If you doubleclick on a jar file, and your Java application does not start, your .jar association has been hijacked. You can fix the problem with Jarfix.
The problem ... The root cause for the problem above is, that a program has stolen the .jar association. If you have installed the Java Runtime Environment the first time, the file type called "jar" is assigned to javaw.exe correctly. "jar" is an abbreviation for "java archive" and javaw.exe is the correct program to execute a .jar. However, on Windows any other program can steal a file type at any time even it is already associated with a program. Many zip/unzip programs prefer to do this, because a jar is stored in the .zip format. If you doubleclick on a .jar, your pack program opens the file, rather than javaw runs the program, because your pack program ignores the meta information which is also stored in a .jar. In the Sun bug database there is report 4912211 "add mechanism to restore hijacked .jar and .jnlp file extensions", but it has been closed as "Closed, will not be fixed".
The workaround ... Reinstall the Java Runtime Environment or fix the Windows Registry manually each time this problem occurs.
The solution ... You can fix this problem very easy with the jarfix.exe program. Just doubleclick on it to restore the .jar association with javaw.exe. It also sets a nice Java icon for your .jar files.
You can start jarfix.exe also with parameter "/s" to run jarfix silently (without the dialog). Run "jarfix.exe /h" to get the program version, the author info and help.