Welcome to JavaRanch! I added some code tags to your post, which make it easier to read. What happens at the front end now? I'm confused by your mixing of Excel and zip files. Also, you set your content type to something that looks like it should be Excel, but then write a zip file?
My advice is to start small and build up. Can you write a working Excel file locally? Maybe start with just one sheet and not zipped. Can you use a servlet to download a simple text file? Once you get the pieces working, you can tie them all together.