Netty：ChannelInitializer

1. 作用

　　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。

2. 实现原理

    @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
            // suprises 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());
            } catch (Throwable cause) {

                exceptionCaught(ctx, cause);
            } finally {
                // 移除自己
                remove(ctx);
            }
            return true;
        }
        return false;
    }

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