J/Link is a toolkit that integrates Mathematica and Java. It lets you call Java from Mathematica in a completely transparent way, and it lets you use and control the Mathematica kernel from a Java program. More information about J/Link can be found at http://www.wolfram.com/solutions/mathlink/jlink.
Mathematica developers at Wolfram Research and elsewhere make heavy use of J/Link as a convenient way to extend the functionality of Mathematica, and also to embed Mathematica inside other applications.
The Wolfram Workbench provides a significant amount of support for developing J/Link applications.
The following topics are available: