Application programmers can not live with out database engines. Especially the Java/J2EE developers. There are of course biggies like Oracle, MS SQL, Sybase, so on and so forth. But, many a times, we may develop smaller lightweight applications that may not need complex database engine at the backend. You may want to embed or use a smaller database engines that are of few MBs or lesser than 10MB in size. Distribution of App becomes easy, less system requirements, probably it will cost nothing to maintain these small databases, the advantages are many. Here is a list of such lightweight Database Engines written entirely in Java and that can be embedded in Java programs.

Apache Derby

Apache Derby, an Apache DB subproject, is an open source relational database implemented entirely in Java and available under the Apache License, Version 2.0. Some key advantages include:

• Derby is tiny - about 2 megabytes for the base engine and embedded JDBC driver.
• Derby is based on the Java, JDBC, and SQL standards.
• Derby provides an embedded JDBC driver that lets you embed Derby in any Java-based solution.
• Derby also supports the more familiar client/server mode with the Derby Network Client JDBC driver and Derby Network Server.
• Derby is easy to install, deploy, and use.
• Active forum for help.

More here.

H2 Database Engine

• Very fast, open source, JDBC API
• Embedded and server modes; in-memory databases
• Browser based Console application
• Small footprint: around 1 MB jar file size

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HSQLDB – HyperSQL

A small, fast, multithreaded engine and server with in-memory and disk based tables, LOBs, transaction isolation, encryption and ACID.

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SmallSQL

• It is embeddable in you application. There is no additional setup required. 100% java!
• It is a very small Java library.
• The quality is tested with JUnit and Emma Coverage.

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Axion

Axion is a small, fast, open source relational database system (RDBMS) supporting SQL and JDBC written in and for the Java programming language.

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TinySQL

tinySQL is a lightweight, 100% Java SQL engine that also includes a JDBC driver. It was featured in the book Java Database Programming.

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McKoi SQL

Mckoi SQL Database is an Open Source SQL Database System written in Java. Mckoi SQL Database was started in 1998, and the goal was to build a database management system in a traditional shared disk/shared memory style architecture. Mckoi SQL Database includes some nice features such as write-ahead-logging. Many of the design ideas implemented in this project were carried through into MckoiDDB, the evolution of this project.

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NeoDatis Database

NeoDatis ODB is a very simple Object Database that currently runs on the Java, .Net, Google Android, Groovy and Scala.

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JODB Mini

Mini is the reduced version of JODB suitable for environments with especially restricted resources. The Android mobile platform is one of the obvious examples of such an environment.

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Jalisto

Jalisto from Xcalia (www.xcalia.com) is a lightweight transactional database written entirely in Java. Jalisto comes in two versions: Jalisto SE (Standard Edition) and Jalisto ME (Micro Edition for J2ME).

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Aviantes

A 100% pure Java, lightweight, single file embeddable database. Similar to a combination of SQLite and BerkeleyDB. Developed for use with PDA and Mobile applications.

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Ashpool

A light weight, Open Source XML based Database system.