What's Java's equivalent of .Net's Interlocked class?

Question

How do I modify an int atomically and thread-safely in Java?

Atomically increment, test & set, etc...?

Answer 1

Use AtomicInteger.

Answer 2

Thread safety can be achieved via synchronized functions. Wrap your int (or such data) in a class which provides the required functionalities via synchronized methods, e.g.

public class X
{
  protected int x;
  public synchronized void set( int value )
  {
    x = value;
  }
}

You can also use classes from the java.util.concurrent.atomic package, e.g. AtomicInteger or AtomicIntegerArray

Why this answer won't work

I just wanted to be sure to point out exactly what is wrong with this answer, in case anyone things that synchronized can be used to solve thread race effects

| Thread A      | Thread B         | 
|---------------|------------------|
| read x (x=4)  |                  |
|               | read x (x=4)     |
| Calculate 4+1 |                  |
| EAX ← 5       |                  |
|               | Calculate 4+1    |
|               | EAX ← 5          |
| Start sync    |                  |
| {             | Start sync       |
| { x ← 5       |    wait          |
| {             |    wait          |
| End sync      |    wait          |
|               | {                | 
|               | { x ← 5          |
|               | {                | 
|               | End sync         |

The end result of the operations:

x = 4;
x += 1;
x += 1;

is that x = 5 rather than 6.

The same issue exists with the volatile keyword. The volatile keyword doesn't save you from thread effects. The volatile keyword only ensures that

• caches are flushed before a variable is read
• caches are flushed after a value is written

Strictly speaking, volatile ensures that memory operations are not reordered around a volatile variable. Which means you still suffer from the:

• read from x
• write to x

problem.