本文我们就开始对 ServerBootstrap 进行源码解读（4.1.51.Final-SNAPSHOT），为什么是 ServerBootstrap，记得在用 Netty 做第一个项目的时候，写的第一行 Code 就是 new ServerBootstrap()，ServerBootstrap 是 Netty Server 的启动类，所以从它开始了解 Netty 是最合适的。
ServerBootstrap
## TcpServer.java
private final EventLoopGroup bossGroup = new NioEventLoopGroup(BIZ_GROUP_SIZE);private final EventLoopGroup workerGroup = new NioEventLoopGroup(BIZ_THREAD_SIZE);
public void init() throws Exception { // Server 服务启动 ServerBootstrap bootstrap = new ServerBootstrap();
bootstrap.group(bossGroup, workerGroup); bootstrap.channel(NioServerSocketChannel.class); bootstrap.childHandler(new ServerChannelInitializer(serverConfig)); // 可选参数 bootstrap.childOption(ChannelOption.WRITE_BUFFER_HIGH_WATER_MARK, 32 * 1024);
// 绑定接口，同步等待成功 ChannelFuture future = bootstrap.bind(port).sync(); ChannelFuture channelFuture = future.addListener(new ChannelFutureListener() { public void operationComplete(ChannelFuture future) throws Exception { } });}
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这是我在做 TCP 网关时写的 Netty Server 的代码片段（https://github.com/SongranZhang/tcp-gateway/blob/master/src/main/java/com/linkedkeeper/tcp/connector/tcp/server/TcpServer.java），可以看到，Netty Server 的初始化首先是通过 ServerBootstrap 的无参构造函数创建一个对象，接着是这个对象的一串链式调用 bootstrap.group().channel().childHandler().childOption()，而服务启动的真正触发点是这段 bootstrap.bind(port).sync()，下面我们就逐一来分析下这里的每个方法。
首先是 group() 方法。
## ServerBootstrap.java
private volatile EventLoopGroup childGroup;
public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) { super.group(parentGroup); if (this.childGroup != null) { throw new IllegalStateException("childGroup set already"); } this.childGroup = ObjectUtil.checkNotNull(childGroup, "childGroup"); return this;}
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这里 workerGroup 赋值给了 ServerBootstrap 的 childGroup，bossGroup 赋值给了父类 AbstractBootstrap 的 group。
## AbstractBootstrap.java
volatile EventLoopGroup group;
public B group(EventLoopGroup group) { ObjectUtil.checkNotNull(group, "group"); if (this.group != null) { throw new IllegalStateException("group set already"); } this.group = group; return self();}
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接下来是 channel() 方法。
## AbstractBootstrap.java
private volatile ChannelFactory<? extends C> channelFactory;
public B channel(Class<? extends C> channelClass) { return channelFactory(new ReflectiveChannelFactory<C>( ObjectUtil.checkNotNull(channelClass, "channelClass") ));}
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();}
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这里 NioServerSocketChannel.class 通过 ReflectiveChannelFactory 进行了实例化，然后赋值给了 AbstractBootstrap 的 channelFactory。
接下来是 childHandler() 方法。
## ServerBootstrap.java
private volatile ChannelHandler childHandler;
public ServerBootstrap childHandler(ChannelHandler childHandler) { this.childHandler = ObjectUtil.checkNotNull(childHandler, "childHandler"); return this;}
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这里是对 ServerBootstrap 的 childHandler 赋值。
最后是 childOption() 方法。
## ServerBootstrap.java
private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap<ChannelOption<?>, Object>();
public <T> ServerBootstrap childOption(ChannelOption<T> childOption, T value) { ObjectUtil.checkNotNull(childOption, "childOption"); synchronized (childOptions) { if (value == null) { childOptions.remove(childOption); } else { childOptions.put(childOption, value); } } return this;}
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这里 childOptions 维护了 TCP 的参数设置。
简言之 bootstrap.group().channel().childHandler().childOption() 就是在构建 Netty Server 的各种参数，下面再来看 bootstrap.bind(port).sync()。
首先是 bind() 方法。
## AbstractBootstrap.java
public ChannelFuture bind(int inetPort) { return bind(new InetSocketAddress(inetPort));}
public ChannelFuture bind(SocketAddress localAddress) { validate(); return doBind(ObjectUtil.checkNotNull(localAddress, "localAddress"));}
public B validate() { if (group == null) { throw new IllegalStateException("group not set"); } if (channelFactory == null) { throw new IllegalStateException("channel or channelFactory not set"); } return self();}
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这里的 validate() 方法对 AbstractBootstrap 的 group 和 channelFactory 进行非空校验，之后调用 doBind() 方法。
## AbstractBootstrap.java
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; }}
