One of the best things about JIRA, if you have a valid license, is that you get to see the source code. To see it, modify it, break it... err, modify it, because you have the license to do it!
Note
Please note that Atlassian does not support building JIRA from source and it should be done only when there is a business-critical requirement that cannot be fulfilled by a third-party or custom add-on.
Following are some of the pre-requisites prior to building JIRA from source:
A valid JIRA license to get access to the source code
An environment with JDK 1.7 or higher
Note
Apache Maven is required to build the JIRA Source, but the JIRA source code (7.x+) now includes the Maven distribution as well. If you would like to use a local Maven version, make sure you update the build scripts accordingly.
If you are building an earlier version of JIRA, make sure you have Maven installed.