JSX serializes Java objects to XML. You can persist objects, evolve them, and send them over the network and between applications. Your object data becomes human-readable and human-writable. You can test it, search it, profile it, audit it, and edit it with ordinary text and XML tools. JSX handles all POJOs and also all classes that require Java's own object serialization.
MUSCLE (Multi User Server Client Linking Environment) is an N-way messaging server and networking API. It includes client-side networking APIs for various languages, including C, C++, C#, Delphi, Java, and Python. MUSCLE lets programs communicate over a network via streams of serialized Message objects. The included server program ("muscled") lets its clients message each other and store information in its server-side hierarchical database. The database supports flexible queries via hierarchical wildcarding, and "live" updates via a subscription mechanism.
ObjectDB for Java/JDO is a powerful yet compact Java Data Objects (JDO) Object Database (ODBMS), written entirely in Java, and compliant with the Java Data Objects (JDO) standard developed by Sun (JSR12). ObjectDB for Java/JDO is designed to efficiently handle databases of various sizes, ranging from a few KBs to hundreds of GBs. Among its features are small footprint, high performance, rapid development, easy deployment, and flexible architecture (1-Tier, 2-Tier, or N-Tier).
The DataBean package is a Java library that provides features for mapping a database table to a Java class (databean). The coding of the databean class must conform to a preset pattern so that the instances of the databean classes can perform common database operations without using much of the ugly SQL. DataBean is functional, stable, light-weight, and easy to use. It can make your application much easier to write when dealing with relatively unsophisticated database operations.
featherCache is a lightweight Java cache implementing a map interface and storing objects on disk. Multiple sections and expiration times are supported. It requires no third party components, has simple integration, and can also be used to store program settings represented by serializable objects. It was used for a long time as a part of the Sight Web agent generator.
Ammentos is a lightweight persistence framework for JDK 5. It does not require any installation nor configuration; just put a jar file into your classpath and start writing code. It is designed so that your persistence code will be dramatically short and so that you won't have to spend a lot of time to learn how to use it. It is about 72Kb large and it does not require any external library except for your database JDBC driver. You can use it in desktop applications or in server-based environments.
With MetaModel, you use a type-safe SQL-like API for querying any datastore. It is a data access framework providing a common interface for exploration and querying of different types of datastores. It isn't a data mapping framework. Instead, it emphasizes abstraction of metadata and the ability to add data sources at runtime, making MetaModel great for generic data processing applications, but less so for applications modeled around a particular domain.
db4o is an object database for Java and all .NET environments. Its features include ACID transactions, automatic class schema recognition, query-by-example, a S.O.D.A. object querying API, native queries, LINQ support on .NET, callback methods, memory files, cascade-on-delete, multi-threaded access, and Java Webstart and servlet support.