Ensure compatibility for elegant shutdown under servlet container. #1998

[ralf0131]

What is the purpose of the change

Ensure compatibility for elegant shutdown under servlet container. #1998

Brief changelog

Consider the following cases:

1. listener class ContextLoaderListener is defined in web.xml.
   servlet container will initialized ContextLoaderListener before this class. Therefore Dubbo's elegant shutdown feature will not be enabled. To enable it, you need to either: 1) remove listener class ContextLoaderListener in your web.xml 2) change listener class to DubboContextLoaderListener in your web.xml

2. listener class DubboContextLoaderListener is defined in web.xml This is automatically enable Dubbo's elegant shutdown feature, even for a servlet container which is not 3.0 compatible

3. no listener class defined in web.xml Dubbo's elegant shutdown feature will be automatically enabled if Dubbo is running under a servlet 3.0+ compatible container see DubboWebApplicationInitializer for more details.

Verifying this change

Run and test the above 3 cases.

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[codecov-io]

Codecov Report

Merging #2126 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2126      +/-   ##
============================================
- Coverage      54.2%   54.15%   -0.05%     
- Complexity     5086     5090       +4     
============================================
  Files           559      559              
  Lines         24922    24934      +12     
  Branches       4442     4442              
============================================
- Hits          13508    13504       -4     
- Misses         9379     9388       +9     
- Partials       2035     2042       +7

Impacted Files	Coverage Δ	Complexity Δ
...nitializer/DubboApplicationContextInitializer.java	100% <100%> (ø)	2 <2> (?)
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-8.48%)	12% <0%> (-1%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	60% <0%> (-6.25%)	21% <0%> (-2%)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	51.89% <0%> (-3.8%)	11% <0%> (-2%)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	8% <0%> (-1%)
...e/dubbo/remoting/transport/netty4/NettyServer.java	72.13% <0%> (-3.28%)	9% <0%> (-1%)
...dubbo/rpc/protocol/dubbo/CallbackServiceCodec.java	77.2% <0%> (-1.48%)	29% <0%> (ø)
...apache/dubbo/rpc/protocol/dubbo/DubboProtocol.java	58.33% <0%> (-0.84%)	30% <0%> (-1%)
.../java/org/apache/dubbo/config/ReferenceConfig.java	46.97% <0%> (-0.72%)	38% <0%> (ø)
... and 2 more

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[chickenlj]

In a pure Spring (non-web) case, do we need to register context.addApplicationListener(new DubboApplicationListener()); mannually?

The configuration various with different use cases, so we'd better provide docs or blogs of how to use covering all use cases.

[ralf0131]

In a pure Spring (non-web) case, do we need to register context.addApplicationListener(new DubboApplicationListener()); mannually?

If we use spring-container provided by Dubbo, it will be there out of the box.
If we use dubbo spring boot starter, it will be there out of the box.
If we use pure spring(non-web), user will need to add that listener manually.

The configuration various with different use cases, so we'd better provide docs or blogs of how to use covering all use cases.

Yes, I am planning to add documentation for that.

[mercyblitz]

I do not recommend this implementation , and suggest:

• To implement WebApplicationInitializer
• To append ServletContext's parameter for ApplicationContextInitializer, whose name is either ContextLoader.GLOBAL_INITIALIZER_CLASSES_PARAM or CONTEXT_INITIALIZER_CLASSES_PARAM , and its value is your ApplicationContextInitializer class.
• To add DubboApplicationListener into ConfigurableApplicationContext in ApplicationContextInitializer#initialize method.

Current approach must be based on Servlet 3.0 + container.

[ralf0131]

@mercyblitz I think your suggestion is more elegant. I have changed my implementation based on your idea.

One thing that doing in a programmatic way can't solve is the case that if user already configured ContextLoaderListener in the web.xml.

I use web-fragment.xml to ensure the compatibility. The following content:

    <context-param>
        <param-name>contextInitializerClasses</param-name>
        <param-value>org.apache.dubbo.config.spring.initializer.DubboApplicationContextInitializer</param-value>
    </context-param>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

will be merged with user's web.xml whether user has specified ContextLoaderListener or not.

Be aware that the current implementation will only works under a servlet 3.0+ container.

If running on a servlet container that is less than 3.0, user should add the above configuration into web.xml.

[kexianjun]

dubbo-config/dubbo-config-spring/src/main/resources/META-INF/web-fragment.xml,This file may have introduced a bug,when running dubbo application in a servlet container,it may load tow ContextLoaderListeners,see #2538 and #2535