netty源码分析之服务端启动

ServerBootstrap与Bootstrap分别是netty中服务端与客户端的引导类，主要负责服务端与客户端初始化、配置及启动引导等工做，接下来咱们就经过netty源码中的示例对ServerBootstrap与Bootstrap的源码进行一个简单的分析。首先咱们知道这两个类都继承自AbstractBootstrap类

 

 

接下来咱们就经过netty源码中ServerBootstrap的实例入手对其进行一个简单的分析。

         // Configure the server.
        EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(); final EchoServerHandler serverHandler = new EchoServerHandler(); try { //初始化一个服务端引导类
            ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) //设置线程组
             .channel(NioServerSocketChannel.class)//设置ServerSocketChannel的IO模型 分为epoll与Nio
             .option(ChannelOption.SO_BACKLOG, 100)//设置option参数，保存成一个LinkedHashMap<ChannelOption<?>, Object>()
             .handler(new LoggingHandler(LogLevel.INFO))//这个hanlder 只专属于 ServerSocketChannel 而不是 SocketChannel。
             .childHandler(new ChannelInitializer<SocketChannel>() { //这个handler 将会在每一个客户端链接的时候调用。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. 启动服务
            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(); }

 接下来咱们主要从服务端的socket在哪里初始化与哪里accept链接这两个问题入手对netty服务端启动的流程进行分析；

 咱们首先要知道，netty服务的启动其实能够分为如下四步：

• 建立服务端Channel
• 初始化服务端Channel
• 注册Selector
• 端口绑定

1、建立服务端Channel

一、服务端Channel的建立，主要为如下流程

咱们经过跟踪代码可以看到

final ChannelFuture regFuture = initAndRegister();// 初始化并建立 NioServerSocketChannel

咱们在initAndRegister()中能够看到channel的初始化。

channel = channelFactory.newChannel(); // 经过 反射工厂建立一个 NioServerSocketChannel

 我进一步看newChannel()中的源码，在ReflectiveChannelFactory这个反射工厂中，经过clazz这个类的反射建立了一个服务端的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);
        }
    }

既然经过反射，咱们就要知道clazz类是什么，那么我咱们来看下channelFactory这个工厂类是在哪里初始化的，初始化的时候咱们传入了哪一个channel。

这里咱们须要看下demo实例中初始化ServerBootstrap时.channel(NioServerSocketChannel.class)这里的具体实现，咱们看下源码

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

经过上面的代码我能够直观的看出正是在这里咱们经过NioServerSocketChannel这个类构造了一个反射工厂。

那么到这里就很清楚了，咱们建立的Channel就是一个NioServerSocketChannel，那么具体的建立咱们就须要看下这个类的构造函数。首先咱们看下一个NioServerSocketChannel建立的具体流程

首先是newsocket(),咱们先看下具体的代码，在NioServerSocketChannel的构造函数中咱们建立了一个jdk原生的ServerSocketChannel

    /**
     * Create a new instance
     */
    public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));//传入默认的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();//能够看到建立的是jdk底层的ServerSocketChannel 
        } catch (IOException e) {
            throw new ChannelException(
                    "Failed to open a server socket.", e);
        }
    }

    

第二步是经过NioServerSocketChannelConfig配置服务端Channel的构造函数，在代码中咱们能够看到咱们把NioServerSocketChannel这个类传入到了NioServerSocketChannelConfig的构造函数中进行配置

    /**
     * Create a new instance using the given {@link ServerSocketChannel}.
     */
    public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);//调用父类构造函数,传入建立的channel
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }

第三步在父类AbstractNioChannel的构造函数中把建立服务端的Channel设置为非阻塞模式

    /**
     * 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;//这个ch就是传入的经过jdk建立的Channel
        this.readInterestOp = readInterestOp;
        try {
            ch.configureBlocking(false);//设置为非阻塞
        } 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);
        }
    }

第四步调用AbstractChannel这个抽象类的构造函数设置Channel的id（每一个Channel都有一个id，惟一标识）,unsafe（tcp相关底层操做），pipeline（逻辑链）等，而无论是服务的Channel仍是客户端的Channel都继承自这个抽象类，他们也都会有上述相应的属性。咱们看下AbstractChannel的构造函数

    /**
     * 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惟一标识 
        unsafe = newUnsafe();//netty封装的TCP 相关操做类
        pipeline = newChannelPipeline();//逻辑链
    }

