netty源碼分析之服務端啓動

ServerBootstrap與Bootstrap分別是netty中服務端與客戶端的引導類，主要負責服務端與客戶端初始化、配置及啓動引導等工做，接下來咱們就經過netty源碼中的示例對ServerBootstrap與Bootstrap的源碼進行一個簡單的分析。首先咱們知道這兩個類都繼承自AbstractBootstrap類

 

 

接下來咱們就經過netty源碼中ServerBootstrap的實例入手對其進行一個簡單的分析。

         // Configure the server.
        EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(); final EchoServerHandler serverHandler = new EchoServerHandler(); try { //初始化一個服務端引導類
            ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) //設置線程組
             .channel(NioServerSocketChannel.class)//設置ServerSocketChannel的IO模型 分爲epoll與Nio
             .option(ChannelOption.SO_BACKLOG, 100)//設置option參數，保存成一個LinkedHashMap<ChannelOption<?>, Object>()
             .handler(new LoggingHandler(LogLevel.INFO))//這個hanlder 只專屬於 ServerSocketChannel 而不是 SocketChannel。
             .childHandler(new ChannelInitializer<SocketChannel>() { //這個handler 將會在每一個客戶端鏈接的時候調用。供 SocketChannel 使用。
 @Override public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); if (sslCtx != null) { p.addLast(sslCtx.newHandler(ch.alloc())); } //p.addLast(new LoggingHandler(LogLevel.INFO));
 p.addLast(serverHandler); } }); // Start the server. 啓動服務
            ChannelFuture f = b.bind(PORT).sync(); // Wait until the server socket is closed.
 f.channel().closeFuture().sync(); } finally { // Shut down all event loops to terminate all threads.
 bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); }

 接下來咱們主要從服務端的socket在哪裏初始化與哪裏accept鏈接這兩個問題入手對netty服務端啓動的流程進行分析；

 咱們首先要知道，netty服務的啓動其實能夠分爲如下四步：

• 建立服務端Channel
• 初始化服務端Channel
• 註冊Selector
• 端口綁定

1、建立服務端Channel

一、服務端Channel的建立，主要爲如下流程

咱們經過跟蹤代碼可以看到

final ChannelFuture regFuture = initAndRegister();// 初始化並建立 NioServerSocketChannel

咱們在initAndRegister()中能夠看到channel的初始化。

channel = channelFactory.newChannel(); // 經過 反射工廠建立一個 NioServerSocketChannel

 我進一步看newChannel()中的源碼，在ReflectiveChannelFactory這個反射工廠中，經過clazz這個類的反射建立了一個服務端的channel。

    @Override
    public T newChannel() {
        try {
            return clazz.getConstructor().newInstance();//反射建立
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + clazz, t);
        }
    }

既然經過反射，咱們就要知道clazz類是什麼，那麼我咱們來看下channelFactory這個工廠類是在哪裏初始化的，初始化的時候咱們傳入了哪一個channel。

這裏咱們須要看下demo實例中初始化ServerBootstrap時.channel(NioServerSocketChannel.class)這裏的具體實現，咱們看下源碼

    public B channel(Class<? extends C> channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
    }

經過上面的代碼我能夠直觀的看出正是在這裏咱們經過NioServerSocketChannel這個類構造了一個反射工廠。

那麼到這裏就很清楚了，咱們建立的Channel就是一個NioServerSocketChannel，那麼具體的建立咱們就須要看下這個類的構造函數。首先咱們看下一個NioServerSocketChannel建立的具體流程

首先是newsocket(),咱們先看下具體的代碼，在NioServerSocketChannel的構造函數中咱們建立了一個jdk原生的ServerSocketChannel

