把标题连在一起，就是这个服务启动所做的事情

• 初始化Channel并注册
  • 初始化Channel
    • 配置ChannelConfig的ChannelOptions
    • 配置Attribute
    • Pipeline添加ChannelHandler
  • 注册过程
    • 选择执行注册的EvenLoop
    • 启动EventLoop
    • 异步执行注册任务
      • 注册到Selector
      • 回调ChannelHandler的handlerAdded
      • 回调ChannelInboundHandler的channelRegistered
      • isActive决定是否回调ChannelInboundHandler的channelActive
  • 异步绑定
    • 回调HeadContext.bind方法
      • NioServerSocketChannel的doBind实现
      • isActive决定是否回调ChannelInboundHandler的channelActive

1 服务端启动Demo

ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup，workerGroup)
    .channel(NioServerSocketChannel.class)
    .childHandler(new ChannelInitializer<SocketChannel>() {
        @Override
        public void initChannel(SocketChannel ch) 
throws Exception {
            ch.pipeline().addLast(handlers);
        }})
    .option(ChannelOption.SO_BACKLOG, 128)
    .childOption(ChannelOption.SO_KEEPALIVE, true);
ChannelFuture f = b.bind(port).sync(); 

2 启动过程源码解读

为了方便阅读和理解，代码做了合并和简化，只保留核心代码

2.1 初始化Channel并注册

 final ChannelFuture initAndRegister() {
    Channel channel = null;
    try {
        channel = channelFactory.newChannel();
        init(channel);
    } catch (Throwable t) {
        // XXX
    }
    ChannelFuture regFuture = config().group().register(channel);
    if (regFuture.cause() != null) {
        // XXX
    }
    return regFuture;
}

2.1.1 初始化Channel

2.1.1.1 配置ChannelConfig的ChannelOptions

final Map<ChannelOption<?>, Object> options = options0();
synchronized (options) {
    for (Map.Entry<ChannelOption<?>, Object> e: options.entrySet()) {
        channel.config().setOption((ChannelOption<Object>) e.getKey(), value)
    }
}

2.1.1.2 配置Attribute

final Map<AttributeKey<?>, Object> attrs = attrs0();
synchronized (attrs) {
    for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
        AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
        channel.attr(key).set(e.getValue());
    }
}

2.1.1.3 Pipeline添加ChannelHandler

ChannelPipeline p = channel.pipeline();
p.addLast(new ChannelInitializer<Channel>() {
    @Override
    public void initChannel(final Channel ch) throws Exception {
        // 添加用户定义的ChannelHandler
        final ChannelPipeline pipeline = ch.pipeline();
        ChannelHandler handler = config.handler();
        if (handler != null) {
            pipeline.addLast(handler);
        }
        
        // 异步添加ServerBootstrapAcceptor
        ch.eventLoop().execute(new Runnable() {
            @Override
            public void run() {
                pipeline.addLast(new ServerBootstrapAcceptor(
                        ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
            }
        });
    }
});

也许你对 ServerBootstrapAcceptor 很感兴趣，那么请延后了解，因为我们本篇文章主要在探究Netty服务加载的过程。
请记住，如果赋予一件事情过多的目的，那么这件事情一定会失败

2.1.2 注册过程

2.1.2.1 选择执行注册的EvenLoop

public EventExecutorChooser newChooser(EventExecutor[] executors) {
    // 非常巧妙，利用补码特性判断2的幂次，此处加鸡腿
    // (val & -val) == val
    if (isPowerOfTwo(executors.length)) {
        return new PowerOfTwoEventExecutorChooser(executors);
    } else {
        return new GenericEventExecutorChooser(executors);
    }
}

• PowerOfTwoEventExecutorChooser

executors[idx.getAndIncrement() & executors.length - 1]

• GenericEventExecutorChooser

executors[Math.abs(idx.getAndIncrement() % executors.length

2.1.2.2 启动EventLoop

public void execute(Runnable task) {
    // 添加任务到队列
    addTask(task);
    executor.execute(new Runnable() {
        @Override
        public void run() {
            try {
                // 执行EvenLoop的run方法
                SingleThreadEventExecutor.this.run();
            } catch (Throwable t) {
                // XXX
            } finally {
                // 循环执行队列中任务
            }
        });
    }
}

