netty服务端客户端启动流程分析

服务端启动流程

我们回顾前面讲解的netty启动流程，服务端这边有两个EventLoopGroup，一个专门用来处理连接，一个用来处理后续的io事件

服务端启动还是跟nio一样，绑定端口进行监听，我们先来看绑定流程

// 绑定端口并同步阻塞直到绑定结束
ChannelFuture cf = serverBootstrap.bind(8080).sync();

private ChannelFuture doBind(final SocketAddress localAddress) {
    // 注册一个NioServerScoketChannel
    final ChannelFuture regFuture = initAndRegister();
    final Channel channel = regFuture.channel();
    if (regFuture.cause() != null) {
        return regFuture;
    }

    if (regFuture.isDone()) {
        // 注册完成 进行端口绑定，并返回一个异步绑定结果
        ChannelPromise promise = channel.newPromise();
        // 绑定端口
        doBind0(regFuture, channel, localAddress, promise);
        return promise;
    } else {
        // 未注册完成 返回带注册结果的promise
        final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
        // 添加一个注册异步监听器 但注册完成会进行绑定操作
        regFuture.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                Throwable cause = future.cause();
                if (cause != null) {
                    // 注册时有发生异常 通知相关监听器
                    promise.setFailure(cause);
                } else {
                    // 设置注册结果为true
                    promise.registered();
                    // 绑定端口
                    doBind0(regFuture, channel, localAddress, promise);
                }
            }
        });
        return promise;
    }
}

final ChannelFuture initAndRegister() {
    Channel channel = null;
    try {
        /**
        * 通过我们之前设置的socket类型进行实例化
        * serverBootstrap.channel(NioServerSocketChannel.class) 会返回一个ReflectiveChannelFactory工厂
        **/
        channel = channelFactory.newChannel(); // 1
        // 初始化channel
        init(channel); // 2
    } catch (Throwable t) {
        if (channel != null) {
            // 发生异常 将内部nio中的channel进行关闭
            channel.unsafe().closeForcibly();
            // 返回一个结果失败的Promise
            return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
        }
        return new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE).setFailure(t);
    }

    // 将channel注册到bossEventLoopGroup上，里面会将channel注册到EventLoop中的selector
    ChannelFuture regFuture = config().group().register(channel); // 3
    if (regFuture.cause() != null) {
        if (channel.isRegistered()) {
            channel.close();
        } else {
            channel.unsafe().closeForcibly();
        }
    }
    return regFuture;
}

private static void doBind0(
        final ChannelFuture regFuture, final Channel channel,
        final SocketAddress localAddress, final ChannelPromise promise) {

    // 添加一个异步绑定任务
    channel.eventLoop().execute(new Runnable() {
        @Override
        public void run() {
            if (regFuture.isSuccess()) {
　　　　　　　　　　// 绑定！！！
                channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE); // 4
            } else {
                promise.setFailure(regFuture.cause());
            }
        }
    });
}

对上述流程进行一个简单的说明: 先创建一个NioServerScoketChannel，然后进行初始化操作，然后注册到bossEventLoop中selector上，nio需要做的流程 netty都要做，然后进行绑定 返回一个绑定异步promise

绑定流程有4个比较重要的操作 我们来一一详解

1、实例化NioServerScoketChannel，内部会创建一个ServerSocketChannel 然后持有，同时创建unsafe和pipeline

/***********************NioServerSocketChannel**********************/
// selector提供者，提供创建selector、ServerSocketChannel、SocketChannel
private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();

public NioServerSocketChannel() {
    this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}

private static ServerSocketChannel newSocket(SelectorProvider provider) {
    try {
        // 返回一个nio中的ServerSocketChannel
        return provider.openServerSocketChannel();
    } catch (IOException e) {
        throw new ChannelException(
                "Failed to open a server socket.", e);
    }
}

public NioServerSocketChannel(ServerSocketChannel channel) {
    // 调用父类构造
    super(null, channel, SelectionKey.OP_ACCEPT);
    // NioServerSocketChannel配置类
    config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}

