Netty源码学习-DefaultChannelPipeline2_JAVA_编程开发_程序员俱乐部

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：

class="java" name="code">

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