Java: wait & notify, await & signal, park & unpark

There are several way to stop a thread in Java, to get awaken later. Here are their usages:

wait & notify

Every object has a .wait() and .notify() method. These methods must be called in a synchronized block. 

When a .notify() happened before .wait(), it will not awake the thread.

await & signal 

With ReentrantLock, a condition object can be pulled from the lock, by `lock.newCondition()`. When the lock is locked, .await() These methods needs to be call when the lock is in lock state.

When a .signal() happened before .await(), it will not awake the thread.

LockSupport: park & unpark

Unlike wait & await, LockSupport.park() and LockSupport.unpark(t) and doesn't need to be in a locked / synchronized block. Since LockSupport is permit based, unpark assign a permit to a thread, which can be later used in park . So the order of park and unpark is not strict. Notice that permit doesn't have a counter - it can only be used in 1 park call.