Netty源码学习-DefaultChannelPipeline2

Netty3的API
http://docs.jboss.org/netty/3.2/api/org/jboss/netty/channel/ChannelPipeline.html
里面提到ChannelPipeline的一个“pitfall”：
如果ChannelPipeline只有一个handler（假设为handlerA）且希望用另一handler（假设为handlerB）
来取代handlerA去完成剩下的工作，那就要先add handlerB再remove handerA：

Pitfall

Due to the internal implementation detail of the current default ChannelPipeline, the following code does not work as expected if FirstHandler is the last handler in the pipeline:

 public class FirstHandler extends SimpleChannelUpstreamHandler {

     @Override
     public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
         // Remove this handler from the pipeline,
         ctx.getPipeline().remove(this);
         // And let SecondHandler handle the current event.
         ctx.getPipeline().addLast("2nd", new SecondHandler());
         ctx.sendUpstream(e);
     }
 }
 

正确的做法是
        ctx.getPipeline().addLast("2nd", new SecondHandler());
ctx.getPipeline().remove(this);

我的测试代码：
https://github.com/bylijinnan/nettyLearn/tree/master/ljn-netty3-learn/src/main/java/com/ljn/handler/pitfall

这是为什么呢？

问题出在ctx.sendUpstream(e)：

public class DefaultChannelPipeline implements ChannelPipeline {

    private final class DefaultChannelHandlerContext implements ChannelHandlerContext {

		public void sendUpstream(ChannelEvent e) {
			
			//下面的this是DefaultChannelHandlerContext
            DefaultChannelHandlerContext next = getActualUpstreamContext(this.next);
            if (next != null) {
                DefaultChannelPipeline.this.sendUpstream(next, e);
            }
        }
	}
	
}
		

当ChannelPipe只有一个handler（也就是只有一个ChannelHandlerContext，代码中的ctx）时，
ctx.next = null
在sendUpstream方法的
DefaultChannelHandlerContext next = getActualUpstreamContext(this.next);
返回的next=null，没有继续往下执行
因此Upstream的处理就中断了

不熟悉ChannelPipeline流程的可以先看看这篇文章：http://bylijinnan.iteye.com/blog/1981763