    /**
     * Create a new instance
     */
    public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));//傳入默認的SelectorProvider
    }

    private static ServerSocketChannel newSocket(SelectorProvider provider) {
        try {
            /**
             *  Use the {@link SelectorProvider} to open {@link SocketChannel} and so remove condition in
             *  {@link SelectorProvider#provider()} which is called by each ServerSocketChannel.open() otherwise.
             *
             *  See <a href="https://github.com/netty/netty/issues/2308">#2308</a>.
             */
            return provider.openServerSocketChannel();//能夠看到建立的是jdk底層的ServerSocketChannel 
        } catch (IOException e) {
            throw new ChannelException(
                    "Failed to open a server socket.", e);
        }
    }

    

第二步是經過NioServerSocketChannelConfig配置服務端Channel的構造函數，在代碼中咱們能夠看到咱們把NioServerSocketChannel這個類傳入到了NioServerSocketChannelConfig的構造函數中進行配置

    /**
     * Create a new instance using the given {@link ServerSocketChannel}.
     */
    public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);//調用父類構造函數,傳入建立的channel
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }

第三步在父類AbstractNioChannel的構造函數中把建立服務端的Channel設置爲非阻塞模式

    /**
     * Create a new instance
     *
     * @param parent            the parent {@link Channel} by which this instance was created. May be {@code null}
     * @param ch                the underlying {@link SelectableChannel} on which it operates
     * @param readInterestOp    the ops to set to receive data from the {@link SelectableChannel}
     */
    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        this.ch = ch;//這個ch就是傳入的經過jdk建立的Channel
        this.readInterestOp = readInterestOp;
        try {
            ch.configureBlocking(false);//設置爲非阻塞
        } catch (IOException e) {
            try {
                ch.close();
            } catch (IOException e2) {
                if (logger.isWarnEnabled()) {
                    logger.warn(
                            "Failed to close a partially initialized socket.", e2);
                }
            }

            throw new ChannelException("Failed to enter non-blocking mode.", e);
        }
    }

第四步調用AbstractChannel這個抽象類的構造函數設置Channel的id（每一個Channel都有一個id，惟一標識）,unsafe（tcp相關底層操做），pipeline（邏輯鏈）等，而無論是服務的Channel仍是客戶端的Channel都繼承自這個抽象類，他們也都會有上述相應的屬性。咱們看下AbstractChannel的構造函數

    /**
     * Creates a new instance.
     *
     * @param parent
     *        the parent of this channel. {@code null} if there's no parent.
     */
    protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();//建立Channel惟一標識 
        unsafe = newUnsafe();//netty封裝的TCP 相關操做類
        pipeline = newChannelPipeline();//邏輯鏈
    }

 二、初始化服務端建立的Channel

init(channel);// 初始化這個 NioServerSocketChannel

咱們首先列舉下init(channel)中具體都作了哪了些功能：

• 設置ChannelOptions、ChannelAttrs ,配置服務端Channel的相關屬性；
• 設置ChildOptions、ChildAttrs，配置每一個新鏈接的Channel的相關屬性；
• Config handler，配置服務端pipeline;
• add ServerBootstrapAcceptor,添加鏈接器，對accpet接受到的新鏈接進行處理，添加一個nio線程；

那麼接下來咱們經過代碼，對每一步設置進行一下分析：

首先是在SeverBootstrap的init()方法中對ChannelOptions、ChannelAttrs 的配置的關鍵代碼

        final Map<ChannelOption<?>, Object> options = options0();//拿到你設置的option
        synchronized (options) {
            setChannelOptions(channel, options, logger);//設置NioServerSocketChannel相應的TCP參數，其實這一步就是把options設置到channel的config中
        }

        final Map<AttributeKey<?>, Object> attrs = attrs0();
        synchronized (attrs) {
            for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
                @SuppressWarnings("unchecked")
                AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
                channel.attr(key).set(e.getValue());
            }
        }

