How can I create classes and Endpoints at runtime?

I would like some directions here.

I need to build a CRM type program (kind of like Salesforce) for a very specific use. However, I need to give the clients the chance of creating new Objects at runtime (like in Salesforce, you have standard objects like Leads, Accounts, Opportunities.. but clients can also create custom objects and fields for these objects at runtime). Besides creating the custom object, Salesforce also creates a SOAP and a REST API for it.

I am programming this solution in Java however I have no idea how I could build the functionality I just mentioned in my program.

Does anyone have an idea?

Thanks.