1. Netty源码研究笔记（1）——开篇
1.1. Netty介绍
Netty是一个老牌的高性能网络框架。在众多开源框架中都有它的身影，比如：grpc、dubbo、seata等。
里面有着非常多值得学的东西：
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I/O模型
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内存管理
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各种网络协议的实现：http、redis、websocket等等
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各种各样有趣的技巧的实现：异步、时间轮、池化、内存泄露探测等等。
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代码风格、设计思想、设计原则等。
1.2. 源码分析方法
我一般是这样进行源码分析的：
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首先是纵向，通过官方提供的demo，进行debug，并记录在一个完整的生命周期下的调用链上，会涉及到哪些组件。
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然后对涉及到的组件拿出来，找出它们的顶层定义（接口、抽象类）。通过其模块/包的划分、类注释、定义的方法及其注释，来大致知晓每个组件是做什么的，以及它们在整个框架中的位置是怎样的。
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第二步完成后，就可以对第一步的调用链流程、步骤、涉及到的组件，进行归纳、划分，从而做到心中有数，知道东南西北了。
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之后就是横向，对这些归纳出来的组件体系，逐个进行分析。
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在分析每个组件体系的时候，也是按照先纵向，再横向的步骤：
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首先是纵向：找出该组件体系中的核心顶层接口、类，然后结合其的所有实现类，捋出继承树，然后弄清楚每个类做的是啥，它是怎么定义的，同一层级的不同实现类之间的区别大致是什么，必要的话，可以将这个继承树记下来，在心中推算几遍。
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然后是横向：将各个类有选择性地拿出来分析。
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当然，所谓的纵向，横向，这两个过程实际是互相交织的，也就是说整个流程不一定就分为前后两半：前面一半都是纵向，后面一半都是横向。
通过纵向的分析，我们能发现整个框架可以分成大致哪几个部分，以及有
1.3. 分析前的准备
- 首先在本地建一个对应的分析学习用的项目，比如：learn_netty，用maven管理依赖
- 然后在maven仓库，中找到我们需要的依赖，比如这里我用的是最新的：
<!-- https://mvnrepository.com/artifact/io.netty/netty-all --> <dependency> <groupId>io.netty</groupId> <artifactId>netty-all</artifactId> <version>4.1.77.Final</version> </dependency> - 将官方提供的demo代码，导入到项目中。
- 学习项目搭建好之后，就尝试编译、运行，没问题后，就命令行
mvn dependency:sources命令（或者通过IDE）来下载依赖的源代码。 - 可选：在github上，将项目同时clone到本地，如果分析中发现问题或者自己有些优化建议，可以尝试为分析的项目贡献代码。
1.4. 分析示例的代码
以一个简单的EchoServer、EchoClient来研究。
public class EchoServer { private final int port; public EchoServer(int port) { this.port = port; } public static void main(String[] args) throws Exception { new EchoServer(8083).start(); } public void start() throws Exception { final EchoServerHandler serverHandler = new EchoServerHandler(); EventLoopGroup group = new NioEventLoopGroup(); try { ServerBootstrap b = new ServerBootstrap(); b.group(group) .channel(NioServerSocketChannel.class) .localAddress(new InetSocketAddress(port)) .childHandler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) { ch.pipeline().addLast(serverHandler); } }); ChannelFuture f = b.bind().sync(); f.channel().closeFuture().sync(); } finally { group.shutdownGracefully().sync(); } } public class EchoServerHandler extends ChannelInboundHandlerAdapter { @Override public void channelRead(ChannelHandlerContext ctx, Object msg) { ByteBuf in = (ByteBuf) msg; System.out.println("Server received: " + in.toString(CharsetUtil.UTF_8)); ctx.write(in); } @Override public void channelReadComplete(ChannelHandlerContext ctx) { ctx.writeAndFlush(Unpooled.EMPTY_BUFFER) .addListener(ChannelFutureListener.CLOSE); } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { cause.printStackTrace(); ctx.close(); } public class EchoClient { public static void main(String[] args) throws Exception { connect("127.0.0.1", 8083); } public static void connect(String host, int port) throws Exception { NioEventLoopGroup group = new NioEventLoopGroup(); Bootstrap bootstrap = new Bootstrap(); try { bootstrap.group(group) .channel(NioSocketChannel.class).remoteAddress(new InetSocketAddress(host, port)) .handler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) { ch.pipeline().addLast(new EchoClientHandler()); } }); ChannelFuture f = bootstrap.connect(); f.channel().closeFuture().sync(); } finally { group.shutdownGracefully(); } } } public class EchoClientHandler extends SimpleChannelInboundHandler<ByteBuf> { @Override public void channelRegistered(ChannelHandlerContext ctx) throws Exception { super.channelRegistered(ctx); } @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { ctx.writeAndFlush(Unpooled.copiedBuffer("Netty Sockets!", CharsetUtil.UTF_8)); } @Override protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception { System.out.println(msg.toString(CharsetUtil.UTF_8)); } } 1.5. 开始分析
分别启动EchoServer、EchoClient，在两个ChannelFuture的位置打断点。
1.5.1. EchoServer启动调用链
进入ServerBootstrap的bind方法，发现该方法定义在父类AbstractBootstrap中：
public ChannelFuture bind() { validate(); SocketAddress localAddress = this.localAddress; if (localAddress == null) { throw new IllegalStateException("localAddress not set"); } return doBind(localAddress); } 接着来看doBind方法，发现也在AbstractBootstrap中：
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; } } 发现doBind中主要做了两件事：
