Java CAS Explained: How It Works

In Java, the compare and swap (CAS) statement is used to implement atomic operations, ensuring data integrity in a multi-threaded environment. CAS statements have several characteristics and usages.

1. Compare and swap: CAS operation involves two operands, one being the memory value to be operated on and the other being the expected value. If the memory value is equal to the expected value, the new value will replace the memory value; otherwise, no operation will be performed.
2. Atomicity: CAS is an atomic operation, so in a multi-threaded environment, using CAS can prevent issues of data races and concurrency conflicts.
3. Lock-free algorithm: CAS operation eliminates the need for locks, thus avoiding the performance overhead and thread blocking caused by using locks.
4. The ABA problem in CAS operations refers to the situation where a memory value is changed to a different value and then changed back to its original value during an operation, which CAS cannot detect. One solution to this issue is to use version numbers or flags to track changes in memory values.

The general steps of using a CAS statement are as follows:

1. To retrieve the memory value to be manipulated and the expected value.
2. Use the CAS operation to compare the memory value with the expected value, if they are equal, then replace the memory value with the new value, otherwise take no action.
3. Handle the corresponding actions based on the results of the CAS operation.

Here is a simple example code:

import java.util.concurrent.atomic.AtomicInteger;

public class CASExample {
    private static AtomicInteger counter = new AtomicInteger(0);
    
    public static void increment() {
        int oldValue, newValue;
        
        do {
            oldValue = counter.get();
            newValue = oldValue + 1;
        } while (!counter.compareAndSet(oldValue, newValue));
        
        System.out.println("Counter: " + counter.get());
    }
    
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                increment();
            }).start();
        }
    }
}

In the example above, the CAS operation is implemented using the compareAndSet method in the AtomicInteger class. Each thread calls the increment method, which atomically increases the counter using CAS operation and outputs the result. Due to the atomicity of the CAS operation, the final output results in increasing order.