How is the CAS implemented in Java?

1 year ago
Ava Mitchell
2 minutes

In Java, CAS (Compare and Swap) is implemented using the Unsafe class, which is a special class in Java that allows for low-level operations to bypass the language restrictions and directly manipulate memory.

CAS operation is atomic and involves three parameters: memory address, old expected value, and new value. The process of executing a CAS operation is as follows:

  1. Obtain the value of the current memory address.
  2. Check if the value at the current memory address is equal to the old expected value. If it is, proceed to step 4. If it is not, proceed to step 3.
  3. Cancel the operation and do nothing.
  4. Update the value of the memory address with a new value.

In Java, you can use the Atomic class to carry out CAS operations. The Atomic class is a set of atomic classes in Java that provide a series of atomic operation methods, including CAS operations. The following is an example using the AtomicInteger class.

import java.util.concurrent.atomic.AtomicInteger;

public class CASExample {
    private static AtomicInteger counter = new AtomicInteger(0);
    
    public static void main(String[] args) {
        int oldValue = counter.get();
        int newValue = oldValue + 1;
        
        while (!counter.compareAndSet(oldValue, newValue)) {
            oldValue = counter.get();
            newValue = oldValue + 1;
        }
        
        System.out.println("Counter: " + counter.get());
    }
}

In the above example, we first retrieve the current value of the counter. Next, we calculate a new value. Then, we use the compareAndSet method for CAS operation. If the CAS operation fails, we retrieve the current value again, calculate a new value, and try the CAS operation again until it succeeds.

CAS operation plays a crucial role in a multi-threaded environment as it ensures there are no conflicts when multiple threads try to modify the same memory address simultaneously.

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