why need to sign a java jar file?

Answer 1

Digital signatures allow clients of your JAR to identify you as its author. The JVM also restricts the functionality of unsigned JARs under certain circumstances such as when executing inside an applet. If I recall an unsigned jar running inside of an applet does not have any access to the local drive of the computer running the applet were as signed jars do.

Answer 2

its for security reasions i think try 1234