Volatile boolean vs AtomicBoolean

Question

What does AtomicBoolean do that a volatile boolean cannot achieve?

Answer 1

If you have only one thread modifying your boolean, you can use a volatile boolean (usually you do this to define a stop variable checked in the thread's main loop).

However, if you have multiple threads modifying the boolean, you should use an AtomicBoolean. Else, the following code is not safe:

boolean r = !myVolatileBoolean;

This operation is done in two steps:

1. The boolean value is read.
2. The boolean value is written.

If an other thread modify the value between #1 and 2#, you might got a wrong result. AtomicBoolean methods avoid this problem by doing steps #1 and #2 atomically.

Answer 2

volatile keyword guarantees happens-before relationship among threads sharing that variable. It doesn't guarantee you that 2 or more threads won't interrupt each other while accessing that boolean variable.

Answer 3

Volatile boolean vs AtomicBoolean

The Atomic* classes wrap a volatile primitive of the same type. From the source:

public class AtomicLong extends Number implements java.io.Serializable {
   ...
   private volatile long value;
   ...
   public final long get() {
       return value;
   }
   ...
   public final void set(long newValue) {
       value = newValue;
   }

So if all you are doing is getting and setting a Atomic* then you might as well just have a volatile field instead.

What does AtomicBoolean do that a volatile boolean cannot achieve?

What the Atomic* classes give you however, are methods that provide more advanced functionality such as incrementAndGet(), compareAndSet(), and others that implement multiple operations (get/increment/set, test/set) without locking. That's why the Atomic* classes are so powerful.

For example, if multiple threads are using the following code using ++, there will be race conditions because ++ is actually: get, increment, and set.

private volatile value;
...
// race conditions here
value++;

However, the following code will work in a multi-threaded environment safely:

private final AtomicLong value = new AtomicLong();
...
value.incrementAndGet();

It's also important to note that wrapping your volatile field using Atomic* class is a good way to encapsulate the critical shared resource from an object standpoint. This means that developers can't just deal with the field assuming it is not shared possibly injecting problems with a field++; or other code that introducing race conditions.

Answer 4

AtomicBoolean has methods that perform their compound operations atomically and without having to use a synchronized block. On the other hand, volatile boolean can only perform compound operations if done so within a synchronized block.

The memory effects of reading/writing to volatile boolean are identical to the get and set methods of AtomicBoolean respectively.

For example the compareAndSet method will atomically perform the following (without a synchronized block):

if (value == expectedValue) {
    value = newValue;
    return true;
} else {
    return false;
}

Hence, the compareAndSet method will let you write code that is guaranteed to execute only once, even when called from multiple threads. For example:

final AtomicBoolean isJobDone = new AtomicBoolean(false);

...

if (isJobDone.compareAndSet(false, true)) {
    listener.notifyJobDone();
}

Is guaranteed to only notify the listener once (assuming no other thread sets the AtomicBoolean back to false again after it being set to true).