模拟CAS

CAS

CAS是一种无锁的非阻塞算法，全称为：Compare-and-swap（比较并交换）算法保证数据的原子性，CAS是硬件对于并发操作共享数据的支持，包含三个操作数，大致思路是：先比较目标对象现值是否和旧值一致，如果一致，则更新对象为新值；如果不一致，则表明对象已经被其他线程修改，直接返回.

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/**
 * @Author: Usher
 * @Description:
 * 模拟CAS
 */
public class TestCompareAndSwap {
    public static void main(String[] args) {
        final CompareAndSwap compareAndSwap = new CompareAndSwap();
        for(int i = 0;i < 10;i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    int expectedValue = compareAndSwap.getValue();
                    boolean b = compareAndSwap.compareAndSet(expectedValue,(int)(Math.random()*101));
                    System.out.println(b);
                }
            }).start();
        }
    }
}

class CompareAndSwap{
    private int value;
    //内存值（旧值）
    public synchronized int getValue(){
        return value;
    }
    //比较
    public synchronized int compareAndSwap(int expectedValue, int newValue){
        int oldValue = value;
      	//预估值
        if (oldValue == expectedValue){
            this.value = newValue;//更新值
        }
        return oldValue;
    }
    //设置
    public synchronized boolean compareAndSet(int expectedValue, int newValue){
        return expectedValue == compareAndSwap(expectedValue,newValue);
    }
}
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//输出
true
false
true
false
false
false
true
true
true
true

Process finished with exit code 0