CAS . . . .

---

一、原子类

java.util.concurrent.atomic 

---

二、没有CAS之前

---

三、使用CAS之后

---

四、CAS是什么？

1. 说明与原理

---

2.硬件基本保证

3.源码分析compareAndSet(int expect, int update)

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }

---

五、CAS底层原理？如何知道，谈谈你对UnSafe的理解

1.UnSafe

2.我们知道i++线程不安全的，那atomicInteger.getAndIncrement()

3.源码分析

---

六、原子引用

---

七、CAS与自旋锁，借鉴CAS思想

1.自旋锁是什么

2.自己实现一个自旋锁SpinLockDemo

/**
 * 题目：实现一个自旋锁，复习CAS思想
 * 自旋锁好处：循环比较获取没有类似wait的阻塞
 * 
 * 通过CAS操作完成自旋锁，A线程先进来调用lock方法，自己持有5秒钟，B随后进来发现
 * 当前线程持有锁，所以只能通过自旋等待，知道A释放锁后B随后抢到。
 * 
 */
public class SpinLockDemo {

    AtomicReference<Thread>  atomicReference=new AtomicReference<>();

    public static void main(String[] args) {
        SpinLockDemo spinLockDemo = new SpinLockDemo();
        new Thread(()->{
            spinLockDemo.lock();
            try { TimeUnit.SECONDS.sleep(5); }catch (Exception e){e.getMessage();}
            spinLockDemo.unLock();
        },"A").start();

        try { TimeUnit.SECONDS.sleep(1); }catch (Exception e){e.getMessage();}
        new Thread(()->{
            spinLockDemo.lock();
            spinLockDemo.unLock();
        },"B").start();
    }

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

        }
    }

    public void unLock(){
        Thread thread = Thread.currentThread();
        atomicReference.compareAndSet(thread,null);
        System.out.println(Thread.currentThread().getName()+" ---task over,unLock...");
    }

}

---

八、CAS缺点

1.循环时间长开销很大

2.ABA问题： 比较+版本号一起上

①ABA问题怎么产生的

②：版本号时间戳原子引用

AtomicStampedReference简单case

public class AtomicStampedDemo {
    public static void main(String[] args) {
        Book javaBook = new Book(1, "Java");
        AtomicStampedReference<Book> stampedReference=new AtomicStampedReference<>(javaBook,1);

        System.out.println(stampedReference.getReference()+" "+stampedReference.getStamp());
        Book mysqlBook = new Book(2, "MySql");

        boolean b;
        b= stampedReference.compareAndSet(javaBook, mysqlBook, stampedReference.getStamp(), stampedReference.getStamp()+1);
        System.out.println(b+" "+stampedReference.getReference()+ " "+stampedReference.getStamp());

        b= stampedReference.compareAndSet(mysqlBook, javaBook, stampedReference.getStamp(), stampedReference.getStamp()+1);
        System.out.println(b+" "+stampedReference.getReference()+ " "+stampedReference.getStamp());
    }
}

---

ABADemo

public class ABADemo {
    static AtomicInteger atomicInteger=new AtomicInteger(100);
    public static void main(String[] args) {
        new Thread(()->{
            atomicInteger.compareAndSet(100,101);
            atomicInteger.compareAndSet(101,100);
        },"t1").start();

        try { TimeUnit.SECONDS.sleep(1); }catch (Exception e){e.getMessage();}

        new Thread(()->{
            boolean b = atomicInteger.compareAndSet(100, 2023);
            System.out.println(b +" "+ atomicInteger.get());
        },"t1").start();

    }
}

true 2023

---

---

public class ABADemo {
    static AtomicInteger atomicInteger=new AtomicInteger(100);
    static AtomicStampedReference<Integer> stampedReference=new AtomicStampedReference(100,1);
    public static void main(String[] args) {
        new Thread(()->{
            int stamp = stampedReference.getStamp();
            System.out.println(Thread.currentThread().getName()+" 首次版本号 "+stamp);
            //等待1秒 保证t2线程获取到和我一样的版本号
            try { TimeUnit.SECONDS.sleep(1); }catch (Exception e){e.getMessage();}
            stampedReference.compareAndSet(100,101,stampedReference.getStamp(),stampedReference.getStamp()+1);
            stampedReference.compareAndSet(101,100,stampedReference.getStamp(),stampedReference.getStamp()+1);
        },"t1").start();
        
        new Thread(()->{
            int stamp = stampedReference.getStamp();
            System.out.println(Thread.currentThread().getName()+" 首次版本号 "+stamp);
            //等待2秒 保证t1线程发声ABA问题
            try { TimeUnit.SECONDS.sleep(2); }catch (Exception e){e.getMessage();}
            boolean b = stampedReference.compareAndSet(100, 2023,stamp,stampedReference.getStamp()+1);
            System.out.println(b +" "+ stampedReference.getReference() + " "+stampedReference.getStamp());
        },"t2").start();

    }
}

t1 首次版本号 1
t2 首次版本号 1
false 100 3