以netty的一个小demo为例（使用的源码版本为4.1.50.Final）
从b.bind(PORT)跟进
最终调用到AbstractBootstrap#doBind这个方法
private ChannelFuture doBind(final SocketAddress localAddress) { final ChannelFuture regFuture = initAndRegister(); final Channel channel = regFuture.channel(); if (regFuture.cause() != null) { return regFuture; } if (regFuture.isDone()) { // At this point we know that the registration was complete and successful. ChannelPromise promise = channel.newPromise(); doBind0(regFuture, channel, localAddress, promise); return promise; } else { // Registration future is almost always fulfilled already, but just in case it‘s not. final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel); regFuture.addListener(new ChannelFutureListener() { @Override public void operationComplete(ChannelFuture future) throws Exception { Throwable cause = future.cause(); if (cause != null) { // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an // IllegalStateException once we try to access the EventLoop of the Channel. promise.setFailure(cause); } else { // Registration was successful, so set the correct executor to use. // See https://github.com/netty/netty/issues/2586 promise.registered(); doBind0(regFuture, channel, localAddress, promise); } } }); return promise; } } 这里就是服务端创建的主要代码
主要概括如下
1.通过demo里传入的NioServerSocketChannel类反射创建channel
2.在NioServerSocketChannel的构造函数里
方法体里第一行代码的initAndRegister方法就是在创建和初始化channel
final ChannelFuture initAndRegister() { Channel channel = null; try { channel = channelFactory.newChannel(); //创建channel，本节分析 init(channel); } catch (Throwable t) { //省略 } ChannelFuture regFuture = config().group().register(channel); if (regFuture.cause() != null) { if (channel.isRegistered()) { channel.close(); } else { channel.unsafe().closeForcibly(); } } return regFuture; } 创建channel就在channelFactory.newChannel()这行代码里，这里会调用到ReflectiveChannelFactory#newChannel这个方法
public T newChannel() { try { return constructor.newInstance(); } catch (Throwable t) { //省略 } } 可以看到这里用反射创建一个channel，也就是说channel是通过channelFactory的constructor来反射创建的，而channelFactory是在demo里调用channel方法时初始化的，如下
ServerBootstrap b = new ServerBootstrap();b.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) //在这里初始化了`channelFactory //省略 在AbstractBootstrap#channel方法里
public B channel(Class<? extends C> channelClass) { return channelFactory(new ReflectiveChannelFactory<C>( ObjectUtil.checkNotNull(channelClass, "channelClass") )); } 最终调用到AbstractBootstrap#channelFactory方法里
public B channelFactory(ChannelFactory<? extends C> channelFactory) { ObjectUtil.checkNotNull(channelFactory, "channelFactory"); if (this.channelFactory != null) { throw new IllegalStateException("channelFactory set already"); } this.channelFactory = channelFactory; return self(); } 既然这里是通过反射创建了一个NioServerSocketChannel，接下来看看NioServerSocketChannel这个类的构造函数
public NioServerSocketChannel() { this(newSocket(DEFAULT_SELECTOR_PROVIDER)); } 先看看NioServerSocketChannel#newSocket这个方法
private static ServerSocketChannel newSocket(SelectorProvider provider) { try { return provider.openServerSocketChannel(); //provider就是DEFAULT_SELECTOR_PROVIDER,为SelectorProvider.provider() } catch (IOException e) { //省略 } } 上面的构造函数调用到了另一个构造函数
public NioServerSocketChannel(ServerSocketChannel channel) { //注意这里传入SelectionKey.OP_ACCEPT，用于之后注册accept事件 super(null, channel, SelectionKey.OP_ACCEPT); config = new NioServerSocketChannelConfig(this, javaChannel().socket()); } super方法调用父类做一些简单的初始化，主要在AbstractNioChannel和AbstractChannel这两个类这种
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) { super(parent); this.ch = ch; this.readInterestOp = readInterestOp; try { ch.configureBlocking(false); } catch (IOException e) { //省略 } } protected AbstractChannel(Channel parent) { this.parent = parent; id = newId(); unsafe = newUnsafe(); pipeline = newChannelPipeline(); } NioServerSocketChannelConfig则是传入java底层的ServerSocketChannel方便以后对做一些配置
先概括一下，主要做了这几件事
接下来分析初始化channel的过程，即init(channel)这行代码
这里会调用到ServerBootstrap#init这个方法
void init(Channel channel) { //newOptionsArray这个方法拿到用户设置的options设置到channel的config里 setChannelOptions(channel, newOptionsArray(), logger); //attrs0这个方法拿到用户设置的attrs设置到channel里 setAttributes(channel, attrs0().entrySet().toArray(EMPTY_ATTRIBUTE_ARRAY)); ChannelPipeline p = channel.pipeline(); final EventLoopGroup currentChildGroup = childGroup; final ChannelHandler currentChildHandler = childHandler; final Entry<ChannelOption<?>, Object>[] currentChildOptions; //拿到用于配置childHandler的childOptions synchronized (childOptions) { currentChildOptions = childOptions.entrySet().toArray(EMPTY_OPTION_ARRAY); } //拿到用于配置childHandler的childAttrs final Entry<AttributeKey<?>, Object>[] currentChildAttrs = childAttrs.entrySet().toArray(EMPTY_ATTRIBUTE_ARRAY); p.addLast(new ChannelInitializer<Channel>() { @Override public void initChannel(final Channel ch) { final ChannelPipeline pipeline = ch.pipeline(); //拿到demo里调用childHandler方法时添加的handler ChannelHandler handler = config.handler(); if (handler != null) { pipeline.addLast(handler); } ch.eventLoop().execute(new Runnable() { @Override public void run() { //ServerBootstrapAcceptor是一个特殊的handler，用于处理新连接接入 pipeline.addLast(new ServerBootstrapAcceptor( ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); } }); } }); } 下篇 Netty服务端启动（二）——注册selector和端口绑定
