Why Lock condition await must hold the lock

Question

I am in doubt with that , in Java language, we need to acquire the lock, before we await some condition to be satisfied.

For example, int java monitor lock:

Answer 1

a sounds-reasonable answer

It is a JVM thing. An Object x has:

• an Entry Set: a queue for threads attempting to synchronized(x)

• a Waiting Set: a queue for threads called x.wait()

When you call x.wait(), JVM adds your current thread into Waiting Set; when you call x.notify()/x.notifyAll(), JVM removes one/all element from Waiting Set.

Multiple threads may call x.wait()/x.notify()/x.notifyAll() to modify the Waiting Set. In order to ensure the Waiting Set thread safety, JVM accepts only one operation from one thread at one time.