Open Source Databases written in Java

• Apache Derby - Derby is an effort undergoing incubation at the Apache Software Foundation. Incubation is required of all newly accepted projects until a further review indicates that the infrastructure, communications, and decision-making process have stabilised in a manner consistent with other successful ASF projects. While incubation status is not necessarily a reflection of the completeness or stability of the code, it does indicate that the project has yet to be fully endorsed by the ASF.
  
  
      
• Ashpool - Ashpool is a lightweight XML Database written in Java(tm). It is goal is to use standard SQL92 syntax to query, add, update, and delete XML documents via a JDBC driver. The XML documents must hold to a table like structure. The reason for the table structure restriction is that SQL was designed to query table data not hierarchical data.
  
  The Ashpool engine via JDBC drivers can be used to query any XML document; however, it is best used with documents that have a table like structure for the first three levels. The column section of the table structure can contain integer, float, string, dateTime or XML fragments. Ashpool can be used to search for XML documents inside of an XML document (for example, as a serialized bean store), or used as a lightweight relational database.
  
      
• Axion - Axion is a small, fast, open source relational database system (RDBMS) supporting SQL and JDBC written in and for the Java programming language.
  
      
• db4o - db4objects develops, commercially licenses and supports db4o, the worlds leading open source object database. With more than 200,000 downloads and many more deployments, db4o offers Java and .NET developers one of the easiest and fastest ways to store objects natively. Development managers at companies such as BMW, Hertz and Bosch rely on db4o to cut development costs and deliver new products to market faster.
  
      
• Excel JDBC Driver - xlSQL JDBC Driver for Excel, CSV and XML files. xlSQL maps a directory of Excel files to a database, workbooks to schemas and sheets to tables. Query documents with either the full HSQL or MySQL dialect. Write objects and data with native xlSQL.
      
• Hypersonic SQL - HyperXtremeSQL™ is a super-fast SQL database engine written in Java. It support SQL and JDBC in J2SE and J2EE contexts.
  
  HyperXtremeSQL is fully compatible with the open-source HSQLDB engine at the application level and can replace the HSQLDB jar in an existing application with no change to the Java code or SQL queries. HyperXtremeSQL is built around a new indexing and persistence core that replaces HSQLDB core classes. The HyperXtreme core breaks new ground as it lowers the code footprint, reduces memory use and at the same time significantly speeds up query performance in all areas of SQL, all modes of database operation and all sizes of database.
  
  
  HyperXtremeSQL is lightweight, featuring full embedded functionality at less than 350K jar size and full client-server functionality at less than 400K.
  
      
• Mckoi SQL Database - Mckoi SQL Database is an SQL (Structured Query Language) Database management system written for the JavaTM platform. Mckoi SQL Database is optimized to run as a client/server database server for multiple clients, however it can also be embedded in an application as a stand-alone database. It is highly multi-threaded and features an extendable object-oriented engine.
  
  Mckoi SQL Database started as an internal project and has since evolved from its inception in 1998. The main goals of the project are a code base that is simple to maintain and extend, ease of use and administration, robustness, multiple concurrent access, and performance.
  
      
• One$DB - One$DB is an Open Source version of Daffodil DB, our commercial Java Database. One$DB is a standards based (J2EE-certified, JDBC 3.0 and SQL 99 compliant), platform independent, footprint size database that can be embedded into any application and requires zero or minimal administration. Daffodil DB is the first Java database that has shown the capability to take on enterprise databases with its high performance in real time environments, Compiere compatibility being the best example so far. One$DB is exactly the same as Daffodil DB barring a few features and has been made available in both Embedded and Network editions.
      
• Ozone - The Ozone Database Project is a open initiative for the creation of an open source, Java based, object-oriented database management system.
  
  The ozone project aims to evolve a database system that allows developers to build pure object-oriented, pure Java database applications. Just program your Java objects and let them run in a transactional database environment.
      
• Quadcap Embeddable Database - QED is a pure Java database that features:
  
  Pure Java: a high performance, small footprint, zero-administration database, perfect for embedding with your application.
  A full set of RDBMS features: transactions, data integrity, failure recovery, comprehensive SQL 92 conformance plus many SQL3 features.
  Support for JDBC 2.0
  Open source, free software, with a single-price UNLIMITED seats commercial redistribution license available.
  QEDs pure Java engine provides very high performance, since database accesses can be performed locally, without the necessity of a network round trip for every operation. Conventional JDBC drivers, no matter how fast the back-end database, incur network overhead -- frequently the bottleneck in a database application. A configurable in-memory cache also helps boost performance by reducing disk operations.
  
  
      
• TinySQL - tinySQL is a lightweight, 100% Java SQL engine that also includes a JDBC driver. It was featured in the book Java Database Programming.
  tinySQL supports the following (from the README file):
  
  SELECT (with joins) joins are buggy in dBase
  UPDATE
  INSERT
  DELETE
  CREATE TABLE
  DROP TABLE
  ALTER TABLE xx ADD coldef dBase only
  ALTER TABLE xx DROP [COLUMN] col dBase only
  ALTER TABLE xx RENAME [COLUMN] foo TO bar dBase only
  WHERE only AND is supported
  
  TinySQL now supports all JDBC-Datatypes and the ODBC-Minimum grammar is
  implemented.
  
  Although tinySQL is not optimized for speed, its architecture makes it possible to build SQL interfaces to non-SQL data sources, such as .DBF files or text files.
  
      
• yaRDBMS - The yaRDBMS (yet another relational database management system) is a Java based, transactional highly concurrent relational database management system with advanced query optimization, indexing features and crash recovery.
  
  The yaRDBMS uses on write-ahead log (WAL) based transaction management. The development is currently in its early stages. It has basic transaction support, an incomplete JDBC driver and basic SQL support (only two-table joins are supported for the time being). What is totally lacking currently is security and crash recovery.
  
  The yaRDBMS uses B+ trees for indexing (currently only for the primary keys.