Question

So I made this (very simple) program with a swing GUI with NetBeans, and I clicked build to make a jar file. When I run it by double clicking it, it tells me it could not find the main class, which, after checking, I am sure is definitely there. But, when I run it from Command Prompt, it works perfectly. Any easily-determinable reason for this strange behaviour (if you want the source code, I can post it here)?

Answer 1

The things that seem to be needed in NetBeans are:

• The project has to be the Main Project (by right clicking on it in the Projects list).
• You have to set the Main Class in the project properties. (Right click, Properties, Run, Main Class.)

Then when you right click on your project and do a “Clean and Build”, a jar will get built into the dist subdirectory.

If that fails to fix the problem, here’s a longer story…

When you double click a jar file to run it, the operating system acts as if you had typed this from the command line:

java -jar filename.jar

(When you say it works for you from the command line, is this what you’re typing?)

At that point, the Java executable looks for a file inside the jar named META-INF/MANIFEST.MF. And then in the contents of that file, it looks for the value of a property, Main-Class. And finally it looks for the class of that name in your jar and runs its static main(String[]) method.

So if your jar is failing to run, you can do the following to debug what’s going on:

• Clean and rebuild your project in NetBeans.
• Double check that your class(es) are actually in the jar:
  • Start a command prompt
  • cd into the dist subdirectory of your project.
  • Use a command like jar tf filename.jar to list what’s in there.
• Double check that the MANIFEST.MF file is correct:
  • Again in a command prompt
  • cd into the dist directory.
  • Use a command like jar xf filename.jar META-INF/MANIFEST.MF to extract the manifest.
  • Look at the contents of that file (e.g. type META-INF\MANIFEST.MF) and make sure Main-Class is set to the appropriate class.

If all of the above check out, then double clicking the file should work.