一、大致介绍

1、Netty这个词，对于熟悉并发的童鞋一点都不陌生，它是一个异步事件驱动型的网络通信框架；
2、使用Netty不需要我们关注过多NIO的API操作，简简单单的使用即可，非常方便，开发门槛较低； 3、而且Netty也经历了各大著名框架的“摧残”，足以证明其性能高，稳定性高； 4、那么本章节就来和大家分享分析一下Netty的服务端启动流程，分析Netty的源码版本为：netty-netty-4.1.22.Final； 

二、简单认识Netty

2.1 何为Netty？

1、是一个基于NIO的客户端、服务器端的网络通信框架；2、是一个以提供异步的、事件驱动型的网络应用工具；3、可以供我们快速开发高性能的、高可靠性的网络服务器与客户端；

2.2 为什么使用Netty？

1、开箱即用，简单操作，开发门槛低，API简单，只需关注业务实现即可，不用关心如何编写NIO；2、自带多种协议栈且预置多种编解码功能，且定制化能力强；3、综合性能高，已历经各大著名框架(RPC框架、消息中间件)等广泛验证，健壮性非常强大；4、相对于JDK的NIO来说，netty在底层做了很多优化，将reactor线程的并发处理提到了极致；5、社区相对较活跃，遇到问题可以随时提问沟通并修复；

2.3 大致阐述启动流程

1、创建两个线程管理组，一个是bossGroup，一个是workerGroup，每个Group下都有一个线程组children[i]来执行任务；2、bossGroup专门用来揽客的，就是接收客户端的请求链接，而workerGroup专门用来干事的，bossGroup揽客完了就交给workerGroup去干活了；3、通过bind轻松的一句代码绑定注册，其实里面一点都不简单，一堆堆的操作；4、创建NioServerSocketChannel，并且将此注册到bossGroup的子线程中的多路复用器上；5、最后一步就是将NioServerSocketChannel绑定到指定ip、port即可，由此完成服务端的整个启动过程；

2.4 Netty服务端启动Demo

/*** Netty服务端启动代码。** @author hmilyylimh * * @version 0.0.1 * * @date 2018/3/25 * */ public class NettyServer { public static final int TCP_PORT = 20000; private final int port; public NettyServer(int port) { this.port = port; } public void start() throws Exception { EventLoopGroup bossGroup = null; EventLoopGroup workerGroup = null; try { // Server 端引导类 ServerBootstrap serverBootstrap = new ServerBootstrap(); // Boss 线程管理组 bossGroup = new NioEventLoopGroup(1); // Worker 线程管理组 workerGroup = new NioEventLoopGroup(); // 将 Boss、Worker 设置到 ServerBootstrap 服务端引导类中 serverBootstrap.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) // 指定通道类型为NioServerSocketChannel，一种异步模式，OIO阻塞模式为OioServerSocketChannel .localAddress("localhost", port)//设置InetSocketAddress让服务器监听某个端口已等待客户端连接。 .childHandler(new ChannelInitializer<Channel>() {//设置childHandler执行所有的连接请求 @Override protected void initChannel(Channel ch) throws Exception { ch.pipeline().addLast(new PacketHeadDecoder()); ch.pipeline().addLast(new PacketBodyDecoder()); ch.pipeline().addLast(new PacketHeadEncoder()); ch.pipeline().addLast(new PacketBodyEncoder()); ch.pipeline().addLast(new PacketHandler()); } }); // 最后绑定服务器等待直到绑定完成，调用sync()方法会阻塞直到服务器完成绑定,然后服务器等待通道关闭，因为使用sync()，所以关闭操作也会被阻塞。 ChannelFuture channelFuture = serverBootstrap.bind().sync(); System.out.println("Server started，port：" + channelFuture.channel().localAddress()); channelFuture.channel().closeFuture().sync(); } finally { // Shut down all event loops to terminate all threads. bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } } public static void main(String[] args) throws Exception { new NettyServer(TCP_PORT).start(); } } 

三、常用的类结构

四、源码分析Netty服务端启动

4.1、创建bossGroup对象

1、源码：// NettyServer.java, Boss 线程管理组, 上面NettyServer.java中的示例代码 bossGroup = new NioEventLoopGroup(1); // NioEventLoopGroup.java /** * Create a new instance using the specified number of threads, {@link ThreadFactory} and the * {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}. */ public NioEventLoopGroup(int nThreads) { this(nThreads, (Executor) null); } // NioEventLoopGroup.java public NioEventLoopGroup(int nThreads, Executor executor) { this(nThreads, executor, SelectorProvider.provider()); } // NioEventLoopGroup.java public NioEventLoopGroup( int nThreads, Executor executor, final SelectorProvider selectorProvider) { this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE); } // NioEventLoopGroup.java public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider, final SelectStrategyFactory selectStrategyFactory) { super(nThreads, executor, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject()); } // MultithreadEventLoopGroup.java /** * @see MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, Executor, Object...) */ protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) { // DEFAULT_EVENT_LOOP_THREADS 默认为CPU核数的2倍 super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args); } // MultithreadEventExecutorGroup.java /** * Create a new instance. * * @param nThreads the number of threads that will be used by this instance. * @param executor the Executor to use, or {@code null} if the default should be used. * @param args arguments which will passed to each {@link #newChild(Executor, Object...)} call */ protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) { this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args); } // MultithreadEventExecutorGroup.java /** * Create a new instance. * * @param nThreads the number of threads that will be used by this instance. * @param executor the Executor to use, or {@code null} if the default should be used. * @param chooserFactory the {@link EventExecutorChooserFactory} to use. * @param args arguments which will passed to each {@link #newChild(Executor, Object...)} call */ protected