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首先看一下 initAndRegister() 方法。
## AbstractBootstrap.java
final ChannelFuture initAndRegister() { Channel channel = null; try { channel = channelFactory.newChannel(); init(channel); } catch (Throwable t) { if (channel != null) { // channel can be null if newChannel crashed (eg SocketException("too many open files")) channel.unsafe().closeForcibly(); // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t); } // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor return new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE).setFailure(t); }
ChannelFuture regFuture = config().group().register(channel); if (regFuture.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 because 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 regFuture;}
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这里 channelFactory.newChannel() 调用的是 ReflectiveChannelFactory 的 newChannel 方法。
## ReflectiveChannelFactory
private final Constructor<? extends T> constructor;
public ReflectiveChannelFactory(Class<? extends T> clazz) {
ObjectUtil.checkNotNull(clazz, "clazz"); try { this.constructor = clazz.getConstructor(); } catch (NoSuchMethodException e) { throw new IllegalArgumentException("Class " + StringUtil.simpleClassName(clazz) + " does not have a public non-arg constructor", e); }}
public T newChannel() { try { return constructor.newInstance(); } catch (Throwable t) { throw new ChannelException("Unable to create Channel from class " + constructor.getDeclaringClass(), t); }}
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这里 constructor.newInstance() 是 NioServerSocketChannel.class 的一个实例。得到 channel 后，调用 init(channel) 进行初始化，一是给 options 和 attrs 赋值，二是构建 pipeline。
## ServerBootstrap.java
void init(Channel channel) { setChannelOptions(channel, newOptionsArray(), logger); 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; synchronized (childOptions) { currentChildOptions = childOptions.entrySet().toArray(EMPTY_OPTION_ARRAY); } 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(); ChannelHandler handler = config.handler(); if (handler != null) { pipeline.addLast(handler); }
ch.eventLoop().execute(new Runnable() { @Override public void run() { pipeline.addLast(new ServerBootstrapAcceptor( ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); } }); } });}
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回到 initAndRegister() 方法中，init(channel) 之后是 register(channel)，该方法在 NioEventLoopGroup 的父类 MultithreadEventLoopGroup 中实现，我们在解读 NioEventLoop 源码时再分析。
## MultithreadEventLoopGroup
public ChannelFuture register(Channel channel) { return next().register(channel);}
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看完 initAndRegister()，再回到 doBind() 接着看 doBind0()。
## AbstractBootstrap
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() { @Override public void run() { if (regFuture.isSuccess()) { channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE); } else { promise.setFailure(regFuture.cause()); } } });}
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这里 regFuture.isSuccess() 会执行 channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);，否者执行 promise.setFailure(regFuture.cause());，这里的 promise 可以认为是一种特殊的 Future 对象。bind 是在 ChannelPipeline 里进行绑定的，我们在解读 ChannelPipeline 源码时再分析。
最后看一下 bootstrap.bind(serverPort).sync() 中的 sync()，bootstrap.bind(serverPort) 返回的是 ChannelFuture，所以 sync() 是调用 DefaultChannelPromise 的方法。
## DefaultChannelPromise
public ChannelPromise sync() throws InterruptedException { super.sync(); return this;}
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这里 super.sync(); 调用了父类的方法。
## DefaultPromise
public Promise<V> sync() throws InterruptedException { await(); rethrowIfFailed(); return this;}
public Promise<V> await() throws InterruptedException { if (isDone()) { return this; }
if (Thread.interrupted()) { throw new InterruptedException(toString()); }
checkDeadLock();
synchronized (this) { while (!isDone()) { incWaiters(); try { wait(); } finally { decWaiters(); } } } return this;}
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这里 while(!isDone()) 会进入循环，调用 sync() 后线程会被阻塞住。
总结
本篇也是写了好久，本文介绍了 ServerBootstrap，它是构建 Netty Server 的主要实现类，ServerBootstrap 里主要是对各种属性进行赋值，并创建 Channel 和 ChannelPipeline，最后绑定本地端口开始监听 IO 事件。
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