1. CAS

compare and swap，比较并交换。

包含三个操作数：内存位置、预期原值及更新值。

执行CAS操作的时候，将内存位置的值与预期原值比较：

• 匹配，更新为新值。
• 不匹配，不进行操作，多个线程同时执行CAS操作只有一个会成功。

CAS是JDK提供的非阻塞原子性操作，通过硬件保证比较-更新的原子性。底层实现为CPU指令cmpxchg，效率更高。通过unsafe类实现。

2. 自旋

线程修改失败重试的过程叫自旋。

public static void main(String[] args) {
        AtomicInteger integer = new AtomicInteger(5);
        System.out.println(integer.compareAndSet(51, 200) + " " + integer.get());
    }

自旋的实现代码

自己实现一个CAS。

public class SingletonLock {
    AtomicReference<Thread> atomicThread = new AtomicReference<>();
    public void Lock() {
        Thread thread = Thread.currentThread();
        System.out.println(thread.getName() + "Lock");
        // 当前不为空, 需要进行自旋
        while (!atomicThread.compareAndSet(null, thread)) {
        }
    }
    public void unLock() {
        Thread thread = Thread.currentThread();
        System.out.println(thread.getName() + "unLock");
        atomicThread.compareAndSet(thread, null);
    }
    public static void main(String[] args) {
        SingletonLock singletonLock = new SingletonLock();

        new Thread(()->{
            singletonLock.Lock();
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            singletonLock.unLock();
        }, "t1").start();

        try {
            TimeUnit.MILLISECONDS.sleep(200);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        new Thread(()->{
            singletonLock.Lock();
            singletonLock.unLock();
        }, "t2").start();
    }
}

3. AtomicReference包装类

class Stu {
    String name;
    Integer age;
    public Stu(String name, Integer age) {
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return name + ": " + age;
    }
}

public class demo03 {
    public static void main(String[] args) {
        Stu zs = new Stu("zs", 18);
        Stu ls = new Stu("ls", 29);

        AtomicReference<Stu> atomicReference = new AtomicReference<>();
        atomicReference.set(zs);
        System.out.println(atomicReference.compareAndSet(zs, ls) + " " + atomicReference.get().toString());
    }
}

4. CAS缺点

4.1 循环时间长，开销大

4.2 ABA问题

解决方案：版本号，戳记流水。

class Stus {
    Integer id;
    String name;
    public Stus(Integer id, String name) {
        this.id = id;
        this.name = name;
    }
    public String getName() {
        return name;
    }
}
public class demo04 {
    public static void main(String[] args) {
        Stus zs = new Stus(1, "zs");
        Stus ls = new Stus(2, "ls");
        AtomicStampedReference<Stus> atomicStampedReference = new AtomicStampedReference<>(zs, 1);
        atomicStampedReference.compareAndSet(zs, ls, atomicStampedReference.getStamp(), 2);
        System.out.println(atomicStampedReference.getReference().getName());
        atomicStampedReference.compareAndSet(ls,zs, atomicStampedReference.getStamp(), 3);
        System.out.println(atomicStampedReference.getReference().getName());
    }
}