[Dubbo-4693] fix EnableDubbo's alias problem (no effect though declared)

[jasonjoo2010]

What is the purpose of the change

fix the issued prompted in #4693

Brief changelog

Use AnnotatedElementUtils.findMergedAnnotation to make inherited annotations' @AliasFor take effects.

Verifying this change

@EnableDubbo(scanBasePackages = "") should bring the property to @EnableDubboConfig

Reference

https://docs.spring.io/spring/docs/4.2.0.RC3_to_4.2.0.RELEASE/Spring%20Framework%204.2.0.RELEASE/org/springframework/core/annotation/AliasFor.html

Similarly, AnnotatedElementUtils supports explicit meta-annotation attribute overrides when @AliasFor is used within an annotation hierarchy. 

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[codecov-io]

Codecov Report

Merging #4895 into master will decrease coverage by 0.06%.
The diff coverage is 73.33%.

@@             Coverage Diff             @@
##             master   #4895      +/-   ##
===========================================
- Coverage     61.17%   61.1%   -0.07%     
+ Complexity      424     423       -1     
===========================================
  Files           919     919              
  Lines         37487   37489       +2     
  Branches       5461    5446      -15     
===========================================
- Hits          22933   22909      -24     
- Misses        12049   12070      +21     
- Partials       2505    2510       +5

Impacted Files	Coverage Δ	Complexity Δ
.../annotation/DubboConfigConfigurationRegistrar.java	88.23% <71.42%> (-11.77%)	0 <0> (ø)
...ontext/annotation/DubboComponentScanRegistrar.java	83.33% <75%> (-8.67%)	0 <0> (ø)
...ache/dubbo/remoting/transport/AbstractChannel.java	75% <0%> (-12.5%)	0% <0%> (ø)
...pache/dubbo/remoting/transport/AbstractServer.java	53.75% <0%> (-3.75%)	0% <0%> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	70.17% <0%> (-3.51%)	8% <0%> (-1%)
...c/main/java/org/apache/dubbo/rpc/RpcException.java	80% <0%> (-3.34%)	0% <0%> (ø)
...g/apache/dubbo/rpc/protocol/rest/RestProtocol.java	68.21% <0%> (-3.11%)	0% <0%> (ø)
.../org/apache/dubbo/qos/legacy/LogTelnetHandler.java	30.3% <0%> (-3.04%)	0% <0%> (ø)
.../dubbo/remoting/transport/netty4/NettyChannel.java	63.36% <0%> (-2.98%)	0% <0%> (ø)
...va/org/apache/dubbo/remoting/exchange/Request.java	84.09% <0%> (-2.28%)	0% <0%> (ø)
... and 9 more

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[beiwei30]

@jasonjoo2010 would you mind to take a look at apache/dubbo-samples@7e25d4e? I cannot reproduce the issue.

[jasonjoo2010]

@jasonjoo2010 would you mind to take a look at apache/dubbo-samples@7e25d4e? I cannot reproduce the issue.

Hi, luo

Sorry for the latency.

I think problem will occur when using spring-boot to initialize the project.
See https://github.com/jasonjoo2010/dubbo-samples/tree/EnableDubbo for sample project dubbo-samples-annotation-springboot.

But surely there is a limitation for this PR. AnnotatedElementUtils.findMergedAnnotationAttributes will only available since spring 4.2. I don't know whether it's acceptable.

[jasonjoo2010]

@beiwei30

You can reproduce it by adding a breakpoint on DubboComponentScanRegistrar.getPackagesToScan (L102) before running AnnotationProviderBootstrap. Break at that point and check the variable basePackages and will find it is empty. It's different to the result in AnnotationConfigApplicationContext.

[jasonjoo2010]

Update:

Recently I just found some ways which will cause the inheriting alias fail but didn't dig the root cause (It maybe not important because we can't make everyone not to do it like this way):

1. Put a @ComponentScan in your bootable class XXX.class (It may be referenced as SpringApplication.run(XXXX.class, argv) )
2. Put a @EnableDubbo in where you like (in the same class or a separated class)

Then the information will not be brought to referenced annotations. And @ComponentScan can be also introduced by introducing @SpringBootApplication.

It will also be reproduced by the pair of @configuration + @EnableAutoConfiguration.
Generally I think if the tag is injected by scanning but not imported manually the issue will occur.

So I think it's a kind of SpringBoot issue. Just like the strange flow of asynchronized servlet in SpringBoot compared SpringMVC in container(standalone tomcat)[1].

Maybe we can't unify them into the same processing flow but to make some changes adapting for the special environment.

You can make more tests in the repository provided in previous comment.

[1] alibaba/Sentinel#310

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[mercyblitz]

Sorry, this PR will not be merged into, thus @org.apache.dubbo.config.spring.context.annotation.DubboComponentScan has to be compatible with Spring Framework 3.2 +, however, @AliasFor is dependent on Spring 4.2 and above.

[jasonjoo2010]

Sorry, this PR will not be merged into, thus @org.apache.dubbo.config.spring.context.annotation.DubboComponentScan has to be compatible with Spring Framework 3.2 +, however, @AliasFor is dependent on Spring 4.2 and above.

I am curious that is this treated a bug or just let users ignore it? (LOST of scanBasePackages)
After all compatibility issue what you just mentioned has many alternative solutions.