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I cant run any jar files

 
Davy Kelly
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Hey everyone,

I cannot run any jar files.

I downloaded Derby today to play around with databases

I get this error all the time:
"Failed to load Main-Class manifest attribute from
C:\Derby_10\lib\derby.jar"

why is this?

Oh and any other Jars I download is the exact same

davy
 
Jeff Mayer
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Davy,

Not every jar file is runnable.
Let's suppose that you create a set of utility classes that you would like to assemble and distribute as a jar file.

It is very probable that none of the classes contained in your jar file has a main method. Maybe it wouldn't even make any sense to have any main method in your classes.

Just to begin with, you cannot directly "run" a jar file if there is no main method to be called in any of its classes. Even if a main method exists, if you would like to run it this way:

java -jar myjarfile.jar

you would need to include a Main-Class entry in the jar's manifest file.

In the case of Derby, you should read its documentation, which explains how you are supposed to use it.
 
It is sorta covered in the JavaRanch Style Guide.
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