Add the following fragment to your pom file to execute the idlj plugin as part of the "generate-sources" phase. Modify the idl file specified to your own idl.
<project> ... <build> <plugins> <plugin> <groupId>org.freehep</groupId> <artifactId>freehep-idlj-plugin</artifactId> <version>2.0-SNAPSHOT</version> <executions> <execution> <goals> <goal>generate</goal> </goals> <configuration> <source>YourFile.idl</source> </configuration> </execution> </executions> </plugin> </plugins> </build> ... </project>