Java lock and happend-before relation

Question

I'm not sure if I'm interpreting the javadoc right. When using a ReentrantLock after calling the lock method and successfully gaining a lock, can you just access any object without any synchronized blocks and the happend-before relationship is magically enforced?

I don't see any connection between the ReentrantLock and the objects I'm working on, that's why it is hard to believe I can work on them safely. But this is the case, or am I reading the javadoc wrong?

score 4 · Accepted Answer · answered Oct 29 '11 at 16:59

If thread A has modified some object inside a code block CB1 guarded by the lock and then releases the lock, and thread B enters in a code block guarded by the same lock, then thread B will see the modifications done by thread A in the code block CB1.

If two threads read and write the same shared state, then every read and write to this state should be guarded by the same lock.

score 2 · Answer 2 · answered Oct 29 '11 at 16:58

2

It's ... a (mutex) lock:

void myMethod()
{

    myLock.lock();  // block until condition holds
    try 
    {
         // Do stuff that only one thread at a time should do
     } 
     finally 
     {
         myLock.unlock()
     }
}

Only one thread can hold the lock at a time, so anything between the lock() and unlock() calls is guaranteed to only be executed by one thread at a time.

The relevant Oracle tutorial can be found here.

answered Oct 29 '11 at 16:58

Brian Roach

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Yeah thanks, I understand that they are executed exclusively but my question was aiming at if the same state will be seen from different threads. – Franz Kafka Oct 29 '11 at 17:13
1

What you're talking about is how java caches variables in each thread. Use of the lock (or use of the `synchronized` keyword) guarantees a synchronization. [This SO question](http://stackoverflow.com/questions/5022100/when-does-java-thread-cache-refresh-happens) I think answers your question more fully. Specifically section 17.1 of the JLS – Brian Roach Oct 29 '11 at 19:38

score 2 · Answer 3 · answered Oct 29 '11 at 16:59

There's no magic in it. You're safe if, and only if, all threads accessing an object use the same lock - be it a ReentrantLock or any other mutex, such as a synchronized block.

The existence ReentrantLock is justified by that it provides more flexibility than synchronized: you can, for example, just try to acquire the lock - not possible with synchronized.

Java lock and happend-before relation

3 Answers3