1.1. CAS自旋锁

unsafe类的compareAndSwap()方法,实现比较并交换

1.1.1. CAS可能造成的问题

• 进入死循环，导致cpu占用过高
• 引发ABA问题

1.1.2. 自己实现自旋锁

尝试获取锁的线程不会立即阻塞，而是采用循环的方式去尝试获取锁。好处：减少线程上下文切换的消耗，缺点：会耗CPU

public class SpinLockDemo {

    /**
     * 原子引用线程
     */
    AtomicReference<Thread> atomicReference = new AtomicReference();

    public void myLock() {
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName() + "come in");
        while (!atomicReference.compareAndSet(null, thread)) {
            System.out.println("尝试");
        }
    }

    public void myUnlock() {
        Thread thread = Thread.currentThread();
        atomicReference.compareAndSet(thread, null);
        System.out.println(Thread.currentThread().getName() + "invoked my unlock");
    }

    public static void main(String[] args) {
        SpinLockDemo spinLockDemo = new SpinLockDemo();
        new Thread(() -> {
            spinLockDemo.myLock();
            try {
                TimeUnit.SECONDS.sleep(5);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            spinLockDemo.myUnlock();
        }, "AA").start();
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        new Thread(() -> {
            spinLockDemo.myLock();
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            spinLockDemo.myUnlock();
        }, "BB").start();
    }
}

1.2. CAS引发的ABA问题

/**
 * 解决ABA问题（原子引用？版本号？）
 */
public class ABADemo {

    /**
     * 原子引用
     */
    static AtomicReference<Integer> atomicReference = new AtomicReference<>(100);


    static AtomicStampedReference atomicStampedReference = new AtomicStampedReference(100, 1);

    public static void main(String[] args) {
        System.out.println("=========================产生ABA问题===========================");
        new Thread(() -> {
            atomicReference.compareAndSet(100, 101);
            atomicReference.compareAndSet(101, 100);
        }, "t1").start();

        new Thread(() -> {
            //保证t1线程完成一次ABA操作
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(atomicReference.compareAndSet(100, 2019) + ":" + atomicReference.get());
        }, "t2").start();

        System.out.println("=========================解决ABA问题===========================");

        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + "初始版本号" + stamp);

            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            atomicStampedReference.compareAndSet(100, 101, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + "2次版本号" + atomicStampedReference.getStamp());

            atomicStampedReference.compareAndSet(101, 100, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1);
            System.out.println(Thread.currentThread().getName() + "3次版本号" + atomicStampedReference.getStamp());
        }, "t3").start();

        new Thread(() -> {
            //等待t3拿到相同的版本号
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + "初始版本号" + stamp);
            try {
                Thread.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean result = atomicStampedReference.compareAndSet(100, 2019, stamp, stamp + 1);
            System.out.println(Thread.currentThread().getName() + ":" + result + ":" + atomicStampedReference.getStamp());
            System.out.println("最新值：" + atomicStampedReference.getReference());
        }, "t4").start();
    }
}