XML DB

As XML is gaining traction in the software industry, using an XML repository to support XML transactions and queries seems logical to improve performance and scalability.

eXist is an open source XML database management system. By storing XML data as XML data model, it uses XQuery and XPath technologies for indexing and querying.

There are two modes of running this eXist: remote and embedded. In embedded mode, the application runs in the same Java Virtual Machine as the client application. Besides using XML:DB APIs, I found out that several other technologies are used to enabled one to embed applications onto another:

jGroups
JGroups is a toolkit for reliable multicast communication.

jLine
JLine is a Java library for handling console input.

XACML
An access control policy evaluation engine written entirely in the Java(tm) programming language.

Quartz
A fully-featured, open-source job scheduling system that can be integrated with or used together with any J2EE or J2SE application virtually.

XML-RPC
It's a specification and a set of implementations that allow software running on disparate operating systems, running in different environments to make procedure calls over the WWW using HTTP as the transport and XML as the encoding.

Commons Pool
Enables pooing of objects

ANTLR (ANother Tool for Language Recognition)
A language tool that provides a framework to construct recognizers, interpreters, compilers and translators from grammatical descriptions specified in various target languages.