So if a thread t1 calls another thread's t2.join() , t2 knows nothing about t1 . So what's actually happening under the hood to make t1 wait for t2 for finish?

Answer 1

By looking at the Java source code:

calling t2.join() from t1 will make t1 wait on t2 object (t2 is a Thread , which is a subclass of Object ). The wait will be forever as long as t1 is alive. When t2 thread finishes its work, it will call Object.notifyAll() so t1 awakens.

Answer 2

The classic implementation of Thread.join (other implementations are possible, is to lock the Thread object, test to see if is alive and if not wait on the Thread object. As a thread exits, it locks its instance and calls notifyAll .

The choice of a public object as the lock is unfortunate.