Netty 服务接收请求&响应

服务端创建连接并响应请求

客户端与服务端通讯必须先创建连接。
在前文介绍服务端初始化的时候说道初始化的时候会向boss线程注册一个OP_ACCEPT事件。客户端请求链接时即会触发此事件。当boss线程池中的NioEventLoop在循环的时候检测到此事件时，会触发NioMessageUnsafe.read方法。

public void read() {
  final ChannelConfig config = config();
  final ChannelPipeline pipeline = pipeline();
  final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
  allocHandle.reset(config);
  boolean closed = false;
  Throwable exception = null;
  try {
      do {
          // NioServerSocketChannel.doReadMessage
          int localRead = doReadMessages(readBuf);
          if (localRead == 0) {
              break;
          }
          if (localRead < 0) {
              closed = true;
              break;
          }
          allocHandle.incMessagesRead(localRead);
      } while (allocHandle.continueReading());
  } catch (Throwable t) {
      exception = t;
  }
  int size = readBuf.size();
  for (int i = 0; i < size; i ++) {
      readPending = false;
      // 触发 ChannelHandler的 channelRead方法，经过LoggingHandler最后到ServerBootstrapAcceptor
      pipeline.fireChannelRead(readBuf.get(i));
  }
  readBuf.clear();
  allocHandle.readComplete();
  pipeline.fireChannelReadComplete();
  if (exception != null) {
      closed = closeOnReadError(exception);
      pipeline.fireExceptionCaught(exception);
  }
  if (closed) {
      inputShutdown = true;
      if (isOpen()) {
          close(voidPromise());
      }
  }
}
protected int doReadMessages(List<Object> buf) throws Exception {
   // 从ServerSocketChannel.accept一个SocketChannel
   SocketChannel ch = SocketUtils.accept(javaChannel());
   if (ch != null) {
       buf.add(new NioSocketChannel(this, ch));
       return 1;
   }
   return 0;
}

和Client端的不同之处即在于doReadMessages方法。此处调用的是NioServerSocketChannel.doReadMessage。
其调用的是原生ServerSocketChannel.accept方法来获取一个SocektChannel然后封装在NioSocetChannel中，并将其传递到ServerBootstrapAcceptor：

public void channelRead(ChannelHandlerContext ctx, Object msg) {
  final Channel child = (Channel) msg;
  // 将初始化时设置的childHandler 加入到pipeline 中
  child.pipeline().addLast(childHandler);
  setChannelOptions(child, childOptions, logger);
  for (Entry<AttributeKey<?>, Object> e: childAttrs) {
      child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
  }
  try {
      // 往reactor线程池中注入，即跟某个`NioEventLoop`绑定。
      childGroup.register(child).addListener(new ChannelFutureListener() {
          @Override
          public void operationComplete(ChannelFuture future) throws Exception {
              if (!future.isSuccess()) {
                  forceClose(child, future.cause());
              }
          }
      });
  } catch (Throwable t) {
      forceClose(child, t);
  }
}

首先，将初始化时配置的childHandler加到Channel的pipeline中，然后将Channel注册到某个 Reactor线程 上，然后开始监听OP_READ读事件。
此后与客户端的通讯工作即会在Reactor线程中进行，其过程与Client中一致，因此这里不再复述。