netty源码分析视频 Netty源码分析之ByteBuf引用计数

引用计数是一种常用的内存管理机制，是指将资源的被引用次数保存起来，当被引用次数变为零时就将其释放的过程。Netty在4.x版本开始使用引用计数机制进行部分对象的管理，其实现思路并不是特别复杂，它主要涉及跟踪某个对象被引用的次数。在Netty具体代码中需要通过引用计数进行内存管理的对象，会基于ReferenceCounted接口实现，其中引用计数大于0时则代表该对象被引用不会释放，当引用计数减少到0时，该对象就会被释放。通过引用计数机制，Netty可以很好的实现内存管理，引用计数减少到0时要么直接释放内存，要么放回内存池中重复利用。
1、基本示例下面先通过一个简单示例看下Netty中引用计数机制的使用
@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) {ByteBuf recvBuffer = (ByteBuf) msg;// 申请ByteBuf 需要主动释放if(recvBuffer.isDirect()){System.err.println(true);}PooledByteBufAllocator allocator = new PooledByteBufAllocator(true);ByteBuf sendBuffer = allocator.buffer();//申请池化直接内存System.err.println("sendBuffer的引用计数:"+sendBuffer.refCnt());sendBuffer.retain();System.err.println("sendBuffer的引用计数:"+sendBuffer.refCnt());sendBuffer.release();System.err.println("sendBuffer的引用计数:"+sendBuffer.refCnt());
try {byte[] bytesReady = new byte[recvBuffer.readableBytes()];recvBuffer.readBytes(bytesReady);System.out.println("channelRead收到数据："+ BytesUtils.toHexString(bytesReady));byte[] sendBytes = new byte[] {0x7E,0x01,0x02,0x7e};sendBuffer.writeBytes(sendBytes);ctx.writeAndFlush(sendBuffer);System.err.println("sendBuffer的引用计数:"+sendBuffer.refCnt());}catch (Exception e) {// TODO: handle exceptionSystem.err.println(e.getMessage());}finally {System.err.println("recvBuffer的引用计数:"+recvBuffer.refCnt());recvBuffer.release(); //此处需要释放System.err.println("recvBuffer的引用计数:"+recvBuffer.refCnt());}}输出结果如下，通过示例可以看出retain方法会增加计数引用，release方法会减少计数引用
truesendBuffer的引用计数:1sendBuffer的引用计数:2sendBuffer的引用计数:1sendBuffer的引用计数:0recvBuffer的引用计数:1recvBuffer的引用计数:0AbstractReferenceCountedByteBuf实现了对ByteBuf的内存管理，以实现内存的回收、释放或者重复利用，AbstractReferenceCountedByteBuf的继承实现关系如下图所示

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2、ReferenceCounted接口定义首先是ReferenceCounted接口的定义
public interface ReferenceCounted {/*** Returns the reference count of this object.If {@code 0}, it means this object has been deallocated.* 返回对象的引用计数*/int refCnt();/*** Increases the reference count by {@code 1}.* 增加引用计数*/ReferenceCounted retain();/*** Increases the reference count by the specified {@code increment}.* 引用计数增加指定值*/ReferenceCounted retain(int increment);/*** Records the current access location of this object for debugging purposes.* If this object is determined to be leaked, the information recorded by this operation will be provided to you* via {@link ResourceLeakDetector}.This method is a shortcut to {@link #touch(Object) touch(null)}.* 记录该对象的当前访问位置，用于调试。* 如果确定该对象被泄露，将提供此操作记录的信息给您*/ReferenceCounted touch();/*** Records the current access location of this object with an additional arbitrary information for debugging* purposes.If this object is determined to be leaked, the information recorded by this operation will be* provided to you via {@link ResourceLeakDetector}.* 记录该对象的当前访问位置，附加信息用于调试。* 如果确定该对象被泄露，将提供此操作记录的信息给您*/ReferenceCounted touch(Object hint);/*** Decreases the reference count by {@code 1} and deallocates this object if the reference count reaches at* {@code 0}.** @return {@code true} if and only if the reference count became {@code 0} and this object has been deallocated* 引用计数减少，如果计数变为了0，则释放对象资源* 如果对象资源被释放，则返回true，否则返回false*/boolean release();/*** Decreases the reference count by the specified {@code decrement} and deallocates this object if the reference* count reaches at {@code 0}.** @return {@code true} if and only if the reference count became {@code 0} and this object has been deallocated* 引用计数-指定值，如果计数变为了0，则释放对象资源或交回到对象池* 如果对象资源被释放，则返回true，否则返回false*/boolean release(int decrement);}3、AbstractReferenceCountedByteBuf源码分析AbstractReferenceCountedByteBuf对ReferenceCounted进行了具体实现，retain与release两个方法通过CAS方式对引用计数refcnt进行操作，下面对其源码进行简单分析
