If more than one thread can access a field should it be marked as volatile?

Question

Reading a few threads (common concurrency problems, volatile keyword, memory model) I\'m confused about concurrency issues in Java.

I have a lot of fields that are acces

Answer 1

If you have to ask, use locks. volatile can be useful in some cases, but it's very, very difficult to get right. For example:

class Foo {
  private volatile int counter = 0;
  int Increment() {
    counter++;
    return counter;
  }
}

If two threads run Increment() at the same time, it's possible for the result to be counter = 1. This is because the computer will first retrieve counter, add one, then save it back. Volatile just forces the save and load to occur in a specific order relative to other statements.

Note that synchronized usually obviates the need for volatile - if all accesses to a given field are protected by the same monitor, volatile will never be needed.

Using volatile to make lockless algorithms is very, very difficult; stick to synchronized unless you have hard evidence that it's too slow already, and have done detailed analysis on the algorithm you plan to implement.