How to write a automation code in Java ?

Hi Team, I have been writing a automation code for testing the API provided by company. Initially, there were few number of APIs but when our features and enhancement grows, API also grows. Now my automation code looks complex and messy. I need to revamp my automation. Brief intro to my automation: My Service provides APIs for all CRUD operations (Create, Retrieve, Update, Delete). All API are implemented in the Web API (an API is typically defined as a set of Hypertext Transfer Protocol (HTTP) request messages, along with a definition of the structure of response messages in XML and JSON markup language) I have been writing the the automation code by only using the core java like (making httpclient request, parsing XML/JSON response, validate the response based on the parameters which i passed and generate the report). Is there any other ways or techniques that make the automation easy to understand, write, cover many test cases. . . ? Looking forward for your reply.