Atomic variables were introduced in Java Version 5 to provide atomic operations on single variables. When you work with a normal variable, each operation that you implement in Java is transformed in several instructions that is understandable by the machine when you compile the program. For example, when you assign a value to a variable, you only use one instruction in Java, but when you compile this program, this instruction is transformed in various instructions in the JVM language. This fact can provide data inconsistency errors when you work with multiple threads that share a variable.
To avoid these problems, Java introduced the atomic variables. When a thread is doing an operation with an atomic variable, if other threads want to do an operation with the same variable, the implementation of the class includes a mechanism to check that the operation is done in one step. Basically,
1) The operation gets the value of the variable, changes the value in a local variable,
2) Then tries to change the old value for the new one.
3) If the old value is still the same, it does the change. If not, the method begins the operation again. This operation is called Compare and Set.
Atomic variables don't use locks or other synchronization mechanisms to protect the access to their values. All their operations are based on the Compare and Set operation. It's guaranteed that several threads can work with an atomic variable at a time without generating data inconsistency errors and its performance is better than using a normal variable protected by a synchronization mechanism.
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