/***********************AbstractNioChannel***********************/
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    super(parent);
    this.ch = ch;
    this.readInterestOp = readInterestOp;
    try {
        // 将ServerSocketChannel设置为非阻塞
        ch.configureBlocking(false);
    } 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);
    }
}

/***********************AbstractChannel*************************/
protected AbstractChannel(Channel parent) {
    this.parent = parent;
    id = newId();
    // 创建一个unsafe 实例为NioMessageUnsafe类型
    unsafe = newUnsafe();
    // 创建pipeline,类型为DefaultChannelPipeline
    pipeline = newChannelPipeline();
}
/***********************DefaultChannelPipeline*************************/
protected DefaultChannelPipeline(Channel channel) {
    this.channel = ObjectUtil.checkNotNull(channel, "channel");
    succeededFuture = new SucceededChannelFuture(channel, null);
    voidPromise =  new VoidChannelPromise(channel, true);

    tail = new TailContext(this);
    head = new HeadContext(this);

    head.next = tail;
    tail.prev = head;
}

2、初始化NioServerScoketChannel

/***********************ServerBootstrap*************************/
void init(Channel channel) throws Exception {
    final Map<ChannelOption<?>, Object> options = options0();
    // 设置我们之前设置的options  serverBootstrap.option(ChannelOption<T> option, T value)
    synchronized (options) {
        setChannelOptions(channel, options, logger);
    }
    // 设置channel自定义属性
    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());
        }
    }

    ChannelPipeline p = channel.pipeline();

    final EventLoopGroup currentChildGroup = childGroup;
    final ChannelHandler currentChildHandler = childHandler;
    final Entry<ChannelOption<?>, Object>[] currentChildOptions;
    final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
    synchronized (childOptions) {
        currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
    }
    synchronized (childAttrs) {
        currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
    }

    // 添加一个ChannelInitializer，用来注册后续的NioSocketChannel
    p.addLast(new ChannelInitializer<Channel>() {
        @Override
        public void initChannel(final Channel ch) throws Exception {
            final ChannelPipeline pipeline = ch.pipeline();
            ChannelHandler handler = config.handler();
            if (handler != null) {
                pipeline.addLast(handler);
            }

            ch.eventLoop().execute(new Runnable() {
                @Override
                public void run() {
                    // 这个handler就是后续监听客户端连接事件后，会将创建的NioSocketChannel 在这里进行注册
                    pipeline.addLast(new ServerBootstrapAcceptor(
                            ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                }
            });
        }
    });
}

private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
ServerBootstrapAcceptor(
                final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
                Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
        this.childGroup = childGroup;
        this.childHandler = childHandler;
        this.childOptions = childOptions;
        this.childAttrs = childAttrs;

        enableAutoReadTask = new Runnable() {
            @Override
            public void run() {
                channel.config().setAutoRead(true);
            }
        };
    }

    @Override
    @SuppressWarnings("unchecked")
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        // NioServerScoketChannel在监听accpet后会创建一个NioSocketChannel 然后调用handler将该channel传递进来,handler那边只是简单的创建 并没有完成注册
        final Channel child = (Channel) msg;
        // 在serverBootstrap.childHandler(new ChannelInitializer<SocketChannel>()添加的handler 加入到NioSocketChannel
        child.pipeline().addLast(childHandler);
        // 设置NioSocketChannel的Options
        setChannelOptions(child, childOptions, logger);
        // 设置NioSocketChannel的attr
        for (Entry<AttributeKey<?>, Object> e: childAttrs) {
            child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
        }

        try {
            // 将NioSocketChannel注册到workerEventLoopGroup上 然后添加一个监听器
            childGroup.register(child).addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    if (!future.isSuccess()) {
                        forceClose(child, future.cause());
                    }
                }
            });
        } catch (Throwable t) {
            forceClose(child, t);
        }
    }

    private static void forceClose(Channel child, Throwable t) {
        child.unsafe().closeForcibly();
        logger.warn("Failed to register an accepted channel: {}", child, t);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        final ChannelConfig config = ctx.channel().config();
        if (config.isAutoRead()) {
            // 将autoread设置为false，一秒后重新设置为true
            config.setAutoRead(false);
            ctx.channel().eventLoop().schedule(enableAutoReadTask, 1, TimeUnit.SECONDS);
        }
        ctx.fireExceptionCaught(cause);
    }
}

