I'm going to create Axis2 web-service on WSO2 AS. AFAIK .aar must contain a service class and its configuration in service.xml. To process requests received by the web-service I'm going to implement some business logic including interaction with DB through Hibernate. In respect to a system architecture it would be wrong to place all this logic into the web-service class. What would be a proper place for the business logic implementation in my case?
You can model this inside the Axis2 project. You should only add the service publishing methods in the service class and no need to implement the core logic there. You can easily model it as project with different hierarchies and aspects.
Look at this example blog post and how the project is modeled which can be found at the end of the tutorial.