ChannelUpstreamHandler理解

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介绍

处理upstream ChannelEvent，同时把ChannelEvent发送给在ChannelPipeline中的上一个或者下一个handler。
upstream ChannelEvent：是在ChannelPipeline中处理的，从第一个handler到最后一个handler。

upstream ChannelEvent

Event name	Event type and condition	Meaning
messageReceived	MessageEvent	a message object (e.g. {@link ChannelBuffer}) was received from a remote peer
exceptionCaught	ExceptionEvent	an exception was raised by an I/O thread or a {@link ChannelHandler}
channelOpen	ChannelStateEvent	a {@link Channel} is open, but not bound nor connected
channelClosed	ChannelStateEvent	a {@link Channel} was closed and all its related resources were released
channelBound	ChannelStateEvent	a {@link Channel} is open and bound to a local address, but not connected
channelUnbound	ChannelStateEvent	a {@link Channel} was unbound from the current local address
channelConnected	ChannelStateEvent	a {@link Channel} is open, bound to a local address, and connected to a remote address
writeComplete	WriteCompletionEvent	omething has been written to a remote peer
channelDisconnected	ChannelStateEvent	a {@link Channel} was disconnected from its remote peer
channelInterestChanged	ChannelStateEvent	a {@link Channel}'s {@link Channel#getInterestOps() interestOps} was changed

Sending the event forward (upstream)
通过ChannelHandlerContext把ChannelEvent传递给下一个handler

 void handleUpstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
      ...
      ctx.sendUpstream(e);
      ...
  }

线程安全

一个IO线程可以用于处理多个Channel NIO
一个IO线程只能处理一个Channel OIO
OrderedMemoryAwareThreadPoolExecutor用来保证同一个Channel的Event的执行顺序
ChannelPipelineCoverage注解：all 代表可以把同一个ChannelHandler加入到多个ChannelPipeline，one代表只能加入到一个ChannelPipeline，其实all代表是线程安全、无状态，one代表线程不安全、有状态。

总结：

最近发现自己在学习业务项目的时候，分析的方式好像不对，事倍功半。比如分析一个复杂的业务系统，一上来自己就下个定义：这个太复杂了，不好理解的，以至于自己好像没有了征服的信心啦。还有就是，太过于专注点了，而没有从面开始，要分析一个大的业务系统，首先要从整体把握，走通整体流程，然后再深入到局部，熟悉具体的业务逻辑。不然只会看到冰山一角，掌控不了全局。
所以有时候，学习方法这个东西，还是要投入点时间思考的，不能一直一味的埋头苦干，适当的停下来，思考一下，优化一下自己的方式、思想。这也就是经常说的，及时的归纳总结。