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initAndRegister（初始化Channel并注册到EventLoop中），这个操作是异步操作，立即返回该操作对应的句柄。 -
拿到
initAndRegister操作的句柄后，对其进行检查。-
如果
initAndRegister已完成那么立即进行doBind0操作（实际的bind操作），并返回doBind0操作对应的句柄。 -
如果
initAndRegister还没有完成，那么就将doBind0操作异步化：initAndRegister操作完成后再触发doBind0。
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然后我们先看initAndRegister，它同样在AbstractBootstrap中：
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; } 忽略对异常的处理，看到有三个步骤：
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使用工厂创建一个
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对这个
channel进行init：由子类实现。 -
将创建的
channel注册（register）到EventLoopGroup中，异步操作，将该操作对应的句柄返回。
看完了initAndRegister后，在回来看doBind0：
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()); } } }); } 发现在doBind0中，最终是通过调用channel的bind方法来完成的。而这个动作是包裹成了一个任务，提交给了channel所注册到的eventloop，由它来执行。
1.5.2. EchoClient启动调用链
首先进入Bootstrap的connect方法中：
public ChannelFuture connect() { validate(); SocketAddress remoteAddress = this.remoteAddress; if (remoteAddress == null) { throw new IllegalStateException("remoteAddress not set"); } return doResolveAndConnect(remoteAddress, config.localAddress()); } 同样忽略validate，直接看doResolveAndConnect。
private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) { final ChannelFuture regFuture = initAndRegister(); final Channel channel = regFuture.channel(); if (regFuture.isDone()) { if (!regFuture.isSuccess()) { return regFuture; } return doResolveAndConnect0(channel, remoteAddress, localAddress, channel.newPromise()); } 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 { // Directly obtain the cause and do a null check so we only need one volatile read in case of a // failure. 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(); doResolveAndConnect0(channel, remoteAddress, localAddress, promise); } } }); return promise; } } 我们发现Bootstrap::doResolveAndConnect和AbstractBootstrap::doBind类似。意思也是说，在initAndRegister完成channel的创建、初始化、绑定到EventLoop之后再进行实际的操作doResolveAndConnect0。
于是我们来看doResolveAndConnect0:
private ChannelFuture doResolveAndConnect0(final Channel channel, SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) { try { final EventLoop eventLoop = channel.eventLoop(); AddressResolver<SocketAddress> resolver; try { resolver = this.resolver.getResolver(eventLoop); } catch (Throwable cause) { channel.close(); return promise.setFailure(cause); } if (!resolver.isSupported(remoteAddress) || resolver.isResolved(remoteAddress)) { // Resolver has no idea about what to do with the specified remote address or it's resolved already. doConnect(remoteAddress, localAddress, promise); return promise; } final Future<SocketAddress> resolveFuture = resolver.resolve(remoteAddress); if (resolveFuture.isDone()) { final Throwable resolveFailureCause = resolveFuture.cause(); if (resolveFailureCause != null) { // Failed to resolve immediately channel.close(); promise.setFailure(resolveFailureCause); } else { // Succeeded to resolve immediately; cached? (or did a blocking lookup) doConnect(resolveFuture.getNow(), localAddress, promise); } return promise; } // Wait until the name resolution is finished. resolveFuture.addListener(new FutureListener<SocketAddress>() { @Override public void operationComplete(Future<SocketAddress> future) throws Exception { if (future.cause() != null) { channel.close(); promise.setFailure(future.cause()); } else { doConnect(future.getNow(), localAddress, promise); } } }); } catch (Throwable cause) { promise.tryFailure(cause); } return promise; } 我们可以看出，doResolveAndConnect0正如其名：
- 首先获取
channel所绑定的eventloop所对应的AddressResolver（从AddressResolverGroup）中拿。 - 拿到
AddressResolver之后，如果它不知道该怎么处理给定的需要连接的地址，或者说这个地址已经被其解析过，那么就直接doConnect。否则使用AddressResolver来解析需要连接的地址（异步操作），并将doConnect操作异步化。
先暂时忽略AddressResolver，我们来看doConnect：
private static void doConnect( final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise connectPromise) { // This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up // the pipeline in its channelRegistered() implementation. final Channel channel = connectPromise.channel(); channel.eventLoop().execute(new Runnable() { @Override public void run() { if (localAddress == null) { channel.connect(remoteAddress, connectPromise); } else { channel.connect(remoteAddress, localAddress, connectPromise); } connectPromise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE); } }); } 我们看到doConnect和之前的doBind0一样，最终也是调用channel的方法，并且将实际的执行交给channel绑定的eventloop来执行。
1.6. 总结
就目前debug的调用链上，我们发现涉及到的组件有：
- Bootstrap系列：脚手架，提供给开发人员使用，类似Spring的ApplicationContext
- Channel系列：连接通道
- EventLoopGroup、EventLoop系列：执行器与事件驱动循环，IO模型。
- AddressResolverGroup、AddressResolver系列：地址解析器
- netty自定义的Future、Promise相关：异步化的基础
我们发现netty的操作全程是异步化的，并且最终要解开其原理的庐山真面目，关键还在于提及的eventloop、channel。
此阶段的纵向分析，目前只解开一隅，待我们看看eventloop、channel后，再来解开更大的谜题。
