Bootstrap的意思就是引导，辅助的意思，在编写服务端或客户端程序时，我们都需要先new一个bootstrap，然后基于这个bootstrap调用函数，添加eventloop和handler，可见对bootstrap进行分析还是有必要的。

1、bootstrap结构图

bootstrap的结构比较简单，涉及的类和接口很少，如下图所示，其中Bootstrap则是客户端程序用的引导类，ServerBootstrap是服务端程序用的引导类。

2、serverbootstrap分析

这部分，专门对serverbootstrap进行分析，bootstrap过程大同小异就不作详细的分析了。下面是我们编写服务端代码的一般化过程，整个分析过程将基于下面这段代码中用到的函数进行。

// Configure the bootstrap.EventLoopGroup bossGroup = new NioEventLoopGroup();EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap b = new ServerBootstrap();b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).childHandler(new HexDumpProxyInitializer(remoteHost, remotePort)).childOption(ChannelOption.AUTO_READ, false).bind(localPort).sync().channel().closeFuture().sync();} finally {bossGroup.shutdownGracefully();workerGroup.shutdownGracefully();}

先看关键代码（注意这里面的部分函数是在AbstractBootstrap中定义的）

    private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap<ChannelOption<?>, Object>();private final Map<AttributeKey<?>, Object> childAttrs = new LinkedHashMap<AttributeKey<?>, Object>();private volatile EventLoopGroup childGroup;private volatile ChannelHandler childHandler;/*** Set the {@link EventLoopGroup} for the parent (acceptor) and the child (client). These* {@link EventLoopGroup}'s are used to handle all the events and IO for {@link SocketChannel} and* {@link Channel}'s.*/public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {super.group(parentGroup);if (childGroup == null) {throw new NullPointerException("childGroup");}if (this.childGroup != null) {throw new IllegalStateException("childGroup set already");}this.childGroup = childGroup;return this;}

属性值ChildGroup，ChildHandler，是用来处理accpt的Channel的。group函数其实就是将parentGroup和ChildGroup进行赋值，其中parentGroup用于处理accept事件，ChildGroup用于处理accpt的Channel的IO事件。

    //channel函数的实现定义在抽象父类中，其实就是通过newInstance函数生成一个具体的channel对象。
<pre name="code" class="java">    /*** The {@link Class} which is used to create {@link Channel} instances from.* You either use this or {@link #channelFactory(ChannelFactory)} if your* {@link Channel} implementation has no no-args constructor.*/public B channel(Class<? extends C> channelClass) {if (channelClass == null) {throw new NullPointerException("channelClass");}return channelFactory(new BootstrapChannelFactory<C>(channelClass));}/*** {@link ChannelFactory} which is used to create {@link Channel} instances from* when calling {@link #bind()}. This method is usually only used if {@link #channel(Class)}* is not working for you because of some more complex needs. If your {@link Channel} implementation* has a no-args constructor, its highly recommend to just use {@link #channel(Class)} for* simplify your code.*/@SuppressWarnings("unchecked")public B channelFactory(ChannelFactory<? extends C> channelFactory) {if (channelFactory == null) {throw new NullPointerException("channelFactory");}if (this.channelFactory != null) {throw new IllegalStateException("channelFactory set already");}this.channelFactory = channelFactory;return (B) this;}<pre name="code" class="java">    private static final class BootstrapChannelFactory<T extends Channel> implements ChannelFactory<T> {private final Class<? extends T> clazz;BootstrapChannelFactory(Class<? extends T> clazz) {this.clazz = clazz;}@Overridepublic T newChannel() {try {return clazz.newInstance();} catch (Throwable t) {throw new ChannelException("Unable to create Channel from class " + clazz, t);}}@Overridepublic String toString() {return clazz.getSimpleName() + ".class";}}

Channel函数比较简单，其实就是通过newInstance函数，生成一个具体的Channel对象，例如服务端的NioServerSocketChannel。

    /*** Set the {@link ChannelHandler} which is used to serve the request for the {@link Channel}'s.*/public ServerBootstrap childHandler(ChannelHandler childHandler) {if (childHandler == null) {throw new NullPointerException("childHandler");}this.childHandler = childHandler;return this;}

上面的函数即给serverbootstrap的childHandler赋值。

    /*** Allow to specify a {@link ChannelOption} which is used for the {@link Channel} instances once they get created* (after the acceptor accepted the {@link Channel}). Use a value of {@code null} to remove a previous set* {@link ChannelOption}.*/public <T> ServerBootstrap childOption(ChannelOption<T> childOption, T value) {if (childOption == null) {throw new NullPointerException("childOption");}if (value == null) {synchronized (childOptions) {childOptions.remove(childOption);}} else {synchronized (childOptions) {childOptions.put(childOption, value);}}return this;}

