netty笔记（3）--netty服务的启动过程

遇到问题时，有时需要带着疑惑分析下源码才能更好的解决问题。
记录下分析netty启动的过程。
启动代码

public void start() throws InterruptedException {
        final EchoServerInHandler inHandler = new EchoServerInHandler();
        final EchoServerOutHandler outHandler=new EchoServerOutHandler();
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        ServerBootstrap bootstrap = new ServerBootstrap();
        try {
            bootstrap.group(bossGroup,workerGroup)
                     .channel(NioServerSocketChannel.class)
                     .localAddress(new InetSocketAddress(port))
                     .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(outHandler);
                            ch.pipeline().addLast(inHandler);
                        }
                    });
            ChannelFuture f = bootstrap.bind().sync();
            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully().sync();
            workerGroup.shutdownGracefully().sync();
        }
    }

分析ChannelFuture f = bootstrap.bind().sync();发生了什么
1、
调用 validate()方法检验参数是否有效。
调用doBind(localAddress)处理核心的逻辑。

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

2、
调用initAndRegister()方法初始化和注册。但是初始化和注册什么呢？看下面

 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;
        }
    }

3、
//通过channelFactory.newChannel()创建一个channel。这里channelFactory是
ReflectiveChannelFactory。
接着调用init(channel)初始化channel。

 final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
        //具体说明看下面，newChannel()发生了什么呢？
            channel = channelFactory.newChannel();
           //初始化channel
            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);
        }
        //注册channel
        ChannelFuture regFuture = config().group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }
        return regFuture;
    }

ReflectiveChannelFactory通过反射创建channel。clazz就是我们设置的NioServerSocketChannel.class。

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

接着看下创建NioServerSocketChannel时发生了什么？

核心类就是SelectorProvider 
private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();
   public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));
    }

通过SelectorProvider 打开一条channel通道

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

看到了SelectionKey.OP_ACCEPT。说明这个channel的作用就是处理建立连接的请求。

 public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }

那 init(channel);又发生了什么
先设置options 和attrs 。最后为NioServerSocketChannel添加一个ServerBootstrapAcceptor，ServerBootstrapAcceptor用于处理建立连接的请求。

    @Override
    void init(Channel channel) throws Exception {
        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()) {
                @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()));
        }

        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() {
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });
    }