Java APIs

Hi people, Am working on a solution that implements an auto update of an application developed using Java. I am using Apache commons package for downloading the updated JAR file from an FTP site. Once I have the new JAR file on my local machine, I double click it and I get the following error. Invalid or corrupt jarfile C:\FIDS\FIDS.jar When I use a different FTP client to download my JAR file, the file works well when double clicked. What could be happening on my JAR file. Thanks and regards.