Netty源码学习（五）ChannelInitializer

0. ChannelInitializer简介

直接用ChannelInitializer的注释吧：A special ChannelInboundHandler which offers an easy way to initialize a Channel once it was registered to its eventLoop.

1. ChannelInitializer类图

需要注意的是：

a. ChannelInitializer继承于ChannelInboundHandler接口

b. ChannelInitializer是一个抽象类，不能直接使用

2. initChannel抽象方法

ChannelInitializer中声明了一个名为initChannel的抽象方法：

/**
 * This method will be called once the {@link Channel} was registered. After the method returns this instance
 * will be removed from the {@link ChannelPipeline} of the {@link Channel}.
 *
 * @param ch            the {@link Channel} which was registered.
 * @throws Exception    is thrown if an error occurs. In that case it will be handled by
 *                      {@link #exceptionCaught(ChannelHandlerContext, Throwable)} which will by default close
 *                      the {@link Channel}.
 */
protected abstract void initChannel(C ch) throws Exception;

ChannelInitializer的实现类必须要重写这个方法，这个方法在Channel被注册到EventLoop的时候会被调用

3. ChannelInitializer什么时候会被调用？

以ServerBootstrap启动这一场景为例

在ServerBootstrap.init()方法中，负责accept新链接的Channel的pipeline被添加了一个ChannelInitializer

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));
            }
        });
    }
});

由于此时这个Channel还没有被register到EventLoop，于是在addLast方法的调用链中，会给pipeline添加一个PendingHandlerAddedTask，其目的是在Channel被register到EventLoop的时候，触发一个回调事件

然后在AbstractBootstrap.initAndRegister()方法中，这个Channel会被register到boss EventLoopGoup，接着会被register到boss EventLoopGoup中的某一个具体的EventLoop

在AbstractChannel.register0()方法中，之前注册的PendingHandlerAddedTask会被调用，经过一系列调用之后，ChannelInitializer.handleAdded()方法会被触发：

    /**
     * {@inheritDoc} If override this method ensure you call super!
     */
    @Override
    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.
            initChannel(ctx);
        }
    }

    @SuppressWarnings("unchecked")
    private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
        if (initMap.putIfAbsent(ctx, Boolean.TRUE) == null) { // Guard against re-entrance.
            try {
                initChannel((C) ctx.channel());//调用子类重写的initChannel方法
            } 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 {
                remove(ctx);//将ChannelInitializer从pipeline中移除
            }
            return true;
        }
        return false;
    }

    /**
     * This method will be called once the {@link Channel} was registered. After the method returns this instance
     * will be removed from the {@link ChannelPipeline} of the {@link Channel}.
     *
     * @param ch            the {@link Channel} which was registered.
     * @throws Exception    is thrown if an error occurs. In that case it will be handled by
     *                      {@link #exceptionCaught(ChannelHandlerContext, Throwable)} which will by default close
     *                      the {@link Channel}.
     */
    protected abstract void initChannel(C ch) throws Exception;

    private void remove(ChannelHandlerContext ctx) {
        try {
            ChannelPipeline pipeline = ctx.pipeline();
            if (pipeline.context(this) != null) {
                pipeline.remove(this);
            }
        } finally {
            initMap.remove(ctx);
        }
    }

大概意思是：

a. 触发ChannelInitializer的initChannel方法，执行子类定义的一系列操作（在ServerBootstrap这个例子中就是将ServerBootstrapAcceptor注册到pipeline中）

b. 将ChannelInitializer从pipeline中移除

4. 总结

ChannelInitializer的主要目的是为程序员提供了一个简单的工具，用于在某个Channel注册到EventLoop后，对这个Channel执行一些初始化操作。ChannelInitializer虽然会在一开始会被注册到Channel相关的pipeline里，但是在初始化完成之后，ChannelInitializer会将自己从pipeline中移除，不会影响后续的操作。

使用场景：

a. 在ServerBootstrap初始化时，为监听端口accept事件的Channel添加ServerBootstrapAcceptor

b. 在有新链接进入时，为监听客户端read/write事件的Channel添加用户自定义的ChannelHandler