上图大致流程：将之前在ServerBootstrap设置的属性在这里相应设置，然后给NioServerScoketChannel添加一个处理器，这个处理器用来接收accept请求时将后续的NioSocketChannel注册到对应的workerEventLoop中，后续的io事件就交给NioSocketChannel来完成

3、channel注册到EventLoop，其实就是将channel注册到EventLoop中的selector上

/************************MultithreadEventLoopGroup*********************/
public ChannelFuture register(Channel channel) {
    return next().register(channel);
}

/***********************MultithreadEventExecutorGroup**********************/
public EventExecutor next() {
    // 选择一个EventLoop
    return chooser.next();
}

/***********************SingleThreadEventLoop**********************/
public ChannelFuture register(Channel channel) {
    return register(new DefaultChannelPromise(channel, this));
}

public ChannelFuture register(final ChannelPromise promise) {
    // 拿到channel对应的Unsafe进行注册
    promise.channel().unsafe().register(this, promise);
    return promise;
}

/***********************AbstractUnsafe**********************/
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
    AbstractChannel.this.eventLoop = eventLoop;

    if (eventLoop.inEventLoop()) {
        register0(promise);
    } else {
        try {
            eventLoop.execute(new Runnable() {
                @Override
                public void run() {
                    register0(promise);
                }
            });
        } catch (Throwable t) {
            closeForcibly();
            closeFuture.setClosed();
            safeSetFailure(promise, t);
        }
    }
}

private void register0(ChannelPromise promise) {
    try {
        // 判断当前注册promise是否被取消或者channel通道是否还处于打开状态
        if (!promise.setUncancellable() || !ensureOpen(promise)) {
            return;
        }
        boolean firstRegistration = neverRegistered;
        // 进行nio注册
        doRegister();
        neverRegistered = false;
        registered = true;

        // 注册好后调用我们之前设置的channelInitializer.initChannel(channel) 方法
        pipeline.invokeHandlerAddedIfNeeded();
        // 设置状态成功
        safeSetSuccess(promise);
        // 注册成功事件传播
        pipeline.fireChannelRegistered();
        // 完成注册正好是active状态
        if (isActive()) {
            if (firstRegistration) {
                pipeline.fireChannelActive();
            } else if (config().isAutoRead()) {
                beginRead();
            }
        }
    } catch (Throwable t) {
        closeForcibly();
        closeFuture.setClosed();
        safeSetFailure(promise, t);
    }
}

/***********************AbstractNioUnsafe**********************/
protected void doRegister() throws Exception {
    boolean selected = false;
    for (;;) {
        try {
            // 将channel注册到EventLoop上的selector
            selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
            return;
        } catch (CancelledKeyException e) {
            if (!selected) {
                eventLoop().selectNow();
                selected = true;
            } else {
                throw e;
            }
        }
    }
}

4、绑定端口,通过pipeline.bind发起绑定，绑定端口是出站事件，由tail像前进行传递，直到执行到head的bind()方法，然后通过其中的unsafe调用NioServerScoketChannel的doBind()进行绑定

