I think you talking about ReentrantLock:
The ReentrantLock provides synchronization to methods while accessing
The code which manipulates the shared resource is surrounded by calls
to lock and unlock method.
This gives a lock to the current working thread and blocks all other
threads which are trying to take a lock on the shared resource.
ReentrantLock allow threads to enter into lock on a resource more than once. When the thread first enters into lock, a hold count is
set to one.
Before unlocking the thread can re-enter into lock again and every
time hold count is incremented by one.
For every unlock request, hold count is decremented by one and when
hold count is 0, the resource is unlocked.
Reentrant Locks also offer a fairness parameter, by which the lock
would abide by the order of the lock request i.e. after a thread
unlocks the resource, the lock would go to the thread which has been
waiting for the longest time. This fairness mode is set up by passing
true to the constructor of the lock.
These locks are used in the following way:
public void method()
//Do some work
The unlock statement is always called in the finally block to ensure
that the lock is released even if an exception is thrown in the method