1. Netty源碼研究筆記（1）——開篇 1.1. Netty介紹 Netty是一個老牌的高性能網路框架。在眾多開源框架中都有它的身影，比如：grpc、dubbo、seata等。裡面有著非常多值得學的東西： I/O模型記憶體管理各種網路協議的實現：http、redis、websocket等等 ...

1. Netty源碼研究筆記（1）——開篇

1.1. Netty介紹

Netty是一個老牌的高性能網路框架。在眾多開源框架中都有它的身影，比如：grpc、dubbo、seata等。

裡面有著非常多值得學的東西：

I/O模型
記憶體管理
各種網路協議的實現：http、redis、websocket等等
各種各樣有趣的技巧的實現：非同步、時間輪、池化、記憶體泄露探測等等。
代碼風格、設計思想、設計原則等。

1.2. 源碼分析方法

我一般是這樣進行源碼分析的：

首先是縱向，通過官方提供的demo，進行debug，並記錄在一個完整的生命周期下的調用鏈上，會涉及到哪些組件。
然後對涉及到的組件拿出來，找出它們的頂層定義（介面、抽象類）。通過其模塊/包的劃分、類註釋、定義的方法及其註釋，來大致知曉每個組件是做什麼的，以及它們在整個框架中的位置是怎樣的。
第二步完成後，就可以對第一步的調用鏈流程、步驟、涉及到的組件，進行歸納、劃分，從而做到心中有數，知道東南西北了。
之後就是橫向，對這些歸納出來的組件體系，逐個進行分析。
在分析每個組件體系的時候，也是按照先縱向，再橫向的步驟：
1. 首先是縱向：找出該組件體系中的核心頂層介面、類，然後結合其的所有實現類，捋出繼承樹，然後弄清楚每個類做的是啥，它是怎麼定義的，同一層級的不同實現類之間的區別大致是什麼，必要的話，可以將這個繼承樹記下來，在心中推算幾遍。
2. 然後是橫向：將各個類有選擇性地拿出來分析。

當然，所謂的縱向，橫向，這兩個過程實際是互相交織的，也就是說整個流程不一定就分為前後兩半：前面一半都是縱向，後面一半都是橫向。

通過縱向的分析，我們能發現整個框架可以分成大致哪幾個部分，以及有

1.3. 分析前的準備

首先在本地建一個對應的分析學慣用的項目，比如：learn_netty，用maven管理依賴
然後在maven倉庫，中找到我們需要的依賴，比如這裡我用的是最新的：

<!-- https://mvnrepository.com/artifact/io.netty/netty-all -->
<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-all</artifactId>
    <version>4.1.77.Final</version>
</dependency>

將官方提供的demo代碼，導入到項目中。
學習項目搭建好之後，就嘗試編譯、運行，沒問題後，就命令行mvn dependency:sources命令（或者通過IDE）來下載依賴的源代碼。
可選：在github上，將項目同時clone到本地，如果分析中發現問題或者自己有些優化建議，可以嘗試為分析的項目貢獻代碼。

1.4. 分析示例的代碼

以一個簡單的EchoServer、EchoClient來研究。

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. 開始分析

分別啟動EchoServer、EchoClient，在兩個ChannelFuture的位置打斷點。

1.5.1. EchoServer啟動調用鏈

進入ServerBootstrap的bind方法，發現該方法定義在父類AbstractBootstrap中：

    public ChannelFuture bind() {
        validate();
        SocketAddress localAddress = this.localAddress;
        if (localAddress == null) {
            throw new IllegalStateException("localAddress not set");
        }
        return doBind(localAddress);
    }

接著來看doBind方法，發現也在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;
        }
    }

發現doBind中主要做了兩件事：

initAndRegister（初始化Channel並註冊到EventLoop中），這個操作是非同步操作，立即返回該操作對應的句柄。
拿到initAndRegister操作的句柄後，對其進行檢查。
1. 如果initAndRegister已完成那麼立即進行doBind0操作（實際的bind操作），並返回doBind0操作對應的句柄。
2. 如果initAndRegister還沒有完成，那麼就將doBind0操作非同步化：initAndRegister操作完成後再觸發doBind0。

然後我們先看initAndRegister，它同樣在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;
    }

忽略對異常的處理，看到有三個步驟：

使用工廠創建一個channel
對這個channel進行init：由子類實現。
將創建的channel註冊（register）到EventLoopGroup中，非同步操作，將該操作對應的句柄返回。

看完了initAndRegister後，在回來看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());
                }
            }
        });
    }

發現在doBind0中，最終是通過調用channel的bind方法來完成的。而這個動作是包裹成了一個任務，提交給了channel所註冊到的eventloop，由它來執行。

1.5.2. EchoClient啟動調用鏈

首先進入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());
    }

同樣忽略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;
        }
    }

我們發現Bootstrap::doResolveAndConnect和AbstractBootstrap::doBind類似。意思也是說，在initAndRegister完成channel的創建、初始化、綁定到EventLoop之後再進行實際的操作doResolveAndConnect0。

於是我們來看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;
    }

我們可以看出，doResolveAndConnect0正如其名：

首先獲取channel所綁定的eventloop所對應的AddressResolver（從AddressResolverGroup）中拿。
拿到AddressResolver之後，如果它不知道該怎麼處理給定的需要連接的地址，或者說這個地址已經被其解析過，那麼就直接doConnect。否則使用AddressResolver來解析需要連接的地址（非同步操作），並將doConnect操作非同步化。

先暫時忽略AddressResolver，我們來看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);
            }
        });
    }

我們看到doConnect和之前的doBind0一樣，最終也是調用channel的方法，並且將實際的執行交給channel綁定的eventloop來執行。

1.6. 總結

就目前debug的調用鏈上，我們發現涉及到的組件有：

Bootstrap系列：腳手架，提供給開發人員使用，類似Spring的ApplicationContext
Channel系列：連接通道
EventLoopGroup、EventLoop系列：執行器與事件驅動迴圈，IO模型。
AddressResolverGroup、AddressResolver系列：地址解析器
netty自定義的Future、Promise相關：非同步化的基礎

我們發現netty的操作全程是非同步化的，並且最終要解開其原理的廬山真面目，關鍵還在於提及的eventloop、channel。

此階段的縱向分析，目前只解開一隅，待我們看看eventloop、channel後，再來解開更大的謎題。

作者：邁吉

出處： https://www.cnblogs.com/stepfortune/

關於作者：邁吉

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