multithreading - Does synchronized keyword prevent reordering in Java?

Question

Suppose I have the following code in Java

a = 5;
synchronized(lock){
    b = 5;
}
c = 5;

Does synchronized prevent reordering? There is no dependency between a, b and c. Would assignment to a first happen then to b and then to c? If I did not have synchronized, the statements can be reordered in any way the JVM chooses right?

Answer 1

Locking the assignment to b will, at the very least, introduce an acquire-fence before the assignment, and a release-fence after the assignment.

This prevents instructions after the acquire-fence to be moved above the fence, and instructions before the release-fence to be moved below the fence.

Using the ↓↑ notation:

a = 5;
↓ 
b = 5;
↑
c = 5;

The ↓ prevents instructions from being moved above it. The ↑ prevents instructions from being moved below it.