上面的函数是指定accpt的channel的属性，channel有很多属性，比如SO_TIMEOUT时间，Buf长度等等。

    /*** Create a new {@link Channel} and bind it.*/public ChannelFuture bind() {validate();SocketAddress localAddress = this.localAddress;if (localAddress == null) {throw new IllegalStateException("localAddress not set");}return doBind(localAddress);}/*** Create a new {@link Channel} and bind it.*/public ChannelFuture bind(int inetPort) {return bind(new InetSocketAddress(inetPort));}/*** Create a new {@link Channel} and bind it.*/public ChannelFuture bind(String inetHost, int inetPort) {return bind(new InetSocketAddress(inetHost, inetPort));}<pre name="code" class="java">    /*** Create a new {@link Channel} and bind it.*/public ChannelFuture bind(SocketAddress localAddress) {validate();if (localAddress == null) {throw new NullPointerException("localAddress");}return doBind(localAddress);}private ChannelFuture doBind(final SocketAddress localAddress) {final ChannelFuture regPromise = initAndRegister();final Channel channel = regPromise.channel();final ChannelPromise promise = channel.newPromise();if (regPromise.isDone()) {doBind0(regPromise, channel, localAddress, promise);} else {regPromise.addListener(new ChannelFutureListener() {@Overridepublic void operationComplete(ChannelFuture future) throws Exception {doBind0(future, channel, localAddress, promise);}});}return promise;}<pre name="code" class="java">    private static void doBind0(final ChannelFuture regFuture, final Channel channel,final SocketAddress localAddress, final ChannelPromise promise) {// This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up// the pipeline in its channelRegistered() implementation.channel.eventLoop().execute(new Runnable() {@Overridepublic void run() {if (regFuture.isSuccess()) {channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);} else {promise.setFailure(regFuture.cause());}}});}

Bind函数层层调用过来之后，最后就调用Channel的bind函数了，下面再看channel的bind函数是如何处理的。定义在AbstractChannel中：

    @Overridepublic ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {return pipeline.bind(localAddress, promise);}

channel的bind函数，最终就是调用pipeline的bind，而pipeline的bind实际上就是调用contexthandler的bind，之个之前分析write和flush的时候说过了。所以这里直接看contexthandler的bind函数。下面是定义：

    @Overridepublic ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {if (localAddress == null) {throw new NullPointerException("localAddress");}validatePromise(promise, false);final DefaultChannelHandlerContext next = findContextOutbound();EventExecutor executor = next.executor();if (executor.inEventLoop()) {next.invokeBind(localAddress, promise);} else {executor.execute(new Runnable() {@Overridepublic void run() {next.invokeBind(localAddress, promise);}});}return promise;}<pre name="code" class="java">    private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {try {((ChannelOutboundHandler) handler).bind(this, localAddress, promise);} catch (Throwable t) {notifyOutboundHandlerException(t, promise);}}

最终调用Handler的bind函数，还记得之前说的outbound类型的事件吗，这类事件提供了默认的实现方法，HeadHandler的bind函数，下面是它的定义：

        @Overridepublic void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)throws Exception {unsafe.bind(localAddress, promise);}

我们又看到了unsafe这个苦力了，最终的操作还是得由它来完成啊，赶紧去看看这个bind函数吧，

        @Overridepublic final void bind(final SocketAddress localAddress, final ChannelPromise promise) {if (!ensureOpen(promise)) {return;}// See: https://github.com/netty/netty/issues/576if (!PlatformDependent.isWindows() && !PlatformDependent.isRoot() &&Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&localAddress instanceof InetSocketAddress &&!((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress()) {// Warn a user about the fact that a non-root user can't receive a// broadcast packet on *nix if the socket is bound on non-wildcard address.logger.warn("A non-root user can't receive a broadcast packet if the socket " +"is not bound to a wildcard address; binding to a non-wildcard " +"address (" + localAddress + ") anyway as requested.");}boolean wasActive = isActive();try {doBind(localAddress);} catch (Throwable t) {closeIfClosed();promise.setFailure(t);return;}if (!wasActive && isActive()) {invokeLater(new Runnable() {@Overridepublic void run() {pipeline.fireChannelActive();}});}promise.setSuccess();}

上面的代码最终调用了Channel的doBind函数，这里我们的Channel是NioServerSocketChannel，所以最终就是调用它的bind函数了，代码如下

    @Overrideprotected void doBind(SocketAddress localAddress) throws Exception {javaChannel().socket().bind(localAddress, config.getBacklog());}

