netty服务端的创建

服务端的创建

示例代码

netty源码中有一个netty-example项目，不妨以经典的EchoServer作为楔子。

// 步骤1
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
final EchoServerHandler serverHandler = new EchoServerHandler();
try {
    // 步骤2
    ServerBootstrap b = new ServerBootstrap();
    b.group(bossGroup, workerGroup)
        .channel(NioServerSocketChannel.class)
        .option(ChannelOption.SO_BACKLOG, 100)
            .attr(AttributeKey.newInstance("attr"), "attr")
        .handler(new LoggingHandler(LogLevel.INFO))
        .childHandler(new channelInitializer<SocketChannel> {
            @Override
            public void initChannel(SocketChannel ch)throws Exception {
                ChannelPipeline p = ch.pipeline();
                if (sslCtx != null) {
                    p.addLast(sslCtx.newHandler(ch.alloc()));
                }
                p.addLast(serverHandler);
            }
        })
        .childOption(ChannelOption.CONNECT_TIMEOUT_MILLIS, 3000)
        .childAttr(AttributeKey.newInstance("childAttr"), "childAttr");
        // 步骤3
        ChannelFuture f = b.bind(PORT).sync();
        // 步骤4
        f.channel().closeFuture().sync();
} finally {
    bossGroup.shutdownGracefully();
    workerGroup.shutdownGracefully();
}

整个流程可以分为几个步骤

1. 创建workerGroup和bossGroup
2. 创建ServerBootstrap，并设置参数
3. 通过Serverbootstrap引导启动，并绑定端口。这里又可以分为初始化、注册、绑定端口、注册感兴趣事件4个小步骤
4. 主线程阻塞
5. 设置优雅关闭

步骤1留待下一节，先看步骤2

步骤2

ServerBootstrap首先设置bossGroup为parentGroup，workerGroup为childGroup，然后在channel方法中创建了一个工厂，该生产通过设置进来的泛型,利用反射来生产对象。此处生产对象为ServerSocketChannel。

除此之外还设置了handler、childHandler、option、childOption、attr、childAttr6个属性，根据group的命名规则，可以猜测不带child的属性是给bossGroup内的nioEventLoop使用，而以child开头的属性是给workGroupo内的nioEventLoop使用的。

public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
    super.group(parentGroup);
    this.childGroup = childGroup;
    return this;
}

public B channel(Class<? extends C> channelClass) {
    return channelFactory(new ReflectiveChannelFactory<C>
            (channelClass)
    ));
}

public ReflectiveChannelFactory(Class<? extends T> clazz) {
    this.constructor = clazz.getConstructor();
}

步骤3

serverBootstrap配置完后,开始服务端真正的启动工作，进入b.bind()方法，一路跳转到AbstractBootstrap.doBind(SocketAddress localAddress)方法。

private ChannelFuture doBind(final SocketAddress localAddress) {
    // 初始化并注册
    final ChannelFuture regFuture = initAndRegister();
    final Channel channel = regFuture.channel();
    //若已经注册完成,则开始绑定，否则添加一个监听器,待注册完成后绑定
    if (regFuture.isDone()) {
        ChannelPromise promise = channel.newPromise();
        doBind0(regFuture, channel, localAddress, promise);
        return promise;
    } else {
        final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
        regFuture.addListener((ChannelFutureListener) future ->
                doBind0(regFuture, channel, localAddress, promise));
        return promise;
    }
}

步骤3.1

先看看initAndRegister()方法。首先是创建NioServerSocketChannel，通过工厂模式实例化一个channel。在channel的构造函数里，传入OP_ACCEPT感兴趣事件，并设置NioServerSocketChannel为非阻塞模式。在顶级抽象父类里，创建好id、unsafe、pipeline。unsafe对象是关联socket或其他进行IO操作组件的一个类，与jdk的unsafe对象类似，一般不需要用户关注

final ChannelFuture initAndRegister() {
    Channel channel  = channelFactory.newChannel();
    init(channel);

    ChannelFuture regFuture = config().group().register(channel);
    return regFuture;
}

public T newChannel() {
     return constructor.newInstance();
}

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

protected AbstractNioChannel(Channel parent, SelectableChannel ch, inteadInterestOp) {
     super(parent);
     this.ch = ch;
     this.readInterestOp = readInterestOp;
     ch.configureBlocking(false);
     }
 }
AbstractChannel(Channel parent) {
     this.parent = parent;
     id = newId();
     unsafe = newUnsafe();
     pipeline = newChannelPipeline();
 }

之后开始执行初始化,将bootstrapServer中的option和attr赋予serverSocketChannel,通过pipeline添加一个channelInitializer，进而通过channelInitializer将childOption、childAttr、workerGroup、childHandler保存在一个叫ServerBootstrapAcceptor的handler中。从名字可以猜测，此handler应该是在客户端连接时来处理相关事件的。而channelInitializer会在完成由子类实现的initChannel方法后将自己从pipeline中移除。

void init(Channel channel) {
    setChannelOptions(channel, options0().entrySet().toArray(newOptionArray(0)), logger);
    setAttributes(channel, attrs0().entrySet().toArray(newAttrArray(0)));
    ChannelPipeline p = channel.pipeline();
    final EventLoopGroup currentChildGroup = childGroup;
    final ChannelHandler currentChildHandler = childHandler;
    final Entry<ChannelOption<?>, Object>[] currentChildOptions = childOptions.entrySet().toArray(newOptionArray(0));
    final Entry<AttributeKey<?>, Object>[] currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
    p.addLast(new ChannelInitializer<Channel>() {
    @Override
    public void initChannel(final Channel ch) {
        final ChannelPipeline pipeline = ch.pipeline();
        ChannelHandler handler = config.handler();
        if (handler != null) {
            pipeline.addLast(handler);
        }
        ch.eventLoop().execute(() -> pipeline.addLast(new ServerBootstrapAcceptor(
                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)));
    }
});
}