 二、初始化服务端建立的Channel

init(channel);// 初始化这个 NioServerSocketChannel

咱们首先列举下init(channel)中具体都作了哪了些功能：

• 设置ChannelOptions、ChannelAttrs ,配置服务端Channel的相关属性；
• 设置ChildOptions、ChildAttrs，配置每一个新链接的Channel的相关属性；
• Config handler，配置服务端pipeline;
• add ServerBootstrapAcceptor,添加链接器，对accpet接受到的新链接进行处理，添加一个nio线程；

那么接下来咱们经过代码，对每一步设置进行一下分析：

首先是在SeverBootstrap的init()方法中对ChannelOptions、ChannelAttrs 的配置的关键代码

        final Map<ChannelOption<?>, Object> options = options0();//拿到你设置的option
        synchronized (options) {
            setChannelOptions(channel, options, logger);//设置NioServerSocketChannel相应的TCP参数，其实这一步就是把options设置到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());
            }
        }

而后是对ChildOptions、ChildAttrs配置的关键代码

        //能够看到两个都是局部变量，会在下面设置pipeline时用到
        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));
        }

第三步对服务端Channel的handler进行配置

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(final Channel ch) throws Exception {
                final ChannelPipeline pipeline = ch.pipeline();
                ChannelHandler handler = config.handler();//拿到咱们自定义的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链接器，这个是netty向服务端Channel自定义添加的一个handler，用来处理新链接的添加与属性配置，咱们来看下关键代码

                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        //在这里会把咱们自定义的ChildGroup、ChildHandler、ChildOptions、ChildAttrs相关配置传入到ServerBootstrapAcceptor构造函数中，并绑定到新的链接上
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });

3、注册Selector

一个服务端的Channel建立完毕后，下一步就是要把它注册到一个事件轮询器Selector上，在initAndRegister()中咱们把上面初始化的Channel进行注册

ChannelFuture regFuture = config().group().register(channel);//注册咱们已经初始化过的Channel

而这个register具体实现是在AbstractChannel中的AbstractUnsafe抽象类中的

 /**
         * 一、先是一系列的判断。
         * 二、判断当前线程是不是给定的 eventLoop 线程。注意：这点很重要，Netty 线程模型的高性能取决于对于当前执行的Thread 的身份的肯定。若是不在当前线程，那么就须要不少同步措施（好比加锁），上下文切换等耗费性能的操做。
         * 三、异步（由于咱们这里直到如今仍是 main 线程在执行，不属于当前线程）的执行 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;//绑定线程

            if (eventLoop.inEventLoop()) {
                register0(promise);//实际的注册过程
            } 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);
                }
            }
        }

 

首先咱们对整个注册的流程作一个梳理

 接下来咱们进入register0()方法看下注册过程的具体实现

        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的底层注册
                neverRegistered = false;
                registered = true;

                // 触发绑定的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()的具体实现就是把jdk底层的channel绑定到eventLoop的selecor上

    @Override
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                //把channel注册到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;
                }
            }
        }
    }

到这里netty就把服务端的channel注册到了指定的selector上，下面就是服务端口的绑定

3、端口绑定

首先咱们梳理下netty中服务端口绑定的流程

咱们来看下AbstarctUnsafe中bind()方法的具体实现

 @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();//判断绑定是否完成
            try {
                doBind(localAddress);//底层jdk绑定端口
            } catch (Throwable t) {
                safeSetFailure(promise, t);
                closeIfClosed();
                return;
            }

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

            safeSetSuccess(promise);
        }

在doBind(localAddress)中netty实现了jdk底层端口的绑定

    @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()中会触发pipeline中的channelActive()方法

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

            readIfIsAutoRead();
        }

在channelActive中首先会把ChannelActive事件往下传播，而后调用readIfIsAutoRead()方法出触发channel的read事件，而它最终调用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为  SelectionKey.OP_ACCEPT
        }
    }

 在doBeginRead()方法，netty会把accept事件注册到Selector上。

到此咱们对netty服务端的启动流程有了一个大体的了解，总体能够归纳为下面四步：

一、channelFactory.newChannel()，其实就是建立jdk底层channel，并初始化id、piepline等属性；

二、init(channel)，添加option、attr等属性，并添加ServerBootstrapAcceptor链接器；

三、config().group().register(channel)，把jdk底层的channel注册到eventLoop上的selector上；

四、doBind0(regFuture, channel, localAddress, promise)，完成服务端端口的监听，并把accept事件注册到selector上；

以上就是对netty服务端启动流程进行的一个简单分析，有不少细节没有关注与深刻，其中若有不足与不正确的地方还望指出与海涵。