其实它最终也是调用了JDK的Channel的socket bind函数。

看到这里，你是否会觉得有点怪异，为什么没有注册accpt事件啊，一般的我们的server socket都是要注册accpt事件到selector，用于监听连接。如果你发现了这个问题，说明你是理解socket的编程的，^_^。实际上是前面在分析bind的时候我们漏掉了一个重要的函数，initAndRegister，下面再来看看它的定义：

    final ChannelFuture initAndRegister() {final Channel channel = channelFactory().newChannel();try {init(channel);} catch (Throwable t) {channel.unsafe().closeForcibly();return channel.newFailedFuture(t);}ChannelPromise regPromise = channel.newPromise();group().register(channel, regPromise);if (regPromise.cause() != null) {if (channel.isRegistered()) {channel.close();} else {channel.unsafe().closeForcibly();}}// If we are here and the promise is not failed, it's one of the following cases:// 1) If we attempted registration from the event loop, the registration has been completed at this point.//    i.e. It's safe to attempt bind() or connect() now beause the channel has been registered.// 2) If we attempted registration from the other thread, the registration request has been successfully//    added to the event loop's task queue for later execution.//    i.e. It's safe to attempt bind() or connect() now://         because bind() or connect() will be executed *after* the scheduled registration task is executed//         because register(), bind(), and connect() are all bound to the same thread.return regPromise;}

在这里，我们看到了我们之前介绍event时说的register函数，它就是用于将Channel注册到eventloop中去的。eventloop经过层层调用，最终调用了SingleThreadEventLoop类中的register函数，下面是它的定义：

    @Overridepublic ChannelFuture register(final Channel channel, final ChannelPromise promise) {if (channel == null) {throw new NullPointerException("channel");}if (promise == null) {throw new NullPointerException("promise");}channel.unsafe().register(this, promise);return promise;}

还是逃离不了unsafe对象的调用，前面说了那么多的unsafe，这个函数猜都可以猜测出执行过程了，这里就不细细的列举代码了。

还有一个init函数，这里需要说明一下，代码如下：

    @Overridevoid init(Channel channel) throws Exception {final Map<ChannelOption<?>, Object> options = options();synchronized (options) {channel.config().setOptions(options);}final Map<AttributeKey<?>, Object> attrs = attrs();synchronized (attrs) {for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {@SuppressWarnings("unchecked")AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();channel.attr(key).set(e.getValue());}}ChannelPipeline p = channel.pipeline();if (handler() != null) {p.addLast(handler());}final EventLoopGroup currentChildGroup = childGroup;final ChannelHandler currentChildHandler = childHandler;final Entry<ChannelOption<?>, Object>[] currentChildOptions;final Entry<AttributeKey<?>, Object>[] currentChildAttrs;synchronized (childOptions) {currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));}synchronized (childAttrs) {currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));}p.addLast(new ChannelInitializer<Channel>() {@Overridepublic void initChannel(Channel ch) throws Exception {ch.pipeline().addLast(new ServerBootstrapAcceptor(currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));}});}

它就是用来处理channel 的pipeline，并添加一个ServerBootstrapAcceptor的handler，继续看看这个handler的定义，我们就会明白它的意图。

    private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {private final EventLoopGroup childGroup;private final ChannelHandler childHandler;private final Entry<ChannelOption<?>, Object>[] childOptions;private final Entry<AttributeKey<?>, Object>[] childAttrs;@SuppressWarnings("unchecked")ServerBootstrapAcceptor(EventLoopGroup childGroup, ChannelHandler childHandler,Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {this.childGroup = childGroup;this.childHandler = childHandler;this.childOptions = childOptions;this.childAttrs = childAttrs;}@Override@SuppressWarnings("unchecked")public void channelRead(ChannelHandlerContext ctx, Object msg) {Channel child = (Channel) msg;child.pipeline().addLast(childHandler);for (Entry<ChannelOption<?>, Object> e: childOptions) {try {if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {logger.warn("Unknown channel option: " + e);}} catch (Throwable t) {logger.warn("Failed to set a channel option: " + child, t);}}for (Entry<AttributeKey<?>, Object> e: childAttrs) {child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());}try {childGroup.register(child);} catch (Throwable t) {child.unsafe().closeForcibly();logger.warn("Failed to register an accepted channel: " + child, t);}}@Overridepublic void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {final ChannelConfig config = ctx.channel().config();if (config.isAutoRead()) {// stop accept new connections for 1 second to allow the channel to recover// See https://github.com/netty/netty/issues/1328config.setAutoRead(false);ctx.channel().eventLoop().schedule(new Runnable() {@Overridepublic void run() {config.setAutoRead(true);}}, 1, TimeUnit.SECONDS);}// still let the exceptionCaught event flow through the pipeline to give the user// a chance to do something with itctx.fireExceptionCaught(cause);}}

上面就是这个handler的全部代码，它重写了ChannelRead函数，它的目的其实是想将server端accept的channel注册到ChildGroup的eventloop中，这样就可以理解，服务端代码workerGroup这个eventloop的作用了，它终于在这里体现出了它的作用了。

3、总结

这篇文章主要是分析了serverbootstrap的全过程，通过对这个的分析，我们清晰的看到了平时编写socket服务端代码时对bind，register事件，以及accept channel等的处理。

http://blog.csdn.net/pingnanlee/article/details/11973769