AtomicIntegerFieldUpdater用法

一个基于反射的工具类，它能对指定类的指定的volatile字段进行原子更新

下面是netty源码中AbstractReferenceCountedByteBuf类的使用

    private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> refCntUpdater;

    static {
        AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> updater =  PlatformDependent.newAtomicIntegerFieldUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
        if (updater == null) {
            updater = AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
        }
        refCntUpdater = updater;
    }

	// 必须用volative声明，保证多线程的可见性
    private volatile int refCnt = 1;

        @Override
    public ByteBuf retain() {
    	//失败重试
        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalReferenceCountException(0, 1);
            }
            if (refCnt == Integer.MAX_VALUE) {
                throw new IllegalReferenceCountException(Integer.MAX_VALUE, 1);
            }
            //使用CAS对refcnt线程安全的操作
            if (refCntUpdater.compareAndSet(this, refCnt, refCnt + 1)) {
                break;
            }
        }
        return this;
    }

        @Override
    public final boolean release() {
        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalReferenceCountException(0, -1);
            }
            // CAS 操作
            if (refCntUpdater.compareAndSet(this, refCnt, refCnt - 1)) {
                if (refCnt == 1) {
                    deallocate();
                    return true;
                }
                return false;
            }
        }
    }