How to reference a resource file correctly for JAR and Debugging?

Once you pack the JAR, your resource files are not files any more, but stream, so getResource will not work!

Use getResourceAsStream.

To get the “file” content, use https://commons.apache.org/proper/commons-io/javadocs/api-release/org/apache/commons/io/IOUtils.html:

static public String getFile(String fileName)
{
        //Get file from resources folder
        ClassLoader classLoader = (new A_CLASS()).getClass().getClassLoader();

        InputStream stream = classLoader.getResourceAsStream(fileName);

        try
        {
            if (stream == null)
            {
                throw new Exception("Cannot find file " + fileName);
            }

            return IOUtils.toString(stream);
        }
        catch (Exception e) {
            e.printStackTrace();

            System.exit(1);
        }

        return null;
}