The Geomajas API project contains some annotations that can be used to indicate which parts (which classes and which (public) methods or fields) are considered as the API. This is particularly useful for projects where you want to mark the API without the need to refactor everything to interfaces and factories.
Resty is a small, convenient API to talk to RESTful services from Java. Its focus is on simplicity and ease-of-use, often requiring only two lines of code to access RESTful Web services. It lets you use much less code than URLConnection, it's simpler than Apache HTTP client, and it's more reliable than Jersey client.
The Apache ODF Toolkit is a set of Java modules that allow programmatic creation, scanning, and manipulation of Open Document Format (ISO/IEC 26300 == ODF) documents. Unlike other approaches that rely on runtime manipulation of heavy-weight editors via an automation interface, the ODF Toolkit is lightweight and ideal for server use.
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.
JRel is a library for the Java language that can be used to construct database queries in a programmatic fashion using a subset of the relational algebra. These queries are expressions in the form of 100% native Java code, and are translated at runtime into compliant ANSI-SQL usable with a variety of SQL databases.
OpenJPA is a 100% compliant feature-rich implementation of the persistence part of Enterprise Java Beans 3.0, also known as the Java Persistence API (JPA). It can be used as a stand-alone POJO persistence layer, or it can be integrated into any EJB3.0 compliant container and many lightweight frameworks.