Is there an open source search engine which uses struts? I heard there is one called "K2 Optimum" or something like that but I can't find it in Internet. Can someone give me a hand?
I don't understand what you want. You want a search engine to use in a project to index files and search in them, or you want a search engine available on the net serving people that has the front end developed in struts?
To expand on what Alaa said, Struts has nothing to do with indexing and searching. So you can use any index/search engine (e.g. the very capable Lucene library) with any web technology you want.
Don't get me started about those stupid light bulbs.