今天又看了一遍boyan的《Nip trick and trip》，又有一些新的收获。比以前看地更深刻了。其中有一处写到Nio的写优化，当Nio的channel设置为nonblocking时，写操作不再阻塞，而是直接返回0，表示当时socket的写缓冲区已满不可写。通常的处理办法是返回本次loop并继续注册op_read或者op_write。这样会带来线程切换的开销，使当前select阻塞，直到有可写发生再次唤醒。Netty是怎么解决的？以前虽然看过Netty的代码但并没有深入到细节中去，看了boyanNetty采用writeSpinCount的介绍才深有感悟。好多知识都在细节里，就看我们对这个领域掌握地深不深，如果蜻蜓点水肯定发现了这些看似简单的代码的玄妙之处。话归正题，writeSpinCount如何解决这个问题的呢？类似自旋锁，还是看看Netty的代码吧，比较形象。

for (;;) {                MessageEvent evt = channel.currentWriteEvent;                SendBuffer buf;                if (evt == null) {                    if ((channel.currentWriteEvent = evt = writeBuffer.poll()) == null) {                       //写事件队列为空，此时应该去除op_write                        removeOpWrite = true;                        channel.writeSuspended = false;                        break;                    }                    channel.currentWriteBuffer = buf = sendBufferPool.acquire(evt.getMessage());                } else {                    buf = channel.currentWriteBuffer;                }                ChannelFuture future = evt.getFuture();                try {                    long localWrittenBytes = 0;                    for (int i = writeSpinCount; i > 0; i --) {//此处为writeSpinCount的解决方式                       //向channel写数据，为了使其fileChannel的transferTo接口一致。这样从file写向socket时避免四次copy，sendfile的功能                        localWrittenBytes = buf.transferTo(ch);                        //写入的数据大于0，跳出本次循环，如为零则循环writeSpinCount，如再次期间localWrittenBytes>0,说明再次可写。                        if (localWrittenBytes != 0) {                            writtenBytes += localWrittenBytes;                            break;                        }                        if (buf.finished()) {                            break;                        }                    }                    if (buf.finished()) {                        // Successful write - proceed to the next message.                        //完整地写完，则继续处理下一个消息                        buf.release();                        channel.currentWriteEvent = null;                        channel.currentWriteBuffer = null;                        evt = null;                        buf = null;                        future.setSuccess();                    } else {                        // Not written fully - perhaps the kernel buffer is full.                        //没能完整地写完当前的message，则继续设置op_write，并跳出外层循环                        addOpWrite = true;                        channel.writeSuspended = true;                        if (localWrittenBytes > 0) {                            // Notify progress listeners if necessary.                            future.setProgress(                                    localWrittenBytes,                                    buf.writtenBytes(), buf.totalBytes());                        }                        break;                    }                } catch (AsynchronousCloseException e) {                    // Doesn't need a user attention - ignore.                } catch (Throwable t) {                    if (buf != null) {                        buf.release();                    }                    channel.currentWriteEvent = null;                    channel.currentWriteBuffer = null;                    buf = null;                    evt = null;                    future.setFailure(t);                    if (iothread) {                        fireExceptionCaught(channel, t);                    } else {                        fireExceptionCaughtLater(channel, t);                    }                    if (t instanceof IOException) {                        open = false;                        close(channel, succeededFuture(channel));                    }                }            }            channel.inWriteNowLoop = false;            // Initially, the following block was executed after releasing            // the writeLock, but there was a race condition, and it has to be            // executed before releasing the writeLock:            //            //     https://issues.jboss.org/browse/NETTY-410            //            if (open) {                if (addOpWrite) {                    setOpWrite(channel);                } else if (removeOpWrite) {                    clearOpWrite(channel);                }            }        }