Netty源码分析（七）ServerBootstrap和Bootstrap

ServerBootstrap和Bootstrap都是Netty的启动类，他们的主要作用就是配置相关参数(IP，端口等)并启动整个Netty服务，不同的是ServerBootstrap用于服务端服务启动，Bootstrap用于客户端。

我们先看一段最简单的ServerBootstrap启动代码

EventLoopGroup bossGroup = new NioEventLoopGroup(1);//用于接受accept事件的group serverSocketChannel 
EventLoopGroup group = new NioEventLoopGroup();  //用于真正读写事件的group socketChannel
try {  
    //create ServerBootstrap instance  
    ServerBootstrap b = new ServerBootstrap();  //启动装载器，用于转载配置
    //Specifies NIO transport, local socket address  
    //Adds handler to channel pipeline  
    //使用NIO通道                                   
    //保持连接，如果不设置则一次通讯后自动断开
    b.group(bossGroup,group).channel(NioServerSocketChannel.class).option(ChannelOption.SO_KEEPALIVE, true).localAddress(port)
                .handler(new SimpleServerHandler())//服务端handler
                    .childHandler(new ChannelInitializer() {  //channel使用的handler
                        @Override  
                        protected void initChannel(Channel ch) throws Exception {  
                            ch.pipeline().addLast(new EchoServerHandler2(),new EchoServerHandler());  
                        }  
                    });  
    //Binds server, waits for server to close, and releases resources  
    ChannelFuture f = b.bind().sync();  
    System.out.println(EchoServer.class.getName() + "started and listen on �" + f.channel().localAddress());  
    f.channel().closeFuture().sync();  
} finally {  
    group.shutdownGracefully().sync();  
}

ServerBootstrap和Bootstrap相比最大的区别，是拥有2个EventLoopGroup，一个用于ServerSocketChannel的accept，一个用于SocketChannel的读写事件。

private volatile EventLoopGroup childGroup;
private volatile ChannelHandler childHandler;
...

public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
    super.group(parentGroup);
    ...
    this.childGroup = childGroup;
    return this;
}

public ServerBootstrap childHandler(ChannelHandler childHandler) {
    ...
    this.childHandler = childHandler;
    return this;
}

ServerBootstrap多了一个childGroup和childHandler
对比下ServerBootstrap和Bootstrap的主要差别：
ServerBootstrap.initChannel()

void init(Channel channel) throws Exception {
    //设置channel参数
    final Map, Object> options = options0();
    synchronized (options) {
        channel.config().setOptions(options);
    }
    final Map, Object> attrs = attrs0();
    synchronized (attrs) {
        for (Entry, Object> e: attrs.entrySet()) {
            @SuppressWarnings("unchecked")
            AttributeKey