netty中pipeline如何添加handler

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pipeline无疑是 netty中非常重要的一环,在io处理中netty通过已存放在pipeline中的handler有序处理socket中的信息,而pipeline中包含的都是一个个的handler，它是怎么添加进去的呢

创建代码如下:其中有handler和childhandler

 public void start() throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            ServerBootstrap sb = new ServerBootstrap();
            sb.option(ChannelOption.SO_BACKLOG, 1024);
            // 绑定线程池
            sb.group(group, bossGroup)
                    // 指定使用的channel
                    .channel(NioServerSocketChannel.class)
                    // 绑定监听端口
                    .localAddress(this.port)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    // 绑定客户端连接时候触发操作
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ChannelPipeline pipeline = ch.pipeline();
                            pipeline.addLast(new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 4, 0, 4));
                            pipeline.addLast(new LengthFieldPrepender(4));
                            //字符串解码
                            pipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
                            //字符串编码
                            pipeline.addLast(new StringEncoder(CharsetUtil.UTF_8));
                            pipeline.addLast(new IdleStateHandler(0, 0, 120, TimeUnit.SECONDS));
                        }
                    });
            // 服务器异步创建绑定
            ChannelFuture cf = sb.bind().sync();
            log.info(NettyServer.class + " 启动正在监听： " + cf.channel().localAddress());
            // 关闭服务器通道
            cf.channel().closeFuture().sync();
        } finally {
            // 释放线程池资源
            group.shutdownGracefully().sync();
            bossGroup.shutdownGracefully().sync();
        }
    }

顺着服务端bind方法点进去会看到如下:创建完channel实例后 会调用一个init方法

final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            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);
            }
            // 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);
        }

        ChannelFuture regFuture = config().group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

        return regFuture;
    }

init方法为抽象方法 服务端实现如下

void init(Channel channel) throws Exception {
//省略若干
        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(0));
        }
        synchronized (childAttrs) {
            currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
        }
		//看这里  添加了一个ChannelInitializer实例 实例中实现了initChannel   获取配置的handler添加到pipeline
        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));
                    }
                });
            }
        });
    }

而pipeline的addlast方法如下

    public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
        final AbstractChannelHandlerContext newCtx;
        synchronized (this) {
            checkMultiplicity(handler);

            newCtx = newContext(group, filterName(name, handler), handler);
			//这方法很简单 添加到链表中  但是此时添加的是上文的ChannelInitializer 而不是我们定义的handler
            addLast0(newCtx);

            // If the registered is false it means that the channel was not registered on an eventLoop yet.
            // In this case we add the context to the pipeline and add a task that will call
            // ChannelHandler.handlerAdded(...) once the channel is registered.
            if (!registered) {
                newCtx.setAddPending();
                callHandlerCallbackLater(newCtx, true);
                return this;
            }

            EventExecutor executor = newCtx.executor();
            if (!executor.inEventLoop()) {
                callHandlerAddedInEventLoop(newCtx, executor);
                return this;
            }
        }
		//这个方法就是关键 将ChannelInitializer中的handler添加到pipeline中 以及删除ChannelInitializer
		//但请注意 首次注册不会走到这里  在前面的判断就会返回  会在注册的逻辑中触发  最终调用的一致
        callHandlerAdded0(newCtx);
        return this;
    }

查看ChannelInitializer中的关键代码如下 OK 通过如下代码我们就看到 调用了ChannelInitializer的initChannel方法 就是在最开始我们定义的那个，然后里面就用继续调用pipeline add我们真正的handler

    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        if (ctx.channel().isRegistered()) {
            // This should always be true with our current DefaultChannelPipeline implementation.
            // The good thing about calling initChannel(...) in handlerAdded(...) is that there will be no ordering
            // surprises if a ChannelInitializer will add another ChannelInitializer. This is as all handlers
            // will be added in the expected order.
            if (initChannel(ctx)) {

                // We are done with init the Channel, removing the initializer now.
                removeState(ctx);
            }
        }
    }
	    private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
        if (initMap.add(ctx)) { // Guard against re-entrance.
            try {
                initChannel((C) ctx.channel());
            } catch (Throwable cause) {
                // Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
                // We do so to prevent multiple calls to initChannel(...).
                exceptionCaught(ctx, cause);
            } finally {
				//看这里删除pipeline中的 ChannelInitializer   ChannelInitializer 其实就是一个handler  只是它重写了handlerAdded方法
                ChannelPipeline pipeline = ctx.pipeline();
                if (pipeline.context(this) != null) {
                    pipeline.remove(this);
                }
            }
            return true;
        }
        return false;
    }

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