Channel.write() 和 ChannelHandlerContext.write() 的区别

看了下netty 源代码, 终于明白了
Channel.write() 和 ChannelHandlerContext.write() 的区别了
网上说的都不是很清楚 

首先注明我的netty版本

	io.netty
	netty-all
	4.0.36.Final

ChannelPipeline处理ChannelHandler的顺序:

pipeline中的handler 处理的请求分为两类: 读 和 写

对于读请求 : 从handler链表的 head  到  tail     挨个的处理, 跳过 ChannelOutboundHandler

对于写请求 : 从handler链表的 tail      到  head 挨个的处理, 跳过 ChannelInboundHandler

1 调用  Channel.write(), 会直接调用ChannelPipeline.write()

    @Override
    public ChannelFuture write(Object msg) {
        return pipeline.write(msg);
    }

    @Override
    public ChannelFuture write(Object msg, ChannelPromise promise) {
        return pipeline.write(msg, promise);
    }

    @Override
    public ChannelFuture writeAndFlush(Object msg) {
        return pipeline.writeAndFlush(msg);
    }

    @Override
    public ChannelFuture writeAndFlush(Object msg, ChannelPromise promise) {
        return pipeline.writeAndFlush(msg, promise);
    }

而 ChannelPipeline .write() 直接调用 tail指向的 最后一个handler的 write

    @Override
    public ChannelFuture write(Object msg) {
        return tail.write(msg);
    }

    @Override
    public ChannelFuture write(Object msg, ChannelPromise promise) {
        return tail.write(msg, promise);
    }

可以看到 调用的是ctx.prev获取前一个, 所以写从tail到head

2 调用ChannelHandlerContext.write() 

首先通过findContextOutbound找到当前ChannelHandlerContext的上一个OutboundContext

再调用它执行具体的写入逻辑

    private void write(Object msg, boolean flush, ChannelPromise promise) {
        AbstractChannelHandlerContext next = findContextOutbound();
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeWrite(msg, promise);
            if (flush) {
                next.invokeFlush();
            }
        } else {
            AbstractWriteTask task;
            if (flush) {
                task = WriteAndFlushTask.newInstance(next, msg, promise);
            }  else {
                task = WriteTask.newInstance(next, msg, promise);
            }
            safeExecute(executor, task, promise, msg);
        }
    }

    private AbstractChannelHandlerContext findContextOutbound() {
        AbstractChannelHandlerContext ctx = this;
        do {
            ctx = ctx.prev;
        } while (!ctx.outbound);
        return ctx;
    }

结论:

Channel.write()  :                        从 tail 到 head 调用每一个outbound 的  ChannelHandlerContext.write

ChannelHandlerContext.write() : 从当前的Context, 找到上一个outbound, 从后向前调用 write