Suggestions needed on Java API automation framework and libraries

So I am about to start developing a framework which will test Java APIs (not rest, not SOAP and not JSON) and am looking for suggestions and best practices to make this task scalable and efficient.

When I say JAVA APIs, I mean that these are Java services which needs to be called by making objects of Classes and then calling methods with parameters as input.

Here there are 2 types of APIs: 1) Publisher API 2) Client API

Publisher API will collect data from Source system and package it into an event (header + collected data) and send it to a sytem where it'll store the header and collected data (let's call payload) in Cache and DB.

Then to retrieve this data from Cache or DB, client API will be called with some parameters which define which data to be extracted. (These parameters are values that matches to those in header)

The payload is mostly in the form of XML/Plain text and hence my automation suite should be able to check the integrity of XML/Plain text throughout the process i.e. Payload should remain consistent throughout Source system -> Publisher API -> Cache -> DB -> Client API

I am looking for help on finding best tools or libraries in Java that can help me accomplish this kind of testing and if you have suggestions on what will be the best way to create a framework here that would also be really helpful. I need: 1) Library or Tools that will help to compare payload (XML/Plain Text) at various junctions while data is moving from Source to Client API. 2) Framework Ideas to test this kind of API service where payload is never touched but once stored in DB it is retrieved based on header fields.

Your suggestions on either tool that can be used with Java to compare XML/Plain text or On framework Ideas would be much appreciated.