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  • 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;
                }
            }
        }
        
        
        
    
    
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  • 原文地址:https://www.cnblogs.com/henuzyy/p/10415026.html
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