AtomicBoolean vs Synchronized block, whats the dif

问题:

I am trying to understand the difference between the two following code blocks

AtomicBoolean ab = new AtomicBoolean(false);  

using the following to get and set state. .
ab.get();
ab.set(X);

vs. 

private boolean ab = false;
private final Object myboollock = new Ojbect();

public void setAB(boolean state)
{
    synchronized(myboollock)
     {
          ab = state;
     }
}

public boolean getAB()
{
 synchronized(myboollock)
 {
         return ab;
 }
}

I need to thread protect a boolean, that is all, and have in the past used the later method, but would like to start to use Atomic objects, (if ) they are safe?,

回答1:

There are a few subtle differences but seen from the outside the two code snippets behave similarly: if you call the set method, the change will be visible to other threads calling get subsequently.

The main differences are:

• performance: depending on the level of contention, you may get better performance with synchronized or AtomicBoolean
• atomicity: if at some stage you want to do more than just setting the boolean value, a synchronized block will allow you to add instructions atomically but AtomicBoolean won't

回答2:

If all you're trying to do is make getting and setting a single boolean value atomic, then yes - you can use AtomicBoolean instead without any synchronization.

Of course, synchronized allows a far wider range of uses, such as performing several actions within the block without losing the lock, or using it for wait/notify. So it's not like AtomicBoolean is a general alternative to synchronization - but in this case you can use it instead of synchronization.