Just a quick note about how to debug and compile Pentaho PDI

1. Get the source from svn e.g. to get the branch 3.1.0 use the following command
   svn co svn://source.pentaho.org/svnkettleroot/Kettle/branches/3.1.0 Kettle3.1.0
2. Make sure you have ant, I have 1.7.x installed and in the path
3. Load the project as an existing Java project in Eclipse, modify source at your will
   
4. Look at the build.xml file at the top level directory where you checked out the source
   e.g. Kettle3.1.0
5. Run the default target or if you changed source file in one of the sub packages such as core or ui etc., there are separate targets to build just those lib