Features: Compliance to Apache TinkerPop 3, supporting Gremlin Schema Metadata Management, including VertexLabel, EdgeLabel, PropertyKey and IndexLabel Multi-type Indexes, supporting exact query, range query and complex conditons combination query Plug-in Backend Store Driver Framework, supporting RocksDB, Cassandra, ScyllaDB and MySQL now and easy to add other backend store driver if needed Integration with Hadoop/Spark

GCHQ's Gaffer comes to mind https://github.com/gchq/Gaffer. The classes are well documented in Javadocs and the provided examples should enable a competent Java programmer to get started easily.

Apart from it, there are a number of competing products listed by Apache Tinkerpop - http://tinkerpop.apache.org/providers.html