MG4J - Managing Gigabytes for Java

MG4J (Managing Gigabytes for Java) is a free full-text search engine for large document collections written in Java. MG4J is a highly customisable, high-performance, full-fledged search engine providing state-of-the-art features (such as BM25/BM25F scoring) and new research algorithms. The main points of MG4J are Powerful indexing, Multi-index interval semantics, Virtual fields, Clustering and lot more.
Release 5.0 has several source and binary incompatibilities, and introduces quasi-succinct indices.