netty服務端輔助類ServerBootstrap如何創建

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ServerBootstrap創建

ServerBootstrap 為 netty 建立服務端的輔助類, 以 NIO為例，創建代碼如下：

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();
    }

核心參數

//配置屬性，如 SO_KEEPALIVE 等private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);
    //acceot 的 子channel所綁定的 事件循環組"
    private volatile EventLoopGroup childGroup;
    private volatile ChannelHandler childHandler;

初始化流程

主要為 綁定本地端口 -> 注冊自身到 EventLoop ， 并注冊 accept 和 read 事件 -> EventLoop的主循環中會不斷的select注冊的channel的事件，并處理。

首先執行綁定

核心邏輯位于 

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();
            //綁定邏輯
            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;
        }
    }

注冊自身到 EventLoop

先來看 initAndRegister ， 核心邏輯就是利用channelFactory初始化一個NioServerSocketChannel實例，并為其設置上config中的參數，然后將其注冊到EventLoop中，實際上是委托的channel的Unsafe來實現注冊的，核心邏輯位于 AbstractUnsafe.register0 中 完成注冊

final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            //本例子中實際調用的是  NioServerSocketChannel的構造參數， 并為其設置感興趣的事件類型為  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 {
         //設置屬性
          ..........

         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() {
                         //為NioServerSocketChannel 設置一個 默認的 channelhandler ： ServerBootstrapAcceptor ， 當發生 accept事件時，將 accept的channel注冊到 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;
                //執行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);

//觸發 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) {
                        //觸發channelActive事件，并會為 channel 綁定上 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);
            }
        }

綁定端口邏輯

initAndRegister注冊成功后,開始執行真正的綁定端口邏輯，核心邏輯位于 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 綁定個成功， 當觸發 ACCEPT 事件時， 會觸發  NioServerSocketChannel.doReadMessages -> ServerBootstrapAcceptor.channelRead , 并將 子channel 注冊到 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 {
                //注冊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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