1. A computer system for generating an On-Demand reconfigurable object-oriented Web application from a Web application framework, which object-oriented Web application provides create/read/update/delete-functionality on data managed in one or more external data sources, the system comprising a processor for executing computer instructions stored on a non-transitory storage medium, the computer instructions providing code for: a) providing an abstraction layer to the external data sources and to one or more computation services, wherein the Web application framework has no predefined connections to data sources from which the data can be read and to which the data can be written, b) providing the Web application framework comprising functionality at the level of a data tier and providing objects implementing a business logic tier, but lacking a presentation tier, c) generating an instance of the object-oriented Web application, comprising: 1) accessing an XML-based definition that is limited to desired create/read/update/delete-functionality in pre-defined form, said XML-based definition serving as a repository that contains all the information for generating the instance of the Web application from the Web application framework, said XML-based definition further containing representations of functionality to be executable in the Web application, which functionality is always related to a specific entity and what can be configured is how data should be updated, 2) adding to the Web application framework the presentation tier for interfacing with elements of the data tier and the business logic tier, thereby providing graphical interfaces on functionality, 3) typing non-typed definitions of data in the external data sources, data logic related to the external data sources and a plurality of relationships between the non-typed definitions, the data logic and the plurality of relationships; and d) constructing the necessary classes on-demand and inserting them into the Web application framework, taking into account a security configuration applied on a BusinessEntity, which instance comprises computer-executable instructions for doing one or both of: presenting one or more graphical interfaces defined in said XML-based definition and comprising data requested in the request, executing the functionality from the XML-based definition that is associated with the action, using the abstraction layer to obtain access to data managed in the external data sources, all necessary connection information being defined in the XML-based definition, and applying On-Demand-Instantiation to type one or more non-typed objects within a context of a definition of the functionality being executed, as defined in the XML-based definition; and e) redefining the generated instance of the Web application.