Question

I want to know whether creating a new thread in Java triggers a cache flush. Suppose I do something like this, in this sequence:

1. A thread runs and sets a variable X.
2. The thread creates a new thread.
3. The new thread accesses X.

My question is this: is the new thread, either at the time it is created or at the time it begins execution, guaranteed to see the update made to X by the old thread in step 1? I understand that if the old thread changes the value of X in the future, it is not guaranteed that the new thread will see these changes. That’s fine. I just want to know whether the new thread will see the right values when it starts without need for explicit synchronization.

When I first decided to look into this topic, I thought a simple google search would immediately reveal the answer, but for some reason, I can’t find any result that addresses this question.

Answer 1

Yes, it is.

In java, there is ‘happens-before’ relation that specifies what memory effects are visible between two actions. If “A happens-before B”, then action B is guaranteed to see all changes done by action A.

Starting a thread creates ‘happens-before’ relation between “thread.start()” call and all code that executes on new thread. New thread is therefore guaranteed to see memory effect of changing variable X on first thread.

For quick overview of happens-before relation, see Memory Visibility part of java.util.concurrent package overview. In your case, interesting bits are:

• Each action in a thread happens-before every action in that thread that comes later in the program’s order.
• A call to start on a thread happens-before any action in the started thread.

More links if you are curious:

• Java Memory Model is described in Chapter 17 of the Java Language Specification
• Java Memory Model FAQ
• Everything else about JMM