When understanding and looking at the source code of AbstractQueuedSynchronizer, compareAndSetHead and compareAndSetTail often appear. The bottom layer of these two methods calls unsafe.compareAndSwapObject ( this , value Offset , expect, update);
This method mainly uses the CAS instruction of the CPU, that is, the CAS algorithm.
The main logic of the method is
if (this == expect) {
this = update
return true;
} else {
return false;
}
In this way we can understand CAS as:
CAS has 3 parameter values, the memory value V(this) , the old value A(expect) and the new value B(update) . Modify memory value V to B if and only if expected value A and memory value V are the same , otherwise do nothing.