Question

If I use synchronize(this) in two methods and one calls the other, will I get stuck in a deadlock situation or will it work because the thread already owns the lock?

Picture the class below:

public class Test {
  public void foo() {
    synchronize(this) {
      bar();
    }
  }

  public void bar() {
    synchronize(this) {
      // do something
    }
  }
}

As you can see, there are two methods foo and bar, which both rely on synchronization.

When calling foo(), a lock will be obtained on (this); will bar try to do the same when called by foo (and thus causing a deadlock) or will it realize that the lock has already been obtained by the same thread?

Hope my explanation is more or less clear 😉

Answer 1

The synchronized block is reentrant (in fact, Java monitors are reentrant, to be perfectly clear), thus no deadlock can happen in your situation.

According to the docs:

Recall that a thread cannot acquire a lock owned by another thread.
But a thread can acquire a lock that it already owns.