初始化引用计数初始值refCnt 使用关键字volatile修饰，保证线程的可见性，同时使用偶数，引用增加通过位移操作实现，提高运算效率。
采用 AtomicIntegerFieldUpdater 对象，通过CAS方式更新refCnt，以实现线程安全，避免加锁，提高效率。
private static final long REFCNT_FIELD_OFFSET;//采用 AtomicIntegerFieldUpdater 对象，CAS方式更新refCntprivate static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> refCntUpdater =AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");//refCnt 实际值为偶数，采用位移操作提高效率// even => "real" refcount is (refCnt >>> 1); odd => "real" refcount is 0@SuppressWarnings("unused")private volatile int refCnt = 2;retain操作上面示例中每调用一次retain方法，引用计数就会累加一次，我们看下源码中retain的具体实现
public ByteBuf retain() {return retain0(1);}@Overridepublic ByteBuf retain(int increment) {return retain0(checkPositive(increment, "increment"));}//计数器增值操作private ByteBuf retain0(final int increment) {// all changes to the raw count are 2x the "real" changeint adjustedIncrement = increment << 1; // overflow OK here真正的计数都是2倍递增int oldRef = refCntUpdater.getAndAdd(this, adjustedIncrement); //通过CAS方式递增并获取原值if ((oldRef & 1) != 0) {//判断奇偶，正常情况这里应该都是偶数throw new IllegalReferenceCountException(0, increment);}// don't pass 0!如果计数小于等于0，以及整型范围越界（0x7fffffff+1）抛出异常if ((oldRef <= 0 && oldRef + adjustedIncrement >= 0)|| (oldRef >= 0 && oldRef + adjustedIncrement < oldRef)) {// overflow caserefCntUpdater.getAndAdd(this, -adjustedIncrement);throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);}return this;}release操作通过调用release方法，对引用计数做减值操作，源码中release的具体实现要注意的是由于引用计数以2倍递增，所以引用次数= 引用计数/2，当decrement=refcnt/2 也就是引用次数=释放次数时，代表ByteBuf不再被引用，执行内存释放或放回内存池的操作。
//计数器减值操作private boolean release0(int decrement) {int rawCnt = nonVolatileRawCnt(), realCnt = toLiveRealCnt(rawCnt, decrement); //对计数器进行除以2操作,也就是引用次数/*** /这里如注意你传入的减值参数decrement= realCnt 时等同于引用次数=释放次数，直接进行释放操作*/if (decrement == realCnt) {if (refCntUpdater.compareAndSet(this, rawCnt, 1)) { //CAS方式置为1deallocate();//内存释放或放回内存池return true;}return retryRelease0(decrement);//进入具体操作}return releaseNonFinal0(decrement, rawCnt, realCnt);}private boolean releaseNonFinal0(int decrement, int rawCnt, int realCnt) {//如果decrement 小于 realCnt，通过CAS方式减去decrement*2if (decrement < realCnt// all changes to the raw count are 2x the "real" change&& refCntUpdater.compareAndSet(this, rawCnt, rawCnt - (decrement << 1))) {return false;}return retryRelease0(decrement);}private boolean retryRelease0(int decrement) {for (;;) {int rawCnt = refCntUpdater.get(this), realCnt = toLiveRealCnt(rawCnt, decrement);if (decrement == realCnt) {if (refCntUpdater.compareAndSet(this, rawCnt, 1)) {deallocate();return true;}} else if (decrement < realCnt) {//如果decrement 小于 realCnt，通过CAS方式减去decrement*2// all changes to the raw count are 2x the "real" changeif (refCntUpdater.compareAndSet(this, rawCnt, rawCnt - (decrement << 1))) {return false;}} else {throw new IllegalReferenceCountException(realCnt, -decrement);}Thread.yield(); // this benefits throughput under high contention}}/*** Like {@link #realRefCnt(int)} but throws if refCnt == 0*/private static int toLiveRealCnt(int rawCnt, int decrement) {if ((rawCnt & 1) == 0) {return rawCnt >>> 1;}// odd rawCnt => already deallocatedthrow new IllegalReferenceCountException(0, -decrement);}4、总结 【netty源码分析视频 Netty源码分析之ByteBuf引用计数】以上我们围绕AbstractReferenceCountedByteBuf对Netty引用计数的具体实现进行了分析，可以看到Netty在实现引用计数的同时，结合CAS、位移计算等方式，保证了运算效率和线程安全，在实际项目中我们遇到类似应用场景也都可以借鉴参考，如数据发送次数，商品剩余数量等计数场景的实现。希望本文对大家能有所帮助，其中如有不足与不正确的地方还望指正与海涵，十分感谢。
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