There are a few Open Source Reporting Tools available in the Java world. Of all the options, the three with the most traction seem to be BIRT, Jasper, and Pentaho. So when evaluating open source alternatives for your Java BI projects you might shortly end up in comparing these three projects with each other.
Two years ago my collegue Tobias Kieninger and myself made a interesting side by side comparison of all the three reporting tools (you can find the corresponding article here).
Unfortunately it’s a bit outdated because it compares BIRT 2.1.1 with iReport 1.2.7 (the WYSWYG editor for Jasper Reports) and Pentaho Report Designer 126.96.36.199. Nevertheless the most of our arctile are so far still valid for the current versions.
In addition Innovent Solutions (a founding member of the BIRT project) made a interesting up to date comparison some weeks ago using the current version of all the tree products. In the first part they compare BIRT with Jasper Reports and in the second part they compare BIRT with Pentaho Reports.
When reading the article please keep in mind that the expertise of Innovent Solutions lies in BIRT. Nevertheless the comparison seems to be quite impartial to my mind.