[Dubbo-1665] Elegant shutdown under tomcat #1665

[ralf0131]

What is the purpose of the change

Fix the issue that Dubbo cannot shutdown elegantly.

Brief changelog

• Introduce a Bootstrap class to easily start/stop Dubbo when running without Spring.
• Introduce DubboApplicationListener to listen to the Spring lifecycle event to start/stop Dubbo under Spring.

Verifying this change

• Verified under embedded mode (without Spring) and under servlet container.

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[codecov-io]

Codecov Report

Merging #1763 into master will decrease coverage by 0.02%.
The diff coverage is 29.16%.

@@             Coverage Diff              @@
##             master    #1763      +/-   ##
============================================
- Coverage     38.77%   38.74%   -0.03%     
+ Complexity     4141     4137       -4     
============================================
  Files           618      620       +2     
  Lines         29728    29725       -3     
  Branches       5241     5239       -2     
============================================
- Hits          11527    11517      -10     
- Misses        16360    16367       +7     
  Partials       1841     1841

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/alibaba/dubbo/config/AbstractConfig.java	71.68% <ø> (-0.15%)	115 <0> (ø)
.../java/com/alibaba/dubbo/config/ProtocolConfig.java	74.61% <ø> (-0.39%)	59 <0> (-4)
.../java/com/alibaba/dubbo/config/RegistryConfig.java	37.64% <0%> (ø)	26 <0> (ø)	⬇️
...ng/initializer/DubboWebApplicationInitializer.java	0% <0%> (ø)	0 <0> (?)
...g/spring/initializer/DubboApplicationListener.java	0% <0%> (ø)	0 <0> (?)
...a/com/alibaba/dubbo/common/utils/ExecutorUtil.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ba/dubbo/registry/multicast/MulticastRegistry.java	44.76% <100%> (+0.23%)	28 <0> (ø)	⬇️
...om/alibaba/dubbo/registry/redis/RedisRegistry.java	32.77% <100%> (+0.18%)	16 <0> (ø)	⬇️
...om/alibaba/dubbo/registry/dubbo/DubboRegistry.java	46.15% <100%> (+1.05%)	8 <1> (ø)	⬇️
...ibaba/dubbo/registry/support/FailbackRegistry.java	55.11% <50%> (-0.22%)	33 <0> (ø)
... and 6 more

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[chickenlj]

I think this is a very good idea refactoring the startup and stop phase of dubbo process. At present, we need a place to highlight this kind of changes, so that users and contributors can easily get noticed and accept the new implementation, for this change, it's especially important for API users.

Further consideration, can discuss later:

1. ReferenceConfig should also involved in DubboBootstrap, also consider ReferenceConfigCache.
2. Make more configuration available in DubboBootstrap.
3. Refactor the ServiceConfig to better cooperate with DubboBootstrap, for example add fluent builder for XxxConfig, support default ApplicationConfig.
4. Provide guide demo introducing usage of DubboBootstrap.

[chickenlj]

If use spring-boot, will DubboApplicationListener still get registered?

[ralf0131]

No. It is only intended for Spring running under servlet container.

For spring-boot support, I am planning to add the support in spring-boot-starter-project.

[ralf0131]

@chickenlj Yes, the current DubboBootstrap class only contains minimal functionality to ensure it works to solve this particular issue, I don't want to bring too much change to this pull request, we need add more later.

[chickenlj]

@ralf0131 I think it's ready to merge, please solve the conflicts first.