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As regards your actual question: jar files by themselves have no provisions for being protected by a password. So whatever password you don't know must have been implemented by some code that is part of the application contained in that jar file. Since we don't know what application that is, the generic advice would be to contact whoever gave you that jar file.
Also: you posted in the "Applets" forum - does the question concern an applet? "Executable jar file" indicates that we may be talking about a desktop application. But either way, the question may not be related to Java at all, but about recovering/resetting a password set by someone or some company.