Java equivalent to Thread.MemoryBarrier

Question

In Java, how can I explicitly trigger a full memory fence/barrier, equal to the invocation of

System.Threading.Thread.MemoryBarrier();

in C#?

I know that since Java 5 reads and writes to volatile variables have been causing a full memory fence, but maybe there is an (efficient) way without volatile .

Answer 1

Compared to MemoryBarrier() , Java's happens-before is a much sharper tool, leaving more leeway for aggressive optimization while maintaining thread safety.

A sharper tool, as you would expect, also requires more care to use properly, and that is how the semantics of volatile variable access could be described. You must write to a volatile variable on the write site and read from the same volatile on each reading site. By implication you can have any number of independent, localized "memory barriers", one per a volatile variable, and each guards only the state reachable from that variable.

The full idiom is usually referred to as "safe publication" (although this is a more general term) and implies populating an immutable object graph which will be shared between threads, then writing a reference to it to a volatile variable.

Answer 2

Java 8, via JEP 108 added another possibility. Access to three fences have been to the Java API, fullFence, loadFence and storeFence.

Answer 3

There are no direct equivalent. Use volatile field or more high level things.