Netty-ServerBootstrap

ServerBootstrap 爲 netty 創建服務端的輔助類, 以 NIO爲例，建立代碼以下：java

public static void main(String[] args) throws Exception { ServerBootstrap bs = new ServerBootstrap(); bs.group(new NioEventLoopGroup(1), new NioEventLoopGroup()) .channel(NioServerSocketChannel.class) .childHandler(new ChannelInitializer<Channel>() { @Override protected void initChannel(Channel ch) throws Exception { ch.pipeline() .addLast(new HttpServerCodec()) .addLast(new HttpObjectAggregator(65535)) .addLast(new Controller()); } }).bind(8080).sync().channel().closeFuture().sync(); }

核心參數以下：git

 //配置屬性，如 SO_KEEPALIVE 等private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);
 //acceot 的 子channel所綁定的 事件循環組" private volatile EventLoopGroup childGroup; private volatile ChannelHandler childHandler;

初始化流程主要爲綁定本地端口 -> 註冊自身到 EventLoop ，並註冊 accept 和 read 事件 -> EventLoop的主循環中會不斷的select註冊的channel的事件，並處理。github

首先執行綁定，核心邏輯位於 io.netty.bootstrap.AbstractBootstrap.doBind(SocketAddress) 和 io.netty.bootstrap.AbstractBootstrap.initAndRegister()中bootstrap

private ChannelFuture doBind(final SocketAddress localAddress) { final ChannelFuture regFuture = initAndRegister(); ..........if (regFuture.isDone()) { // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();
 //綁定邏輯 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; } }

先來看 initAndRegister ，核心邏輯就是利用channelFactory初始化一個NioServerSocketChannel實例，併爲其設置上config中的參數，而後將其註冊到EventLoop中，其實是委託的channel的Unsafe來實現註冊的，核心邏輯位於 AbstractUnsafe.register0 中完成註冊promise

final ChannelFuture initAndRegister() { Channel channel = null; try {
 //本例子中實際調用的是 NioServerSocketChannel的構造參數， 併爲其設置感興趣的事件類型爲 OP_ACCEPT 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); } ChannelFuture regFuture = config().group().register(channel); if (regFuture.cause() != null) { if (channel.isRegistered()) { channel.close(); } else { channel.unsafe().closeForcibly(); } }  return regFuture; }

1 void init(Channel channel) throws Exception { 2         //設置屬性
15 .......... 17         p.addLast(new ChannelInitializer<Channel>() { 30 @Override 31             public void initChannel(final Channel ch) throws Exception { 32                 final ChannelPipeline pipeline = ch.pipeline(); 33                 ChannelHandler handler = config.handler(); 34                 if (handler != null) { 35 pipeline.addLast(handler); 36 } 37 
38                 ch.eventLoop().execute(new Runnable() { 39 @Override 40                     public void run() { //爲NioServerSocketChannel 設置一個 默認的 channelhandler ： ServerBootstrapAcceptor ， 當發生 accept事件時，將 accept的channel註冊到 childEventLoop中
41                         pipeline.addLast(new ServerBootstrapAcceptor( 42 ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); 43 } 44 }); 45 } 46 }); 47     }

private void register0(ChannelPromise promise) { try { // check if the channel is still open as it could be closed in the mean time when the register // call was outside of the eventLoop
                if (!promise.setUncancellable() || !ensureOpen(promise)) { return; } boolean firstRegistration = neverRegistered;
 //執行channel到 eventloop的 selector doRegister(); neverRegistered = false; registered = true; // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the // user may already fire events through the pipeline in the ChannelFutureListener.
 pipeline.invokeHandlerAddedIfNeeded(); safeSetSuccess(promise);

 //觸發 InboundChannelHnader.channelRegistered 事件
 pipeline.fireChannelRegistered();

 // Only fire a channelActive if the channel has never been registered. This prevents firing // multiple channel actives if the channel is deregistered and re-registered. if (isActive()) { if (firstRegistration) {
 //觸發channelActive事件，並會爲 channel 綁定上 read 事件 pipeline.fireChannelActive(); } else if (config().isAutoRead()) { // This channel was registered before and autoRead() is set. This means we need to begin read // again so that we process inbound data. // // See https://github.com/netty/netty/issues/4805  beginRead(); } } } catch (Throwable t) { // Close the channel directly to avoid FD leak.  closeForcibly(); closeFuture.setClosed(); safeSetFailure(promise, t); } }

initAndRegister註冊成功後,開始執行真正的綁定端口邏輯，核心邏輯位於 NioSocketChannel.doBind0(SocketAddress) 中socket

private void doBind0(SocketAddress localAddress) throws Exception { if (PlatformDependent.javaVersion() >= 7) { SocketUtils.bind(javaChannel(), localAddress); } else { SocketUtils.bind(javaChannel().socket(), localAddress); } }

至此綁定個成功，當觸發 ACCEPT 事件時，會觸發 NioServerSocketChannel.doReadMessages -> ServerBootstrapAcceptor.channelRead , 並將子channel 註冊到 childEventLoop中ide

public void channelRead(ChannelHandlerContext ctx, Object msg) { final Channel child = (Channel) msg; child.pipeline().addLast(childHandler); setChannelOptions(child, childOptions, logger); for (Entry<AttributeKey<?>, Object> e: childAttrs) { child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue()); } try {
 //註冊channel childGroup.register(child).addListener(new ChannelFutureListener() { @Override public void operationComplete(ChannelFuture future) throws Exception { if (!future.isSuccess()) { forceClose(child, future.cause()); } } }); } catch (Throwable t) { forceClose(child, t); } }