netty服务端辅助类ServerBootstrap如何创建

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ServerBootstrap创建

ServerBootstrap 为 netty 建立服务端的辅助类, 以 NIO为例，创建代码如下：

public static void main(String[] args) throws Exception {
        ServerBootstrap bs = new ServerBootstrap();
        bs.group(new NioEventLoopGroup(1), new NioEventLoopGroup())
                .channel(NioServerSocketChannel.class)
                .childHandler(new ChannelInitializer<Channel>() {
                    @Override
                    protected void initChannel(Channel ch) throws Exception {
                        ch.pipeline()
                        .addLast(new HttpServerCodec())
                        .addLast(new HttpObjectAggregator(65535))
                        .addLast(new Controller());
                    }
                }).bind(8080).sync().channel().closeFuture().sync();
    }

核心参数

//配置属性，如 SO_KEEPALIVE 等private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);
    //acceot 的 子channel所绑定的 事件循环组"
    private volatile EventLoopGroup childGroup;
    private volatile ChannelHandler childHandler;

初始化流程

主要为 绑定本地端口 -> 注册自身到 EventLoop ， 并注册 accept 和 read 事件 -> EventLoop的主循环中会不断的select注册的channel的事件，并处理。

首先执行绑定

核心逻辑位于 

io.netty.bootstrap.AbstractBootstrap.doBind(SocketAddress) 和  io.netty.bootstrap.AbstractBootstrap.initAndRegister()中

private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        ..........if (regFuture.isDone()) {
            // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();
            //绑定逻辑
            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;
        }
    }

注册自身到 EventLoop

先来看 initAndRegister ， 核心逻辑就是利用channelFactory初始化一个NioServerSocketChannel实例，并为其设置上config中的参数，然后将其注册到EventLoop中，实际上是委托的channel的Unsafe来实现注册的，核心逻辑位于 AbstractUnsafe.register0 中 完成注册

final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            //本例子中实际调用的是  NioServerSocketChannel的构造参数， 并为其设置感兴趣的事件类型为  OP_ACCEPT
            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);
        }
        ChannelFuture regFuture = config().group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

         return regFuture;
    }

 void init(Channel channel) throws Exception {
         //设置属性
          ..........

         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() {
                         //为NioServerSocketChannel 设置一个 默认的 channelhandler ： ServerBootstrapAcceptor ， 当发生 accept事件时，将 accept的channel注册到 childEventLoop中
                         pipeline.addLast(new ServerBootstrapAcceptor(
                                 ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                     }
                 });
             }
         });
     }

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;
                //执行channel到 eventloop的 selector
                doRegister();
                neverRegistered = false;
                registered = true;

                // 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);

//触发 InboundChannelHnader.channelRegistered 事件
                  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) {
                        //触发channelActive事件，并会为 channel 绑定上 read 事件 
                        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);
            }
        }

绑定端口逻辑

initAndRegister注册成功后,开始执行真正的绑定端口逻辑，核心逻辑位于 NioSocketChannel.doBind0(SocketAddress) 中

private void doBind0(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) {
            SocketUtils.bind(javaChannel(), localAddress);
        } else {
            SocketUtils.bind(javaChannel().socket(), localAddress);
        }
    }

至此 绑定个成功， 当触发 ACCEPT 事件时， 会触发  NioServerSocketChannel.doReadMessages -> ServerBootstrapAcceptor.channelRead , 并将 子channel 注册到 childEventLoop中

public void channelRead(ChannelHandlerContext ctx, Object msg) {
            final Channel child = (Channel) msg;
            child.pipeline().addLast(childHandler);
            setChannelOptions(child, childOptions, logger);
            for (Entry<AttributeKey<?>, Object> e: childAttrs) {
                child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
            }
            try {
                //注册channel
                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);
            }
        }

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