How Java java.util.concurrent.locks.Lock guarantees memory sync between threads?

Question

Check out this java code snippet:

 Lock l = ReentrantLock();
 l.lock();
 try {
     counter++;
 } finally {
     l.unlock();
 }

This code guarantees that only one thread will execute the code in the try block in a particular time.

My question is: how the lock/unlock mechanism guarantees memory visibility between the threads? (what is the mechanism that will make sure that the counter result will be flushed into the main memory by Thread X, and will be loaded from the main memory by Thread Y that will come afterward?)

jn1kk · Accepted Answer · 2015-03-24 20:51:50Z

1

http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5

17.4.5. Happens-before Order

Two actions can be ordered by a happens-before relationship. If one action happens-before another, then the first is visible to and ordered before the second.

...

It follows from the above definitions that:

An unlock on a monitor happens-before every subsequent lock on that monitor.

...

In a happens-before consistent set of actions, each read sees a write that it is allowed to see by the happens-before ordering.

The seeing is implied to be seeing from a different thread.

edited Mar 24, 2015 at 20:51

answered Mar 24, 2015 at 20:46

jn1kk

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2 Comments

Shay Tsadok Over a year ago

Thanks for the reference, it explains there is happens-before relation between unlock and the next lock (which is reasonable), but it still does not explain why the data between the original lock to the unlock is the one to be synced to the next thread..

jn1kk Over a year ago

The reason why Thread B, may see a different value than Thread A is because of instruction reordering, among other things. This page explains why a Happens-Before relationship ensures memory consistency: docs.oracle.com/javase/tutorial/essential/concurrency/…

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