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服务端启动DEMO
先从一个简单的服务端启动`DEMO`开始，以下是一个标准的`Netty`服务端代码
public final class NettyServer {public static void main(String[] args) throws Exception { EventLoopGroup bossGroup = new NioEventLoopGroup(); EventLoopGroup workerGroup = new NioEventLoopGroup();try { ServerBootstrap serverBootstrap = new ServerBootstrap(); serverBootstrap.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) .childHandler(new ChannelInitializer() {@Overridepublic void initChannel(SocketChannel channel) { ChannelPipeline channelPipeline = channel.pipeline(); channelPipeline.addLast("decoder", new StringDecoder()); channelPipeline.addLast("encoder", new StringEncoder()); channelPipeline.addLast("handler", new ServerHandler()); } }); ChannelFuture channelFuture = serverBootstrap.bind(8888).sync(); channelFuture.channel().closeFuture().sync(); } finally { bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } }}br 注:ServerBootstrap.childHandler()用于指定处理新连接数据的读写处理逻辑，同时ServerBootstrap还提供handler()用于指定在服务端启动过程中的一些逻辑，通常情况下我们用不着这个方法
`ServerHandler`代码如下:
public class ServerHandler extends ChannelInboundHandlerAdapter { @Override public void channelActive(ChannelHandlerContext ctx) { System.out.println("channelActive"); } @Override public void channelRegistered(ChannelHandlerContext ctx) { System.out.println("channelRegistered"); } @Override public void handlerAdded(ChannelHandlerContext ctx) { System.out.println("handlerAdded"); } @Override public void channelReadComplete(ChannelHandlerContext ctx) throws Exception { System.out.println("channelReadComplete"); } @Override public void channelInactive(ChannelHandlerContext ctx) throws Exception { System.out.println("channelInactive"); } @Override public void channelRead(final ChannelHandlerContext ctx, Object msg) throws Exception { System.out.println("service receive msg:" + msg); }}br 当有新连接接入时，控制台打印出
reStructuredTexthandlerAddedchannelRegisteredchannelActivebr 但接收到新消息时，控制台打印出
reStructuredTextservice receive msg:xxxchannelReadCompletebr 本文主要分析服务端的启动过程，而新连接接入 新消息的读取会在后续章节中说明
服务端启动源码分析
`ServerBootstrap`是`Netty`为方便开发者使用而设计的一个启动类，其核心代码入口在`bind()`，代码如下
public ChannelFuture bind(int inetPort) {return bind(new InetSocketAddress(inetPort));}//通过端口号创建一个`InetSocketAddress`，然后继续调用重载的`bind()`public ChannelFuture bind(SocketAddress localAddress) {// ...return doBind(localAddress);}br 由于博客篇幅有限所以有些健壮性分支会以`// ...`略过，健壮性分支不会影响对`Netty`主流程的理解。
`Netty`服务端启动最后会调用到`doBind()`方法，代码如下
private ChannelFuture doBind(final SocketAddress localAddress) {//...final ChannelFuture regFuture = initAndRegister();//...final Channel channel = regFuture.channel();//... doBind0(regFuture, channel, localAddress, promise);//...return promise;br 在`doBind()`中我们关注两个核心方法`initAndRegister()`以及`doBind0()`
服务端`Channel`创建
final ChannelFuture initAndRegister() { Channel channel = null;// 新建Channel channel = channelFactory.newChannel();// 初始化Channelinit(channel);// 将这个Channel Register到某个对象 ChannelFuture regFuture = config().group().register(channel);return regFuture;}br `Channel`是`Netty`的核心概念之一，它是`Netty`网络通信的主体由它负责同对端进行网络通信、注册和数据操作等功能。
`Channel`的创建是由`channelFactory.newChannel()`完成的，接下来跟踪`channelFactory`是在何时被初始化，我们层层回溯最终发现是在这个函数中
public B channel(Class channelClass) {if (channelClass == null) {throw new NullPointerException("channelClass"); }return channelFactory(new ReflectiveChannelFactory(channelClass));}br 在`Demo`程序调用`.channel()`方法并传入`NioServerSocketChannel.class`
public class ReflectiveChannelFactory implements ChannelFactory {private final Class clazz;public ReflectiveChannelFactory(Class clazz) {if (clazz == null) {throw new NullPointerException("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); } }}br 即在`Netty`服务端启动的时候通过反射方式(调用默认构造函数)来创建一个`NioServerSocketChannel`对象，
加下来我们继续跟进`NioServerSocketChannel`的默认构造函数
