Happens-Before relation in Java Memory Model

Question

Regarding JLS ch17 Threads and Locks, it says \"if one action happens-before another, then the first is visible to and ordered before the second\"; I wonder:

(1) Wha

Answer 1

What does it really mean by saying "ordered before"? Because even if action_a happens-before action_b, action_a can be executed after action_b in some implementation, right?

A Happens before relationship creates a memory barrier which prevents the execution of action-b before action-a. Thus some underlying JVM optimizations cannot be applied. So, NO action-a cannot be executed after or along with action-b.

If action_a happens-before action_b, does it mean action_a MUST NOT see action_b? Or action_a may see or may not see action_b?

It means action-b must see all the changes brought about by action-a.

If action_a does NOT happen-before action_b, and action_b does NOT happen-before action_a, does it mean action_a may see or may not see action_b?

Happens-before is a transitive relationship. So, if action-a happens before action-b which happens before action-c ... so on upto action-y, and action-y happens before action-z, then action-a happens before action-z.

A happens before relationship ensures that the actions which follow the current action, will see the changes made by the current action. If the changes are not seen, then a happens before doesn't exist.

There could not be any cyclic happens-before, right?

Right, If action-a happens before action-b,action-c, action-d, then none among b,c,d can happen before action-a.

Edit :

The JLS says It should be noted that the presence of a happens-before relationship between two actions does not necessarily imply that they have to take place in that order in an implementation. If the reordering produces results consistent with a legal execution, it is not illegal.. So, if action-a has a happens before relationship with action-b, then action-b can execute first provided the final is equivalent to the sate if action a had executed before action b. This is implementation specific. The JIT might decide to run action-b earlier than action a provided this change in order does not affect the final result.

2. Well, action-a is independent of action-b. atleast theoretically :)

3. Happens before specifies, sequential actions. If the actions are parallel, then a happens before doesn't exist.

Note : All this confusion is because of the removal of happens before by the the JIT if there is no dependency between two actions. Please read about Escape analysis.