步骤3.2

初始化完毕后便是注册,首先判断是否注册以及传进来的eventLoop参数是否属于NioEventLoop类，然后判断当前线程是否是NioEventLoop线程，若是，则直接注册，否则添加到eventLoop的线程池中,通过创建nioEventLoop线程来执行注册。在register0——doRegister方法链中，我们看到netty最终调用了jdk底层的channel绑定了selector，由于此时还未绑定端口，所以ops即感兴趣事件是0。同时，把this，即NioServerSocketChannel作为attachment添加到selectionKey上，这是为了之后在select出事件时，可以获取channel进行操作。当注册完毕后，调用pipeline进行注册事件传播。如果设置了自动读取，还会立即开始一次读取。

// 通过unsafe对象进行注册
public ChannelFuture register(final ChannelPromise promise) {
    promise.channel().unsafe().register(this, promise);
    return promise;
}

// 如果当前线程是之前nioEventLoop绑定的线程则直接注册，否则添加到eventLoop的线程池中
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
    AbstractChannel.this.eventLoop = eventLoop;
    if (eventLoop.inEventLoop()) {
        register0(promise);
    } else {
        eventLoop.execute(() -> register0(promise));
    }
}

// 注册后调用pipeline进行一些事件传播
private void register0(ChannelPromise promise) {
   boolean firstRegistration = neverRegistered;
   doRegister();
   neverRegistered = false;
   registered = true;
   pipeline.invokeHandlerAddedIfNeeded();
   pipeline.fireChannelRegistered();
   if (isActive()) {
       if (firstRegistration) {
           pipeline.fireChannelActive();
       } else if (config().isAutoRead()) {
           beginRead();
       }
   }
}

// 最终调用jdk底层channel进行注册
protected void doRegister() throws Exception {
    for (;;) {
        selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
        return;
    }
}

步骤3.3

回到AbstractBootStrap, 开始执行doBind0方法。这也是需要由nioEventLoop执行的，所以也丢到了线程池里。

private static void doBind0(final ChannelFuture regFuture, final Channel channel,
    final SocketAddress localAddress, final ChannelPromise promise) {
    channel.eventLoop().execute(new Runnable() {
        public void run() {
            if (regFuture.isSuccess()) {
                channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
            } else {
                promise.setFailure(regFuture.cause());
            }
        }
    });
}
// 调用pipeline-tail进行绑定
public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
    return pipeline.bind(localAddress, promise);
}
public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
    return tail.bind(localAddress, promise);
}

@Override
public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
    // 调用下一个OutBoundHandlerContext执行,默认传递到headContext
    final AbstractChannelHandlerContext next = findContextOutbound(MASK_BIND);
    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;
}
// 传到headContext，调用unsafe来执行。
@Override
public void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) {
    unsafe.bind(localAddress, promise);
}

// AbstractUnsafe的bind
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
    doBind(localAddress);
    if (!wasActive && isActive()) {
        invokeLater(new Runnable() {
        @Override
        public void run() {
            pipeline.fireChannelActive();
        }
    });
}
// unsafe最终调用jdk的channel完成绑定操作
protected void doBind(SocketAddress localAddress) throws Exception {
    javaChannel().bind(localAddress, config.getBacklog());
}

nioEventLoop通过pipeline的方式绑定,pipeline调用tailContext的bind方法，tailContext又会不断寻找下一个OutBoundHandler来执行bind方法，默认会传到headContext，headContext再调用底层的unsafe来执行bind。unsafe完成bind后，会通知pipeline调用fireChannelActive方法。这里绑定端口便完成了

步骤3.4

绑定端口后还需要注册感兴趣事件，这是通过fireChannelActive触发的。active事件首先会传递到headContext，

public void channelActive(ChannelHandlerContext ctx) {
    ctx.fireChannelActive();
    readIfIsAutoRead();
}

headContext继续将active事件传播，然后调用readIfIsAutoRead方法。此方法会调用channel的read方法，继而调用pipeline的read事件进行传播，又回到headContext，headContext又调用unsafe的beginRead如下:

protected void doBeginRead() throws Exception {
    final SelectionKey selectionKey = this.selectionKey;
    readPending = true;
    final int interestOps = selectionKey.interestOps();
    if ((interestOps & readInterestOp) == 0) {
        selectionKey.interestOps(interestOps | readInterestOp);
    }
}

在beginRead方法中,将channel注册感兴趣事件为创建NioServerSocketChannel时传入的OP_ACCEPT事件。

至此，初始化、注册selector、绑定、注册感兴趣事件4个小步骤都完成了

步骤4

主线程调用DefaultChannelPromise的sync方法，sync方法调用父类的await方法,阻塞在此处, 从此任由netty自由发挥，知道netty关闭了线程

步骤5

当netty报错或关闭后，主线程结束阻塞，开始执行netty优雅关闭机制。待续