Volatile boolean vs AtomicBoolean

Question

What does AtomicBoolean do that a volatile boolean cannot achieve?

Answer 1

You can't do compareAndSet, getAndSet as atomic operation with volatile boolean (unless of course you synchronize it).

Answer 2

Boolean primitive type is atomic for write and read operations, volatile guarantees the happens-before principle. So if you need a simple get() and set() then you don't need the AtomicBoolean.

On the other hand if you need to implement some check before setting the value of a variable, e.g. "if true then set to false", then you need to do this operation atomically as well, in this case use compareAndSet and other methods provided by AtomicBoolean, since if you try to implement this logic with volatile boolean you'll need some synchronization to be sure that the value has not changed between get and set.

Answer 3

I use volatile fields when said field is ONLY UPDATED by its owner thread and the value is only read by other threads, you can think of it as a publish/subscribe scenario where there are many observers but only one publisher. However if those observers must perform some logic based on the value of the field and then push back a new value then I go with Atomic* vars or locks or synchronized blocks, whatever suits me best. In many concurrent scenarios it boils down to get the value, compare it with another one and update if necessary, hence the compareAndSet and getAndSet methods present in the Atomic* classes.

Check the JavaDocs of the java.util.concurrent.atomic package for a list of Atomic classes and an excellent explanation of how they work (just learned that they are lock-free, so they have an advantage over locks or synchronized blocks)

Answer 4

Remember the IDIOM -

READ - MODIFY- WRITE this you can't achieve with volatile

Answer 5

If there are multiple threads accessing class level variable then each thread can keep copy of that variable in its threadlocal cache.

Making the variable volatile will prevent threads from keeping the copy of variable in threadlocal cache.

Atomic variables are different and they allow atomic modification of their values.

Answer 6

They are just totally different. Consider this example of a volatile integer:

volatile int i = 0;
void incIBy5() {
    i += 5;
}

If two threads call the function concurrently, i might be 5 afterwards, since the compiled code will be somewhat similar to this (except you cannot synchronize on int):

void incIBy5() {
    int temp;
    synchronized(i) { temp = i }
    synchronized(i) { i = temp + 5 }
}

If a variable is volatile, every atomic access to it is synchronized, but it is not always obvious what actually qualifies as an atomic access. With an Atomic* object, it is guaranteed that every method is "atomic".

Thus, if you use an AtomicInteger and getAndAdd(int delta), you can be sure that the result will be 10. In the same way, if two threads both negate a boolean variable concurrently, with an AtomicBoolean you can be sure it has the original value afterwards, with a volatile boolean, you can't.

So whenever you have more than one thread modifying a field, you need to make it atomic or use explicit synchronization.

The purpose of volatile is a different one. Consider this example

volatile boolean stop = false;
void loop() {
    while (!stop) { ... }
}
void stop() { stop = true; }

If you have a thread running loop() and another thread calling stop(), you might run into an infinite loop if you omit volatile, since the first thread might cache the value of stop. Here, the volatile serves as a hint to the compiler to be a bit more careful with optimizations.