A quick and dirty solution is to just generate  a CSV file. excel  will load
that just like a spreadsheet with no formatting... The other option is to do
some googling. Once upon a time, I played around with a java  package that
could read/write excel files, but I have no idea where I found it.

Please see jasper report, it is open source, I ever used it, its very
convenient and easy
see http://jasperreports.sourceforge.net/ it can generate  excell file, csv,
pdf, etc.

Try using POI .
....................................

Use apache poi..........................

Yo can use POI API, a free development made by Jakarta
group: http://jakarta.apache.org/poi/

You can use simreport library in this link http://jsimreport.com. THis library requires in some code lines and some files to config, it is visual.