Question

In Java, do ReentrantLock.lock() and ReetrantLock.unlock() use the same locking mechanism as synchronized()?

My guess is “No,” but I’m hoping to be wrong.

Example:

Imagine that Thread 1 and Thread 2 both have access to:

ReentrantLock lock = new ReentrantLock();

Thread 1 runs:

synchronized (lock) {
    // blah
}

Thread 2 runs:

lock.lock();
try {
    // blah
}
finally {
    lock.unlock();
}

Assume Thread 1 reaches its part first, then Thread 2 before Thread 1 is finished: will Thread 2 wait for Thread 1 to leave the synchronized() block, or will it go ahead and run?

Answer 1

No, Thread 2 can lock() even when Thread 1 is synchronized on the same lock. This is what the documentation has to say:

Note that Lock instances are just
normal objects and can themselves be
used as the target in a synchronized
statement. Acquiring the monitor lock
of a Lock instance has no specified
relationship with invoking any of the
lock() methods of that instance. It
is recommended that to avoid confusion
you never use Lock instances in this
way, except within their own
implementation.