由前面的分析可知NioEventLoop的run()方法是一个无限循环，NioEventLoop会不断的调用Selector的select(timeout)方法查询是否有新的IO事件，所以当一个客户端连接进入的时候会被Boss线程select到，故新连接接入流程的入口为Bose线程的select方法。

select(boolean oldWakenUp)方法如下所示(仅保留相关代码)：

private void select(boolean oldWakenUp) throws IOException {......int selectedKeys = selector.select(timeoutMillis);......if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {break;}   ......}

当客户端发起连接请求时，int selectedKeys = selector.select(timeoutMillis);返回的selectedKeys的值将会从0变为1。从而满足后面selectedKeys != 0 的判断，进而跳出select()方法，执行后续的操作。

select到IO事件后会进入NIOEventLoop的processSelectedKeys()方法，因为默认情况netty会对选择器Selector的SelectionKeys做相关优化（将原先的HashSet通过反射的方式修改为数组），故会执行NIOEventLoop的processSelectedKeysOptimized方法。

NIOEventLoop的processSelectedKeysOptimized()如下所示，因为当前是服务端，所以当前ServerSocketChannel是AbstractNioChannel类的子类，故进而由调用processSelectedKey(SelectionKey k, AbstractNioChannel ch)方法处理

private void processSelectedKeysOptimized() {for (int i = 0; i < selectedKeys.size; ++i) {final SelectionKey k = selectedKeys.keys[i];selectedKeys.keys[i] = null;final Object a = k.attachment();if (a instanceof AbstractNioChannel) {processSelectedKey(k, (AbstractNioChannel) a);} else {@SuppressWarnings("unchecked")NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;processSelectedKey(k, task);}if (needsToSelectAgain) {selectedKeys.reset(i + 1);selectAgain();i = -1;}}
}

NIOEventLoop的processSelectedKey(SelectionKey k, AbstractNioChannel ch)方法如下所示

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {final AbstractNioChannel.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();}if ((readyOps & SelectionKey.OP_WRITE) != 0) {ch.unsafe().forceFlush();}if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {unsafe.read();}} catch (CancelledKeyException ignored) {unsafe.close(unsafe.voidPromise());}
}

该方法的主要作用是根据SelectionKey中就绪事件的类型，执行不同的逻辑。因为当前分析的是服务端处理新连接的过程，由下面的代码片段可知，对于SelectionKey.OP_ACCEPT类型的事件，将会调用Unsafe接口的read()方法进行处理，这里实际调用的是AbstractNioMessageChannel的read()方法。

if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {unsafe.read();
}

AbstractNioMessageChannel的read()方法如下：

read()方法中有三个核心的步骤，即：

​ int localRead = doReadMessages(readBuf);

**pipeline.fireChannelRead(readBuf.get(i));****pipeline.fireChannelReadComplete();**	

public void read() {assert eventLoop().inEventLoop();//断言当前线程为ServerSocketChannel绑定的EventLoopfinal ChannelConfig config = config();final ChannelPipeline pipeline = pipeline();final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();allocHandle.reset(config);boolean closed = false;Throwable exception = null;try {try {do {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;pipeline.fireChannelRead(readBuf.get(i));//触发通道读时间}readBuf.clear();allocHandle.readComplete();pipeline.fireChannelReadComplete();//触发通道读取完成时间if (exception != null) {closed = closeOnReadError(exception);pipeline.fireExceptionCaught(exception);}if (closed) {inputShutdown = true;if (isOpen()) {close(voidPromise());}}} finally {if (!readPending && !config.isAutoRead()) {removeReadOp();}}
}

doReadMessages(List buf)

@Override
protected int doReadMessages(List<Object> buf) throws Exception {SocketChannel ch = SocketUtils.accept(javaChannel());//调用JDK的accpt方法获取一个客户端连接通道try {if (ch != null) {buf.add(new NioSocketChannel(this, ch));//创建一个netty的客户端通道类，并加入到集合中return 1;}} catch (Throwable t) {logger.warn("Failed to create a new channel from an accepted socket.", t);try {ch.close();} catch (Throwable t2) {logger.warn("Failed to close a socket.", t2);}}return 0;
}

fireChannelRead(Object msg)方法

该方法会触发Pipeline上所有的ChannelHandler的channelRead方法，最终会触发ServerBootstrapAcceptor类的channelRead方法，该实例是在ServerSocketChannel实例化的时候添加到pipeline的尾部的(ServerBootStrap的init(Channel channel)方法)

@Override
public final ChannelPipeline fireChannelRead(Object msg) {AbstractChannelHandlerContext.invokeChannelRead(head, msg);return this;
}

接着分析ServerBootstrapAcceptor的channelRead(ChannelHandlerContext ctx, Object msg)方法

该方法主要逻辑为：

1、根据ServerBootStrap中的配置，对客户端通道SocketChannel进行设置

2、在work线程组中选择一个NioEventLoop，并将当前客户连接注册在NioEventLoop的Selector上

当客户端后续再有read事件被触发，则会由当前work线程组中获得的NioEventLoop进行处理

public void channelRead(ChannelHandlerContext ctx, Object msg) {final Channel child = (Channel) msg;//客户端连接channelchild.pipeline().addLast(childHandler);//添加childHandlersetChannelOptions(child, childOptions, logger);//设置通道的option//设置socketChannel的属性for (Entry<AttributeKey<?>, Object> e: childAttrs) {child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());}//在work线程组里挑选一个EventLoop,并将当前的SocketChannel注册到EventLoop的Selector上try {childGroup.register(child).addListener(new ChannelFutureListener() {@Overridepublic void operationComplete(ChannelFuture future) throws Exception {if (!future.isSuccess()) {forceClose(child, future.cause());}}});} catch (Throwable t) {forceClose(child, t);}
}

pipeline.fireChannelReadComplete();

pipeline.fireChannelReadComplete();方法同上面fireChannelRead方法的流程类似，将Pipeline中的所有ChannelHandler中的channelReadComplete(ChannelHandlerContext ctx)方法都执行一遍