for a real example.
This is great for starting application phase. In this scenario you have a minimalist plugin framework with one class loader
-(the application class loader), similar with Java ServiceLoader(https://docs.oracle.com/javase/7/docs/api/java/util/ServiceLoader.html)
+(the application class loader), similar with Java [ServiceLoader](https://docs.oracle.com/javase/7/docs/api/java/util/ServiceLoader.html)
but with the following benefits:
- no need to write provider-configuration files in the resource directory `META-INF/services`, you using the elegant
`@Extension` annotation from PF4J