Hi there. As I understand, you want to build an interface as if it was a real API provided by the site's owner, but instead the API will use scraping techniques to communicate to the server via their webpage.
I specialize in web scraping but I always use the Scrapy framework, which is written in Python. Either way, this means I'm proficient in all the technologies required for scraping as HTTP, HTML, CSS selectors, AJAX, regular expressions, etc. On the other hand, I have worked with Java for many years (I can show you some open source projects I have); just not for web scraping. I would gladly study a Java framework for HTML parsing (jsoup definitely seems to be the most popular one) for your project, but this might mean that I will need a little more time than someone who already works with web scraping in Java.
Anyway, feel free to contact me.