/************************tailHandler*********************/
public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
    final AbstractChannelHandlerContext next = findContextOutbound();
    EventExecutor executor = next.executor();
    if (executor.inEventLoop()) {
        next.invokeBind(localAddress, promise);
    } else {
        safeExecute(executor, new Runnable() {
            @Override
            public void run() {
                next.invokeBind(localAddress, promise);
            }
        }, promise, null);
    }
    return promise;
}

private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
    if (invokeHandler()) {
        try {
            ((ChannelOutboundHandler) handler()).bind(this, localAddress, promise);
        } catch (Throwable t) {
            notifyOutboundHandlerException(t, promise);
        }
    } else {
        bind(localAddress, promise);
    }
}

/************************headHandler*********************/
public void bind(
        ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
        throws Exception {
    unsafe.bind(localAddress, promise);
}

/************************AbstractUnsafe*********************/
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
    assertEventLoop();

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

    boolean wasActive = isActive();
    try {
        doBind(localAddress);
    } catch (Throwable t) {
        safeSetFailure(promise, t);
        closeIfClosed();
        return;
    }

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

    safeSetSuccess(promise);
}

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

服务端的启动流程就讲的差不多了，对上述流程大致进行一个梳理，进行端口绑定时首先对创建一个NioServerScoketChannel 用来处理accpet，然后注册到BossEventLoop上，通过给NioServerScoketChannel添加一个处理器 将来发生accpet时，将生成的NioSocketChannel注册到WorkerEventLoop上，后续的io事件就在该NioSocketChannel完成

客户端启动流程

客户端这边启动流程和服务端大致类似，连接ip端口时，创建一个用来和服务端通信的NioSocketChannel（内部包裹着SocketChannel），然后注册到对应的EventLoop上的selector开启事件监听

ChannelFuture cf = bootstrap.connect("127.0.0.1", 8080).sync();
cf.channel().closeFuture().sync();

/************************Bootstrap*********************/
private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
    // 创建一个NioSocketChannel然后进行注册，流程跟NioServerScoketChannel差不多，返回一个注册future
    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 {
        // 还未注册完成 添加一个监听器 用作将来注册成功后进行连接操作
        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) {
                    promise.setFailure(cause);
                } else {
                    promise.registered();
                    doResolveAndConnect0(channel, remoteAddress, localAddress, promise);
                }
            }
        });
        return promise;
    }
}

private ChannelFuture doResolveAndConnect0(final Channel channel, SocketAddress remoteAddress,
                                           final SocketAddress localAddress, final ChannelPromise promise) {
    try {
        final EventLoop eventLoop = channel.eventLoop();
        final AddressResolver<SocketAddress> resolver = this.resolver.getResolver(eventLoop);

        //解析器无法解析ip地址或者已经被解析
        if (!resolver.isSupported(remoteAddress) || resolver.isResolved(remoteAddress)) {
            doConnect(remoteAddress, localAddress, promise);
            return promise;
        }

        final Future<SocketAddress> resolveFuture = resolver.resolve(remoteAddress);

        if (resolveFuture.isDone()) {
            // 解析完成 进行连接
            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;
        }

        // 添加一个监听器用来将来地址解析成功然后进行连接
        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;
}

private static void doConnect(
        final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise connectPromise) {

    final Channel channel = connectPromise.channel();
    // 提交一个连接任务到EventLoop上
    channel.eventLoop().execute(new Runnable() {
        @Override
        public void run() {
            if (localAddress == null) {
                channel.connect(remoteAddress, connectPromise);
            } else {
                channel.connect(remoteAddress, localAddress, connectPromise);
            }
            // 添加一个关闭或失败的监听器,会将channel进行关闭
            connectPromise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
        }
    });
}

// 设置channel的options和attrs
void init(Channel channel) throws Exception {
    ChannelPipeline p = channel.pipeline();
    p.addLast(config.handler());

    final Map<ChannelOption<?>, Object> options = options0();
    synchronized (options) {
        setChannelOptions(channel, options, logger);
    }

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

至此我们服务端和客户端的启动流程都分析完了，我们前面说过EventLoop可以当做一个线程来执行， channel.eventLoop().execute(new Runnable(），后续的io事件监听和自定义任务处理都在EventLoop内执行，我们下节就来剖析其内部实现