Why aren't variables in Java volatile by default?

Question

Possibly similar question:

Do you ever use the volatile keyword in Java?

---

Today I was debugging my game; It had a very difficult threading problem that would show up every few minutes, but was difficult to reproduce. So first I added the synchronized keyword to each of my methods. That didn't work. Then I added the volatile keyword to every field. The problem seemed to just fix itself.

After some experimentation I found that the field responsible was a GameState object which kept track of my game's current state, which can be either playing or busy. When busy, the game ignores user input. What I had was a thread that constantly changed the state variable, while the Event thread reads the state variable. However, after one thread changes the variable, it takes several seconds for the other thread to recognize the changes, which ultimately causes the problem.

It was fixed by making the state variable volatile.

Why aren't variables in Java volatile by default and what's a reason not to use the volatile keyword?

Answer 1

To make a long story short, volatile variables--be they in Java or C#--are never cached locally within the thread. This doesn't have much of an implication unless you're dealing with a multiprocessor/multicore CPU with threads executing on different cores, as they'd be looking at the same cache. When you declare a variable as volatile, all reads and writes come straight from and go straight to the actual main memory location; there's no cache involved. This has implications when it comes to optimization, and to do so unnecessarily (when most variables don't need to be volatile) would be inflicting a performance penalty (paltry as it may or may not be) for a relatively small gain.