How can I use the volatile keyword in Java correctly?

Question

Say I have two threads and an object. One thread assigns the object:

public void assign(MyObject o) {
    myObject = o;
}

Another thread uses t

Answer 1

I have spent quite a lot of time trying to understanding the volatile keyword. I think @aleroot has given the best and simplest example in the world.

This is in turn my explanation for dummies (like me :-)):

Scenario1: Assuming the stop is not declared as volatile then a given thread does and 'thinks' the following:

1. stopWork() is called: I have to set the stop to true
2. Great, I did it in my local stack now I have to update the main heap of JVM.
3. Oops, JVM tells me to give a way in CPU to another thread, I have to stop for a while...
4. OK, I am back. Now I can update the main heap with my value. Updating ...

Scenario2: Now let the stop be declared as volatile:

1. stopWork() is called: I have to set the stop to true
2. Great, I did it in my local stack now I have to update the main heap of JVM.
3. Sorry guys, I have to do (2) NOW - I am told it is volatile. I have to occupy CPU a bit longer...
4. Updating the main heap ...
5. OK, I am done. Now I can yield.

No synchronization, just a simple idea...

Why not to declare all variables volatile just in case? Because of Scenario2/Step3. It is a bit inefficient but still better than regular synchronization.