相比于 compareAndSet 的实现，这里将 for 循环去掉

public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf {

private volatile int refCnt;

protected AbstractReferenceCountedByteBuf(int maxCapacity) {
    super(maxCapacity);
    // 引用计数在初始的时候还是为 1 
    refCntUpdater.set(this, 1);
}

private ByteBuf retain0(final int increment) {
    // 相比于 compareAndSet 的实现，这里将 for 循环去掉
    // 并且每次是先对 refCnt 增加计数 increment
    int oldRef = refCntUpdater.getAndAdd(this, increment);
    // 增加完 refCnt 计数之后才去判断异常情况
    if (oldRef <= 0 || oldRef + increment < oldRef) {
        // Ensure we don't resurrect (which means the refCnt was 0) and also that we encountered an overflow.
        // 如果原来的 refCnt 已经为 0 或者 refCnt 溢出，则对 refCnt 进行回退，并抛出异常
        refCntUpdater.getAndAdd(this, -increment);
        throw new IllegalReferenceCountException(oldRef, increment);
    }
    return this;
}

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public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf { // 获取 refCnt 字段在 ByteBuf 对象内存中的偏移 // 后续通过 Unsafe 对 refCnt 进行操作 private static final long REFCNT_FIELD_OFFSET = ReferenceCountUpdater.getUnsafeOffset(AbstractReferenceCountedByteBuf.class, "refCnt");

// 获取 refCnt 字段 的 AtomicFieldUpdater
// 后续通过 AtomicFieldUpdater 来操作 refCnt 字段
private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> AIF_UPDATER =
        AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
        
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// 创建 ReferenceCountUpdater，对于引用计数的所有操作最终都会代理到这个类中
private static final ReferenceCountUpdater<AbstractReferenceCountedByteBuf> updater =
        new ReferenceCountUpdater<AbstractReferenceCountedByteBuf>() {
    @Override
    protected AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> updater() {
        // 通过 AtomicIntegerFieldUpdater 操作 refCnt 字段
        return AIF_UPDATER;
    }
    @Override
    protected long unsafeOffset() {
        // 通过 Unsafe 操作 refCnt 字段
        return REFCNT_FIELD_OFFSET;
    }
};
// ByteBuf 中的引用计数，初始为 2 （偶数）
private volatile int refCnt = updater.initialValue();

}