腳踏實地的Netty原始碼研究筆記(1)——開篇

邁吉發表於2022-05-20

1. 腳踏實地的Netty原始碼研究筆記(1)——開篇

1.1. Netty介紹

Netty是一個老牌的高效能網路框架。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、記憶體洩露探測等等。

  • 程式碼風格、設計思想、設計原則等。

1.2. 原始碼分析方法

我一般是這樣進行原始碼分析的:

  1. 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週期下的呼叫鏈上,會涉及到哪些元件。

  2. 然後對涉及到的元件拿出來,找出它們的頂層定義(介面、抽象類)。通過其模組/包的劃分類註釋定義的方法及其註釋,來大致知曉每個元件是做什麼的,以及它們在整個框架中的位置是怎樣的。

  3. 第二步完成後,就可以對第一步的呼叫鏈流程、步驟、涉及到的元件,進行歸納、劃分,從而做到心中有數,知道東南西北了。

  4. 之後就是橫向,對這些歸納出來的元件體系,逐個進行分析。

  5. 在分析每個元件體系的時候,也是按照先縱向,再橫向的步驟:

    1. 首先是縱向:找出該元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

  1. 首先在本地建一個對應的分析學習用的專案,比如:learn_netty,用maven管理依賴
  2. 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>
  1. 將官方提供的demo程式碼,匯入到專案中。
  2. 學習專案搭建好之後,就嘗試編譯、執行,沒問題後,就命令列mvn dependency:sources命令(或者通過IDE)來下載依賴的原始碼。
  3. 可選:在github上,將專案同時clone到本地,如果分析中發現問題或者自己有些優化建議,可以嘗試為分析的專案貢獻程式碼。

1.4. 分析示例的程式碼

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啟動呼叫鏈

進入ServerBootstrapbind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:

  1. 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控制程式碼。

  2. 拿到initAndRegister操作的控制程式碼後,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操作對應的控制程式碼。

    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:

  1. 使用工廠建立一個channel

  2. 對這個channel進行init:由子類實現。

  3. 將建立的channel註冊(register)到EventLoopGroup中,非同步操作,將該操作對應的控制程式碼返回。

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,最終是通過呼叫channelbind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啟動呼叫鏈

首先進入Bootstrapconnect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::doResolveAndConnectAbstractBootstrap::doBind類似。意思也是說,在initAndRegister完成channel的建立、初始化、繫結到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:

  1. 首先獲取channel所繫結的eventloop所對應的AddressResolver(從AddressResolverGroup)中拿。
  2. 拿到AddressResolver之後,如果它不知道該怎麼處理給定的需要連線的地址,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來解析需要連線的地址(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,最終也是呼叫channel的方法,並且將實際的執行交給channel繫結的eventloop來執行。

1.6. 總結

就目前debug的呼叫鏈上,我們發現涉及到的元件有:

  • Bootstrap系列:腳手架,提供給開發人員使用,類似Spring的ApplicationContext
  • Channel系列:連線通道
  • 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。

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