0. ChannelInitializer簡介

直接用ChannelInitializer的註釋吧：A special ChannelInboundHandler which offers an easy way to initialize a Channel once it was registered to its eventLoop.

1. ChannelInitializer類圖

需要註意的是：

a. ChannelInitializer繼承於ChannelInboundHandler接口

b. ChannelInitializer是一個抽象類，不能直接使用

2. initChannel抽象方法

ChannelInitializer中聲明了一個名為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的實現類必須要重寫這個方法，這個方法在Channel被註冊到EventLoop的時候會被調用

3. ChannelInitializer什麽時候會被調用？

以ServerBootstrap啟動這一場景為例

在ServerBootstrap.init()方法中，負責accept新鏈接的Channel的pipeline被添加了一個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));
            }
        });
    }
});

由於此時這個Channel還沒有被register到EventLoop，於是在addLast方法的調用鏈中，會給pipeline添加一個PendingHandlerAddedTask，其目的是在Channel被register到EventLoop的時候，觸發一個回調事件

然後在AbstractBootstrap.initAndRegister()方法中，這個Channel會被register到boss EventLoopGoup，接著會被register到boss EventLoopGoup中的某一個具體的EventLoop

在AbstractChannel.register0()方法中，之前註冊的PendingHandlerAddedTask會被調用，經過一系列調用之後，ChannelInitializer.handleAdded()方法會被觸發：

    /**
     * {@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());//調用子類重寫的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);//將ChannelInitializer從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. 觸發ChannelInitializer的initChannel方法，執行子類定義的一系列操作（在ServerBootstrap這個例子中就是將ServerBootstrapAcceptor註冊到pipeline中）

b. 將ChannelInitializer從pipeline中移除

4. 總結

ChannelInitializer的主要目的是為程序員提供了一個簡單的工具，用於在某個Channel註冊到EventLoop後，對這個Channel執行一些初始化操作。ChannelInitializer雖然會在一開始會被註冊到Channel相關的pipeline裏，但是在初始化完成之後，ChannelInitializer會將自己從pipeline中移除，不會影響後續的操作。

使用場景：

a. 在ServerBootstrap初始時，為監聽端口accept事件的Channel添加ServerBootstrapAcceptor

b. 在有新鏈接進入時，為監聽客戶端read/write事件的Channel添加用戶自定義的ChannelHandler

Netty源碼學習（五）ChannelInitializer