Lucene

Lucene is a full-text search library completely written in Java. It adds the content to full-text index and then allows you to perform queries on this index, returning results ranked by either the relevance to the query or sorted by an arbitrary field such as a document’s last modified date.

Lucene index is inverted index, by design because it inverts a page-centric data structure (page->words) to a keyword-centric data structure (word->pages).