public NioServerSocketChannel() {this(newSocket(DEFAULT_SELECTOR_PROVIDER));}private static ServerSocketChannel newSocket(SelectorProvider provider) {try {// 利用SelectorProvider产生一个ServerSocketChannel对象return provider.openServerSocketChannel(); } catch (IOException e) {throw new ChannelException("Failed to open a server socket.", e); }}br 通过`newSocket(DEFAULT_SELECTOR_PROVIDER)`创建一条`server`端`channel`，然后进入到以下方法
public NioServerSocketChannel(ServerSocketChannel channel) {super(null, channel, SelectionKey.OP_ACCEPT); config = new NioServerSocketChannelConfig(this, javaChannel().socket());}br 该方法主要完成两个功能，首先是调用父类的构造方法然后初始化`NioServerSocketChannelConfig`属性
protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent, ch, readInterestOp);}protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {super(parent);this.ch = ch;// 设置SelectionKey.OP_ACCEPT事件this.readInterestOp = readInterestOp;// 设置ServerSocketChannel为非阻塞的 ch.configureBlocking(false);}br 这里将前面通过`provider.openServerSocketChannel()`创建出来的`ServerSocketChannel`保存到成员变量，然后调用将该`channel`为非阻塞模式，这是个标准的`JDK NIO`编程的玩法。这里的`readInterestOp`即前面层层传入的`SelectionKey.OP_ACCEPT`，接下来继续跟进`super(parent)`
protected AbstractChannel(Channel parent) {this.parent = parent; id = newId();unsafe = newUnsafe(); pipeline = newChannelPipeline();}br 在`AbstractChannel`的构造方法中主要是初始化了`id`，`unsafe`，`pipeline`属性
服务端`Channel`初始化
在创建完`Channel`后，我们在`init`方法中对`Channel`进行初始化操作，代码如下
void init(Channel channel) throws Exception {// 给channel设置optionfinal Map, Object> options = options0();synchronized (options) { channel.config().setOptions(options); }// 给channel设置attrfinal Map, Object> attrs = attrs0();synchronized (attrs) {for (Entry, Object> e: attrs.entrySet()) { AttributeKey key = (AttributeKey) e.getKey(); channel.attr(key).set(e.getValue()); } } ChannelPipeline p = channel.pipeline();final EventLoopGroup currentChildGroup = childGroup;final ChannelHandler currentChildHandler = childHandler;final Entry, Object>[] currentChildOptions;final Entry, Object>[] currentChildAttrs;synchronized (childOptions) {// 设置新接入channel的options currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size())); }synchronized (childAttrs) {// 设置新接入channel的attrs currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size())); } p.addLast(new ChannelInitializer() {@Overridepublic void initChannel(Channel ch) throws Exception {final ChannelPipeline pipeline = ch.pipeline();// 设置handler到pipeline上// 这里的handler()返回的就是.handler()所设置的值 ChannelHandler handler = config.handler();if (handler != null) { pipeline.addLast(handler); } ch.eventLoop().execute(new Runnable() {@Overridepublic void run() {// p.addLast()向serverChannel的流水线处理器中加入了一个ServerBootstrapAcceptor// 从名字上就可以看出来这是一个接入器，专门接受新请求，把新的请求扔给某个事件循环器 pipeline.addLast(new ServerBootstrapAcceptor(currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); } }); } });}br 以上代码主要完成如下功能:
为`channel`设置`option`及`attr` 初始化服务端`channel`的`pipeline` 添加`ServerBootstrapAcceptor`(请求接入器) `NioEventLoop.execute()`方法为`Netty Reactor`线程执行的入口，关于`Netty Reactor`线程我们将在下一篇中介绍。我们总结一下发现代码执行到这里`Netty`并未真正启动服务，只是初始化了一些基本的配置和属性以及在`pipeline`上加入了一个接入器用来专门接受新连接 ChannelFuture regFuture = config().group().register(channel);```此处`config().group()`返回的对象为`NioEventLoopGroup````java@Overridepublic ChannelFuture register(Channel channel) {// 调用了NioEventLoop对象中的register方法// NioEventLoop extends SingleThreadEventLoopreturn next().register(channel);}br 在`next`方法中返回一个`EventLoop`对象，每一个`EventLoop`都与一个`selector`绑定 在之前的代码中`EventLoop`中的`Selector`一直没有任何`Channel`注册，所以每次`select`操作都是空，但从这行代码开始这个`selector`中开始有`Channel`注册 public ChannelFuture register(Channel channel) { return register(new DefaultChannelPromise(channel, this));}public ChannelFuture register(final ChannelPromise promise) { ObjectUtil.checkNotNull(promise, "promise"); promise.channel().unsafe().register(this, promise); return promise;}br 这里可以看到`register`操作是委托给`Channel`中的`Unsafe`对象来执行的，这里的`Unsafe`对象对上文稍有印象的同学应该能知道这个就是创建`NioServerSocketChannel`的时候创建的`Unsafe`对象 public final void register(EventLoop eventLoop, final ChannelPromise promise) {// ... AbstractChannel.this.eventLoop = eventLoop;// ... register0(promise);}br 先将`EventLoop`事件循环器绑定到该`NioServerSocketChannel`上，然后调用`register0()`代码如下 