Apache Lucene is an open-source Java library with full text search capabilities using an inverted index. Enhanced analysis and tokenization functionalities are also part of this library, but also things like spell checking.

Apache Lucene is especially good at handling a large amount of different text sources with a lot of contents.

Inverted index means that it does not store keywords or terms of each and every source, but stores certain keywords and point to the sources containing these keywords.

Instead of doing this:

Apache Lucene does that:

Apache Lucene Fundamentals Tutorial