CAS

什么是cas

全称compare and swap(compare and exchange) 比较并交换

CAS图

场景1

场景：代码对着某个int属性加一，单线程没问题，但是多线程的时候会有问题，因此要加锁

假如不加锁synchrnoized的情况下，可以用上面的cas

但是这种操作会引发ABA问题

解决方法：加版本号进行判断(每次改的时候加个版本号，然后返回时进行判断)

在java中的应用

public class AtomicIntegerTest {
    //AtomicInteger典型的cas
    private static AtomicInteger m = new AtomicInteger ( 0) ;
    public static void main(String[] args) throws Exception {
        Thread[] threads = new Thread[100];
        CountDownLatch latch = new CountDownLatch(threads.length);
        Object o = new Object();
        for (int i = 0; i < threads.length; i++) {
            threads[i] = new Thread(() -> {
                for (int j = 0; j < 10000; j++) {
                    m.incrementAndGet(); //m++
                }
                latch.countDown();
            });
        }
        Arrays.stream(threads).forEach((t) -> t.start());
        latch.await();
        System.out.println(m);
    }
}

incrementAndGet()点进去看

    /**
     * Atomically increments by one the current value.
     *
     * @return the updated value
     */
    public final int incrementAndGet() {
        return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
    }

getAndAddInt点进去看

    public final int getAndAddInt(Object var1, long var2, int var4) {
        int var5;
        do {
            var5 = this.getIntVolatile(var1, var2);
        } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));

        return var5;
    }

compareAndSwapInt这里用到了cas
compareAndSwapInt点进去看

public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);

场景2

synchrnoized里面也有用到