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Synchronization vs Lock

java.util.concurrent API provides a class called as Lock, which would basically serialize the control in order to access the critical resource. It gives method such as park() and unpark().

We can do similar things if we can use synchronized keyword and using wait() and notify() notifyAll() methods.

I am wondering which one of these is better in practice and why?

Answer*

There are 4 main factors into why you would want to use `synchronized` or `java.util.concurrent.Lock`.

Note: Synchronized locking is what I mean when I say intrinsic locking.

1. When Java 5 came out with
    ReentrantLocks, they proved to have
    quite a noticeble throughput
    difference then intrinsic locking. 
    If youre looking for faster locking
    mechanism and are running 1.5
    consider j.u.c.ReentrantLock.  Java
    6's intrinsic locking is now
    comparable.
    
 2. j.u.c.Lock has different mechanisms
        for locking. Lock interruptable -
        attempt to lock until the locking
        thread is interrupted; timed lock -
        attempt to lock for a certain amount
        of time and give up if you do not
        succeed; tryLock - attempt to lock,
        if some other thread is holding the
        lock give up.  This all is included
        aside from the simple lock. 
        Intrinsic locking only offers simple
        locking
 3. Style.  If both 1 and 2 do not fall
        into categories of what you are
        concerned with most people,
        including myself, would find the
        intrinsic locking semenatics easier
        to read and less verbose then
        j.u.c.Lock locking.
 4. Multiple Conditions.  An object you
    lock on can only be notified and
    waited for a single case.  Lock's
    newCondition method allows for a
    single Lock to have mutliple reasons
    to await or signal.  I have yet to
    actually need this functionality in
    practice, but is a nice feature for
    those who need it.

Edit Summary*

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I liked the details on your comment. I would add one more bullet point - the ReadWriteLock provides useful behavior if you are dealing with several threads, only some of which need to write to the object. Multiple threads can be concurrently reading the object and are only blocked if another thread is already writing to it.
– Sam Goldberg
Commented Jul 3, 2017 at 14:24
To add to the 4th point - In j.u.c.ArrayBlockingQueue, the lock has 2 reasons to await: queue non-empty and queue non-full. For this reason, j.u.c.ArrayBlockingQueue uses the explicit lock and lock.newCondition().
– yiksanchan
Commented Nov 30, 2020 at 16:52

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Synchronization vs Lock

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