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;

            }

        }

    }