6. Netty解析：服务端如何接受连接并后续处理读写事件

前言

  之前的文章分析到了服务端NioServerSocketChannel的创建注册及注册accept事件。到现在为止，关于服务端，我们还有多个疑问未解开：例如当有客户端连接过来时，服务端要怎么处理，以及后续的读写如何进行的？之前的分析都是在服务端的父事件循环组中，那么子事件循环组又是怎么起作用的？之前提到了子处理器childHandler被封装在了ServerBootStrapAcceptor中添加到了流水线，那么子处理器又是怎么工作的，有没有被添加到pipeline中？本文将对这些问题进行解答。

服务端接收到连接后做了什么

  我们不得不再次回到下面这个方法，当有read或者accept事件到来时，执行unsaf.read()（对于连接事件和read事件，因为其所对应的通道一个是NioServerSocketChannel一个是NioSocketChannel，两者的unsafe是不同的，对于连接事件，unsafe.read位于AbstractNioUnsafe中）。

    private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            final EventLoop eventLoop;
            try {
                eventLoop = ch.eventLoop();
            } catch (Throwable ignored) {
             
                return;
            }
            if (eventLoop != this || eventLoop == null) {
                return;
            }
            unsafe.close(unsafe.voidPromise());
            return;
        }

        try {
            int readyOps = k.readyOps();
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }

            // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }
           if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }

    @Override
    public void read() {
        /*省略代码*/
        try {
            try {
                for (;;) {
                    int localRead = doReadMessages(readBuf);
                    if (localRead == 0) {
                        break;
                    }
                    if (localRead < 0) {
                        closed = true;
                        break;
                    }

                    // stop reading and remove op
                    if (!config.isAutoRead()) {
                        break;
                    }

                    if (readBuf.size() >= maxMessagesPerRead) {
                        break;
                    }
                }
            } catch (Throwable t) {
                exception = t;
            }
            setReadPending(false);
            int size = readBuf.size();
            for (int i = 0; i < size; i ++) {
                pipeline.fireChannelRead(readBuf.get(i));
            }
            readBuf.clear();
            pipeline.fireChannelReadComplete();
            /*省略代码*/
        } finally {
            /*省略代码*/
        }
    }


    /*进行连接的接收*/
    @Override
    protected int doReadMessages(List