NetBeans-made jar file won't work
java, netbeans
Solution
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.
Problem
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)?