atomatic包

大致看了下atomatic包下的類，都直到是用的CAS樂觀鎖，有的方法會重試直到成功，有的方法不管成功和失敗都會立刻返回。

AtomicInteger 類

comparaAnSet

JDK1.7和1.8一樣，調用unsafe的compareAndSwapInt，失敗返回false，不會重試

public final boolean compareAndSet(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }

getAndIncrement

JDK1.7和1.8有細微的差別。總體來說都會循環執行unsafe的compareAndSwapInt方法，直到成功爲止。

• 1.8

public final int getAndIncrement() {
        return unsafe.getAndAddInt(this, valueOffset, 1);
    }

 //Unsafe 的方法
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;
    }

• 1.7

public final int getAndIncrement() {
        for (;;) {
            int current = get();
            int next = current + 1;
            if (compareAndSet(current, next))
                return current;
        }
    }
 public final boolean compareAndSet(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }