private void handleWrites(SelectionKey sk, MemcachedNode qa)
    throws IOException {
    // 填充写缓冲区
    qa.fillWriteBuffer(shouldOptimize);
    boolean canWriteMore = qa.getBytesRemainingToWrite() > 0;
    while (canWriteMore) {
      int wrote = qa.writeSome();
      qa.fillWriteBuffer(shouldOptimize);
      // 如果wrote等于零，表示没有写出数据，那么不再尝试写，等待下次线程外层循环注册write事件
      canWriteMore = wrote > 0 && qa.getBytesRemainingToWrite() > 0;
    }

public final int writeSome() throws IOException {
    int wrote = channel.write(wbuf);
    // 写入多少个字节，toWrite就减去对应的数量
    toWrite -= wrote;
    return wrote;
}

public final int getSelectionOps() {
    int rv = 0;
    if (getChannel().isConnected()) {
      if (hasReadOp()) {
        rv |= SelectionKey.OP_READ;
      }
      // 如果toWrite大于0，说明由于某种异常原因上次写入还未完成；hasWriteOp()用于判断写队列是否还有元素。这两种情况下，需要注册写事件。本文讨论的是toWrite>0的情况。
      if (toWrite > 0 || hasWriteOp()) {
        rv |= SelectionKey.OP_WRITE;
      }
    } else {
      rv = SelectionKey.OP_CONNECT;
    }
    return rv;
}

protected void write0(AbstractNioChannel<?> channel) {
    boolean open = true;
    boolean addOpWrite = false;
    boolean removeOpWrite = false;
    boolean iothread = isIoThread(channel);

    long writtenBytes = 0;

    final SocketSendBufferPool sendBufferPool = this.sendBufferPool;
    final WritableByteChannel ch = channel.channel;
    final Queue<MessageEvent> writeBuffer = channel.writeBufferQueue;
    final int writeSpinCount = channel.getConfig().getWriteSpinCount();
    List<Throwable> causes = null;

    synchronized (channel.writeLock) {
        channel.inWriteNowLoop = true;
        for (;;) {
            MessageEvent evt = channel.currentWriteEvent;
            SendBuffer buf = null;
            ChannelFuture future = null;
            try {
                if (evt == null) {
                    if ((channel.currentWriteEvent = evt = writeBuffer.poll()) == null) {
                        // 如果无数据可写，则需要删除可写事件的注册
                        removeOpWrite = true;
                        channel.writeSuspended = false;
                        break;
                    }
                    future = evt.getFuture();

                    channel.currentWriteBuffer = buf = sendBufferPool.acquire(evt.getMessage());
                } else {
                    future = evt.getFuture();
                    buf = channel.currentWriteBuffer;
                }

                long localWrittenBytes = 0;
                // 通过writeSpinCount来控制尝试写的次数，如果最终还是无法写入，就注册写事件
                for (int i = writeSpinCount; i > 0; i --) {
                    // 写数据
                    localWrittenBytes = buf.transferTo(ch);
                    //  如果写入数据不等于零，表明写入成功，跳出循环
                    if (localWrittenBytes != 0) {
                        writtenBytes += localWrittenBytes;
                        break;
                    }
                    // 如果buf的数据都写完了，则跳出循环
                    if (buf.finished()) {
                        break;
                    }
                }

                if (buf.finished()) {
                    // Successful write - proceed to the next message.
                    buf.release();
                    channel.currentWriteEvent = null;
                    channel.currentWriteBuffer = null;
                    // Mark the event object for garbage collection.
                    //noinspection UnusedAssignment
                    evt = null;
                    buf = null;
                    future.setSuccess();
                } else {
                    // Not written fully - perhaps the kernel buffer is full.
                    addOpWrite = true;
                    channel.writeSuspended = true;

                    if (writtenBytes > 0) {
                        // Notify progress listeners if necessary.
                        future.setProgress(
                                localWrittenBytes,
                                buf.writtenBytes(), buf.totalBytes());
                    }
                    break;
                }
            }
        }
        channel.inWriteNowLoop = false;

        if (open) {
            if (addOpWrite) {
                // 注册写事件
                setOpWrite(channel);
            } else if (removeOpWrite) {
               // 删除写事件
                clearOpWrite(channel);
            }
        }
    }
}

public static long flushChannel(SocketChannel socketChannel, ByteBuffer bb, long writeTimeout)
        throws IOException {
    SelectionKey key = null;
    Selector writeSelector = null;
    int attempts = 0;
    int bytesProduced = 0;
    try {
        while (bb.hasRemaining()) {
            int len = socketChannel.write(bb);
            // 类似Netty的spinCount
            attempts++;
            if (len < 0) {
                throw new EOFException();
            }
            bytesProduced += len;
            if (len == 0) {
                if (writeSelector == null) {
                   // 获取一个新的selector
                    writeSelector = SelectorFactory.getSelector();
                    if (writeSelector == null) {
                        // Continue using the main one
                        continue;
                    }
                }
                // 在新selector上注册写事件，而不是在主selector上注册
                key = socketChannel.register(writeSelector, key.OP_WRITE);
                // 利用writeSelector.select()来阻塞当前线程，等待可写事件发生，总共等待可写事件的时长是3*writeTimeout
                if (writeSelector.select(writeTimeout) == 0) {
                    if (attempts > 2)
                        throw new IOException("Client disconnected");
                } else {
                    attempts--;
                }
            } else {
                attempts = 0;
            }
        }
    } 
    return bytesProduced;
}