2.1.2.3异步执行注册任务

private void register0(ChannelPromise promise) {
    try {
        doRegister();
        pipeline.invokeHandlerAddedIfNeeded();
        pipeline.fireChannelRegistered();
        if (isActive()) {
            if (firstRegistration) {
                pipeline.fireChannelActive();
            } else if (config().isAutoRead()) {
                beginRead();
            }
        }
    } catch (Throwable t) {
        // Close the channel directly to avoid FD leak.
    }
}

2.1.2.3.1 注册到Selector

• AbstractNioUnsafe的实现

protected void doRegister() throws Exception {
    boolean selected = false;
    for (;;) {
        try {
            selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
            return;
        } catch (CancelledKeyException e) {
            if (!selected) {
                // Force the Selector to select now as the "canceled" SelectionKey may still be
                // cached and not removed because no Select.select(..) operation was called yet.
                eventLoop().selectNow();
                selected = true;
            } else {
                // We forced a select operation on the selector before but the SelectionKey is still cached
                // for whatever reason. JDK bug ?
                throw e;
            }
        }
    }
}

2.1.2.3.2 回调ChannelHandler的handlerAdded

final void invokeHandlerAddedIfNeeded() {
    assert channel.eventLoop().inEventLoop();
    if (firstRegistration) {
        firstRegistration = false;
        PendingHandlerCallback task = pendingHandlerCallbackHead;
        while (task != null) {
            if (executor.inEventLoop()) {
                ctx.setAddComplete();
                ctx.handler().handlerAdded(ctx);
            } else {
                // 异步执行
            }
            task = task.next;
        }
    }
}

2.1.2.3.3 回调ChannelInboundHandler的channelRegistered

@Override
public final ChannelPipeline fireChannelRegistered() {
    EventExecutor executor = next.executor();
    if (executor.inEventLoop()) {
        invokeChannelRegistered : if (invokeHandler()) {
            try {
                ((ChannelInboundHandler) handler()).channelRegistered(this);
            }
        } else {
            // 查找下一个Handler
            AbstractChannelHandlerContext ctx = this;
            do {
                ctx = ctx.next;
            } while (!ctx.inbound);

            EventExecutor executor2 = ctx.executor();
            if (executor2.inEventLoop()) {
                ctx.invokeChannelRegistered();
            } else {
                // 异步执行
            });
        }
    } else {
        // 异步执行
    }
}

2.1.2.3.4 isActive决定是否回调ChannelInboundHandler的channelActive

@Override
public final ChannelPipeline fireChannelActive() {
    EventExecutor executor = next.executor();
    if (executor.inEventLoop()) {
        invokeChannelActive : if (invokeHandler()) {
            try {
                ((ChannelInboundHandler) handler()).channelActive(this);
            }
        } else {
            AbstractChannelHandlerContext ctx = this;
            do {
                ctx = ctx.next;
            } while (!ctx.inbound);
            if (ctx.executor().inEventLoop()) {
                ctx.invokeChannelActive();
            } else {
                // 异步执行
            }
        }
    } else {
        // 异步处理
    }
}

2.2 异步绑定

2.2.1 回调HeadContext.bind方法

@Override
public void bind(
        ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
        throws Exception {
    unsafe.bind(localAddress, promise);
}

2.2.1.1 NioServerSocketChannel的doBind实现

@Override
protected void doBind(SocketAddress localAddress) throws Exception {
    if (PlatformDependent.javaVersion() >= 7) {
        javaChannel().bind(localAddress, config.getBacklog());
    } else {
        javaChannel().socket().bind(localAddress, config.getBacklog());
    }
}

2.2.1.2 isActive决定是否回调ChannelInboundHandler的channelActive

事实上，在执行doBind操作后，isActive才为true，所以channelActive实在这一步回调的

if (!wasActive && isActive()) {
    invokeLater(new Runnable() {
        @Override
        public void run() {
            pipeline.fireChannelActive();
        }
    });
}