[GitHub] [skywalking] lujiajing1126 commented on pull request #6973: Fix ClassCast issue for RequestHolder/ResponseHolder

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lujiajing1126 commented on pull request #6973:
URL: https://github.com/apache/skywalking/pull/6973#issuecomment-845838177


   I would summarize the idea,
   
   - `InvocableHandlerMethodInstrumentation` has switched to Plugin API v2. Thus we can take advantage of the performance improvement while sharing Reference between `InvokeInterceptor.beforeMethod` and  `InvokeInterceptor.afterMethod`
   
   After some investigation for the reactive scenario, the execution order of the `InvokeInterceptor` and `AbstractMethodInterceptor` is listed below,
   
   1. `InvokeInterceptor.beforeMethod`
   2. `InvokeInterceptor.afterMethod`
   3. `AbstractMethodInterceptor.beforeMethod`
   4. `AbstractMethodInterceptor.afterMethod`
   
   In order to record the real span of the async execution in the controller, we need to add a hook with the help of `Mono.doFinally`, which will be executed asynchronously.
   
   So we need to pass the span from `AbstractMethodInterceptor.afterMethod` to `InvokeInterceptor.afterMethod` which seems impossible. But we can do it with a Reference Container. 
   
   - In the past, the author used `ResponseHolder`, but due to the ClassLoader issue, it should be removed.
   - Now, I use a primitive array (`[]AbstractSpan`) as a slot.
   
   We follows the previous idea,
   
   - First, we put the slot in the RuntimeContext, as well as the MIC. So that we can access it in the InvokeInterceptor.afterMethod`
   - Then, we fetch the slot from `AbstractMethodInterceptor.afterMethod` and put the real `AbstractSpan` into it, after that we can safely remove the RuntimeContext.
   - Finally, when the lambda passed to the `Mono.doFinally` is executed, we can take it out from the MIC.


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