DataNucleus AccessPlatform is a standards-compliant Java persistence product. It is fully compliant with the JDO1, JDO2, JDO2.1, JDO2.2, JDO3, JPA1, and JPA2 Java standards, and provides a REST API. It complies with the OGC Simple Feature Spec for persistence of geospatial Java types. It allows access to all popular RDBMS available today, together with the MongoDB, LDAP, NeoDatis, JSON, Excel/ODF spreadsheets, XML, BigTable, HBase, and Neo4j databases.
Butterfly Persistence is a simple, no nonsense Java persistence API. It aims to provide a simple relational persistence API. Its features include automatic/manual connection management, easier JDBC operations via JDBC templates (Spring style), simple object relational mapping, and map reading for dynamic queries. It provides a simple and pragmatic approach to persistence and will either help you, or get out of the way and let you do the job manually.
Ujorm is an easy-to-use ORM framework based on key-value objects. The framework was designed for rapid Java development with great performance and a small footprint. The key features are type safe queries, relation mapping by Java code, no entity states, and a memory overloading protection cache.