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

The contents of Maven resource folders are copied to target/classes and from there to the root of the resulting Jar file. That is the expected behaviour.

What I don't understand is what the problem is in your scenario. Referencing a Resource through getClass().getResource("/filename.txt") starts at the root of the classpath, whether that (or an element of it) is target/classes or the JAR's root. The only possible error I see is that you are using the wrong ClassLoader.

Make sure that the class that uses the resource is in the same artifact (JAR) as the resource and do ThatClass.class.getResource("/path/with/slash") or ThatClass.class.getClassLoader().getResource("path/without/slash").

But apart from that: if it isn't working, you are probably doing something wrong somewhere in the build process. Can you verify that the resource is in the JAR?

I had a similar problem. After a full day of trying every combination and debugging I tried getClass().getResourceAsStream("resources/filename.txt") and got it to work finally. Nothing else helped.

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;
}