Poi Problem
I had a problem again today with Ben Nadel's poi solution for uploading excel spreadsheets and inserting the data in your database. The error I got was:  An exception occurred while instantiating a Java object. The class must not be an interface or an abstract class. Error: '.

The solution was to delete several of the rows at the end of the spreadsheet. Somehow something gets into one of the blank rows that you can't see. Deleting rows after your data always seems to fix the problem.