I'm currently researching Java libraries to help in building a meta type search engine in the sense of being able to replace any given search engine in the back-end of the application or to simultaneously search using multiple search engines. I'm not interested in the GUI part here, just the generalization of search engine APIs and usage.

I'd like to know about the common libraries used to achieve this task and if there are any common patterns used in this case.

I imagined that this problem is common enough to be able to find plenty of stuff on Google, but it seems like search is a very proprietary domain and not much information is fed back to the community.