private void register0(ChannelPromise promise) { try { boolean firstRegistration = neverRegistered; doRegister(); neverRegistered = false; registered = true; pipeline.invokeHandlerAddedIfNeeded(); safeSetSuccess(promise); pipeline.fireChannelRegistered(); if (isActive()) { if (firstRegistration) { pipeline.fireChannelActive(); } else if (config().isAutoRead()) { beginRead(); } } } catch (Throwable t) { closeForcibly(); closeFuture.setClosed(); safeSetFailure(promise, t); }}//这一段其实也很清晰，先调用`doRegister()`进行注册protected void doRegister() throws Exception { boolean selected = false; for (;;) { try { selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this); return; } catch (CancelledKeyException e) { // ... } }}br 在这里我们终于看到`JDK`底层的`Channel`注册到`Selector`的过程，但是这里的`OPS`为0即不关心任何事件，而我们期望`OPS`的值为`SelectionKey.OP_ACCEPT`，所以到了这里代码还没有结束。在执行完`Channel`注册后接着执行了几个`pipeline`相关的方法，我们后面详细剖析`pipeline`的时候再讲 private static void doBind0(final ChannelFuture regFuture, final Channel channel, final SocketAddress localAddress, final ChannelPromise promise) { 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()); } } });}//在`dobind0()`方法中通过`EventLoop`执行一个任务，接下来我们进入到`channel.bind()`方法public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {return pipeline.bind(localAddress, promise);}public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {return tail.bind(localAddress, promise);}br 关于`Pipeline`相关的内容将在后续博客中介绍，当前一个比较好的方式就是`Debug`单步进入。 跟踪调用链最后我们来到了`DefaultChannelPipeline.HeadContext`的`bind()`，代码如下 public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception {unsafe.bind(localAddress, promise);}//这里的`unsafe`就是前面提到的`AbstractUnsafe`, 准确点应该是`NioMessageUnsafe`@Overridepublic final void bind(final SocketAddress localAddress, final ChannelPromise promise) {// ... boolean wasActive = isActive();// ... doBind(localAddress);if (!wasActive && isActive()) { invokeLater(new Runnable() { @Overridepublic void run() { pipeline.fireChannelActive(); } }); } safeSetSuccess(promise);}//在`doBind`方法中完成绑定操作，代码如下protected void doBind(SocketAddress localAddress) throws Exception {if (PlatformDependent.javaVersion() >= 7) { javaChannel().bind(localAddress, config.getBacklog()); } else { javaChannel().socket().bind(localAddress, config.getBacklog()); }}br 最终调用到了`JDK`里面的`bind`方法真正进行了端口的绑定。按照正常流程我们前面已经分析到`isActive()`方法返回`false`，进入到`doBind()`之后如果`channel`被激活了，就发起`pipeline.fireChannelActive()`调用
public void channelActive(ChannelHandlerContext ctx) throws Exception { ctx.fireChannelActive(); readIfIsAutoRead();}//`pipeline.channelActive`会逐一调用`pipeline`中每一个节点的`channelActive`方法，所以`HeadContext`的`channelActive`将会被调用，即`readIfIsAutoRead`方法将会被调用public void channelActive(ChannelHandlerContext ctx) throws Exception { ctx.fireChannelActive(); readIfIsAutoRead();}private void readIfIsAutoRead() {if (channel.config().isAutoRead()) { channel.read(); }}//最终这个方法会调用到`AbstractNioChannel`的`doBeginRead`方法protected void doBeginRead() throws Exception {final SelectionKey selectionKey = this.selectionKey;if (!selectionKey.isValid()) {return; } readPending = true;final int interestOps = selectionKey.interestOps();if ((interestOps & readInterestOp) == 0) { selectionKey.interestOps(interestOps | readInterestOp); }}br 即在上文中提到的`SelectionKey.OP_ACCEPT`，即让事件轮询器关注`Accept`事件，至此完成了`Channel`对`ACCEPT`事件的注册过程 到目前为止我们看到的代码相当于传统NIO编程中的如下代码 ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();// 建NioServerSocketChannelserverSocketChannel.configureBlocking(false);//AbstractNioChannel中ch.configureBlocking(false);serverSocketChannel.bind(new InetSocketAddress("localhost", 8888));// NioServerSocketChannel.doBind()Selector selector = Selector.open();// NioEventLoop.openSelector()serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT) // AbstractNioChannel.doBeginRead()br 服务端启动完成的主要功能为创建一个`Channel`，并且将`Channel`注册到`NioEventLoop`的`Selector`上. “Netty是如何绑定端口和启动服务的”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注创新互联网站，小编将为大家输出更多高质量的实用文章！
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