jar file building

My program runs fine 开发者_如何学运维under NetBean. So I tried to build a jar file. I set everything up with project properties and picked the Main class by "browsing the Main class" so that I do not make any typing mistakes. Everything seems fine and a jar file is generated. But the jar file just could not run after showing the SlashScreen. The program keeps saying there is no main class but it is right there in the class! I checked the MANIFEST.MF file, Main-Class is set just right there too. It really confused me. What could be wrong here?

Thanks.