step to reproduce @Chanel-Li 's problem：
1. clone the latest version code dubbo-2.7.0
2. run `org.apache.dubbo.container.Main` throw above exception.
This is not a bug. `org.apache.dubbo.container.Main` will default loads `spring
container`, while the source code of moudle `dubbo-container-api` doesn't add
dependency of `dubbo-container-spring`, so
`ExtensionLoader.getExtensionLoader(Container.class).getExtension("spring")`
will throw exception.
suggestion:
1. You shouldn't run `org.apache.dubbo.container.Main` directly from source
code.More details you can browse the doc
[service-container.html](https://dubbo.apache.org/zh-cn/docs/user/demos/service-container.html)
2. If you want to unit test `spring-container`, ask
`org.apache.dubbo.container.spring.SpringContainerTest` for help.
The simplest way to deal with your problem.Add below dependency to your project
```
<dependency>
<groupId>org.apache.dubbo</groupId>
<artifactId>dubbo-container-spring</artifactId>
<version>${project.parent.version}</version>
</dependency>
```
and the try again.
[ Full content available at:
https://github.com/apache/incubator-dubbo/issues/2478 ]
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