Netty源码研究笔记（1）——开篇

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：

• I/O模型

• 内存管理

• 各种网络协议的实现：http、redis、websocket等等

• 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。

• 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：

1. 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。

2. 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。

3. 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。

4. 之后就是横向，对这些归纳出来的组件体系，逐个进行分析。

5. 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：

   1. 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。

   2. 然后是横向：将各个类有选择性地拿出来分析。

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

1. 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
2. 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>

3. 将官方提供的demo代码，导入到项目中。
4. 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。
5. 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：

1. 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。

2. 拿到initAndRegister操作的句柄后，对其进行检查。

   1. 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。

   2. 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。

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：

1. 使用工厂创建一个channel

2. 对这个channel进行init：由子类实现。

3. 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：

1. 首先获取channel所绑定的eventloop所对应的AddressResolver（从AddressResolverGroup）中拿。
2. 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。