而後是對ChildOptions、ChildAttrs配置的關鍵代碼

        //能夠看到兩個都是局部變量，會在下面設置pipeline時用到
        final Entry<ChannelOption<?>, Object>[] currentChildOptions;
        final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
        synchronized (childOptions) {
            currentChildOptions = childOptions.entrySet().toArray(newOptionArray(0));
        }
        synchronized (childAttrs) {
            currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
        }

第三步對服務端Channel的handler進行配置

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(final Channel ch) throws Exception {
                final ChannelPipeline pipeline = ch.pipeline();
                ChannelHandler handler = config.handler();//拿到咱們自定義的hanler
                if (handler != null) {
                    pipeline.addLast(handler);
                }

                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });

第四步添加ServerBootstrapAcceptor鏈接器，這個是netty向服務端Channel自定義添加的一個handler，用來處理新鏈接的添加與屬性配置，咱們來看下關鍵代碼

                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        //在這裏會把咱們自定義的ChildGroup、ChildHandler、ChildOptions、ChildAttrs相關配置傳入到ServerBootstrapAcceptor構造函數中，並綁定到新的鏈接上
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });

3、註冊Selector

一個服務端的Channel建立完畢後，下一步就是要把它註冊到一個事件輪詢器Selector上，在initAndRegister()中咱們把上面初始化的Channel進行註冊

ChannelFuture regFuture = config().group().register(channel);//註冊咱們已經初始化過的Channel

而這個register具體實現是在AbstractChannel中的AbstractUnsafe抽象類中的

 /**
         * 一、先是一系列的判斷。
         * 二、判斷當前線程是不是給定的 eventLoop 線程。注意：這點很重要，Netty 線程模型的高性能取決於對於當前執行的Thread 的身份的肯定。若是不在當前線程，那麼就須要不少同步措施（好比加鎖），上下文切換等耗費性能的操做。
         * 三、異步（由於咱們這裏直到如今仍是 main 線程在執行，不屬於當前線程）的執行 register0 方法。
         */
        @Override
        public final void register(EventLoop eventLoop, final ChannelPromise promise) {
            if (eventLoop == null) {
                throw new NullPointerException("eventLoop");
            }
            if (isRegistered()) {
                promise.setFailure(new IllegalStateException("registered to an event loop already"));
                return;
            }
            if (!isCompatible(eventLoop)) {
                promise.setFailure(
                        new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
                return;
            }

            AbstractChannel.this.eventLoop = eventLoop;//綁定線程

            if (eventLoop.inEventLoop()) {
                register0(promise);//實際的註冊過程
            } else {
                try {
                    eventLoop.execute(new Runnable() {
                        @Override
                        public void run() {
                            register0(promise);
                        }
                    });
                } catch (Throwable t) {
                    logger.warn(
                            "Force-closing a channel whose registration task was not accepted by an event loop: {}",
                            AbstractChannel.this, t);
                    closeForcibly();
                    closeFuture.setClosed();
                    safeSetFailure(promise, t);
                }
            }
        }

 

首先咱們對整個註冊的流程作一個梳理

 接下來咱們進入register0()方法看下注冊過程的具體實現

        private void register0(ChannelPromise promise) {
            try {
                // check if the channel is still open as it could be closed in the mean time when the register
                // call was outside of the eventLoop
                if (!promise.setUncancellable() || !ensureOpen(promise)) {
                    return;
                }
                boolean firstRegistration = neverRegistered;
                doRegister();//jdk channel的底層註冊
                neverRegistered = false;
                registered = true;

                // 觸發綁定的handler事件
                // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                // user may already fire events through the pipeline in the ChannelFutureListener.
                pipeline.invokeHandlerAddedIfNeeded();

               
                safeSetSuccess(promise);
                pipeline.fireChannelRegistered();
                // Only fire a channelActive if the channel has never been registered. This prevents firing
                // multiple channel actives if the channel is deregistered and re-registered.
                if (isActive()) {
                    if (firstRegistration) {
                        pipeline.fireChannelActive();
                    } else if (config().isAutoRead()) {
                        // This channel was registered before and autoRead() is set. This means we need to begin read
                        // again so that we process inbound data.
                        //
                        // See https://github.com/netty/netty/issues/4805
                        beginRead();
                    }
                }
            } catch (Throwable t) {
                // Close the channel directly to avoid FD leak.
                closeForcibly();
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }

AbstractNioChannel中doRegister()的具體實現就是把jdk底層的channel綁定到eventLoop的selecor上

    @Override
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                //把channel註冊到eventLoop上的selector上
                selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
                return;
            } catch (CancelledKeyException e) {
                if (!selected) {
                    // Force the Selector to select now as the "canceled" SelectionKey may still be
                    // cached and not removed because no Select.select(..) operation was called yet.
                    eventLoop().selectNow();
                    selected = true;
                } else {
                    // We forced a select operation on the selector before but the SelectionKey is still cached
                    // for whatever reason. JDK bug ?
                    throw e;
                }
            }
        }
    }

到這裏netty就把服務端的channel註冊到了指定的selector上，下面就是服務端口的綁定

3、端口綁定

首先咱們梳理下netty中服務端口綁定的流程

咱們來看下AbstarctUnsafe中bind()方法的具體實現

 @Override
        public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
            assertEventLoop();

            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            // See: https://github.com/netty/netty/issues/576
            if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
                localAddress instanceof InetSocketAddress &&
                !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&
                !PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) {
                // Warn a user about the fact that a non-root user can't receive a
                // broadcast packet on *nix if the socket is bound on non-wildcard address.
                logger.warn(
                        "A non-root user can't receive a broadcast packet if the socket " +
                        "is not bound to a wildcard address; binding to a non-wildcard " +
                        "address (" + localAddress + ") anyway as requested.");
            }

            boolean wasActive = isActive();//判斷綁定是否完成
            try {
                doBind(localAddress);//底層jdk綁定端口
            } catch (Throwable t) {
                safeSetFailure(promise, t);
                closeIfClosed();
                return;
            }

            if (!wasActive && isActive()) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireChannelActive();//觸發ChannelActive事件
                    }
                });
            }

            safeSetSuccess(promise);
        }

在doBind(localAddress)中netty實現了jdk底層端口的綁定

    @Override
    protected void doBind(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) {
            javaChannel().bind(localAddress, config.getBacklog());
        } else {
            javaChannel().socket().bind(localAddress, config.getBacklog());
        }
    }

在 pipeline.fireChannelActive()中會觸發pipeline中的channelActive()方法

        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            ctx.fireChannelActive();

            readIfIsAutoRead();
        }

在channelActive中首先會把ChannelActive事件往下傳播，而後調用readIfIsAutoRead()方法出觸發channel的read事件，而它最終調用AbstractNioChannel中的doBeginRead()方法

    @Override
    protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        final SelectionKey selectionKey = this.selectionKey;
        if (!selectionKey.isValid()) {
            return;
        }

        readPending = true;

        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            selectionKey.interestOps(interestOps | readInterestOp);//readInterestOp爲  SelectionKey.OP_ACCEPT
        }
    }

 在doBeginRead()方法，netty會把accept事件註冊到Selector上。

到此咱們對netty服務端的啓動流程有了一個大體的瞭解，總體能夠歸納爲下面四步：

一、channelFactory.newChannel()，其實就是建立jdk底層channel，並初始化id、piepline等屬性；

二、init(channel)，添加option、attr等屬性，並添加ServerBootstrapAcceptor鏈接器；

三、config().group().register(channel)，把jdk底層的channel註冊到eventLoop上的selector上；

四、doBind0(regFuture, channel, localAddress, promise)，完成服務端端口的監聽，並把accept事件註冊到selector上；

以上就是對netty服務端啓動流程進行的一個簡單分析，有不少細節沒有關注與深刻，其中若有不足與不正確的地方還望指出與海涵。