Java CAS原理和用法总结

if (V == A) {
V = B;
return true;
} else {
return false;
}

import java.util.concurrent.atomic.AtomicInteger;
public class CASDemo {
    public static void main(String[] args) throws InterruptedException {
        AtomicInteger atomicInt = new AtomicInteger(0);
        Runnable task = () -> {
            for (int i = 0; i < 1000; i++) {
                // 内部通过循环CAS操作实现原子递增
                atomicInt.incrementAndGet();
            }
        };
        Thread thread1 = new Thread(task);
        Thread thread2 = new Thread(task);
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        // 结果总是 2000，保证了原子性
        System.out.println("Final Count: " + atomicInt.get()); 
    }
}

AtomicInteger atomicInt = new AtomicInteger(0);
int oldValue, newValue;
do {
    oldValue = atomicInt.get(); // 获取当前值作为预期值
    newValue = oldValue + 1;    // 计算新值
} while (!atomicInt.compareAndSet(oldValue, newValue)); 
// 如果CAS失败（oldValue已不是当前值），则循环重试

import java.util.concurrent.atomic.AtomicStampedReference;
public class ABADemo {
    // 初始值为 100，版本号（Stamp）为 0
    static AtomicStampedReference<Integer> atomicStampedRef = 
            new AtomicStampedReference<>(100, 0);
    public static void main(String[] args) throws InterruptedException {
        int initialStamp = atomicStampedRef.getStamp(); // 获取初始版本号
        // 线程1模拟ABA操作
        Thread thread1 = new Thread(() -> {
            // 先改成 101，版本号+1
            atomicStampedRef.compareAndSet(100, 101, initialStamp, initialStamp + 1);
            // 再改回 100，版本号再+1
            atomicStampedRef.compareAndSet(101, 100, initialStamp + 1, initialStamp + 2);
        });
        // 线程2尝试修改
        Thread thread2 = new Thread(() -> {
            // 先睡一会儿，确保线程1完成了ABA操作
            try { Thread.sleep(1000); } catch (InterruptedException e) {}
            // 尝试修改。虽然期望值还是100，但版本号已经从0变成了2，所以CAS会失败！
            boolean success = atomicStampedRef.compareAndSet(
                    100, 
                    202, 
                    initialStamp, // 传入旧的版本号0
                    initialStamp + 1
            );
            System.out.println("CAS successful? " + success); // 输出：false
        });
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
    }
}