Is a write to a volatile a memory-barrier in Java

Question

I recently heard in a talk that a write to a volatile triggers a memory-barrier for each variable that the thread has written to. Is that really correct? From the JLS, it seems that only the variable concerned gets flushed out, but not others. Does anybody know what is actually correct? Can one point me a concrete location in the JLS?

Answer 1

Yes, it will initiate a barrier. You can read more here. There are 4 types, LoadLoad LoadStore StoreStore StoreLoad.

As far as your question

From the JLS, it seems that only the variable concerned gets flushed out, but not others. Does anybody know what is actually correct?

All writes that occur before a volatile store are visible by any other threads with the predicate that the other threads load this new store. However, writes that occur before a volatile load may or may not be seen by other threads if they do not load the new value.

For a practical example

volatile int a =0; int b = 0;  Thread-1 b = 10; a = 3;  Thread-2 if(a == 0){   // b can b 10 or 0 }  if(a == 3){    // b is guaranteed to be 10 (according to the JMM) }