ExtensionLoader is a core API to extend the features of Dubbo components, it's based on the Dubbo's Service Provider mechanism that will search three classpaths in order :

1. META-INF/dubbo/internal/
2. META-INF/dubbo/
3. META-INF/services/

However, Dubbo framework just loads only one SPI file if found, does not support the overriding rule, for example :

The implementation in META-INF/services/ can override META-INF/dubbo/,
META-INF/dubbo/ can override META-INF/dubbo/internal/ .