Intellij Idea Resource Loader - "Resources not found in plugin" (Intelij Idea) (idea plugin) (java)! It may not be the best solution, but it works. resource loading even in unit tests.

IntellijIdeaResourceLoader.java

import io.github.classgraph.ClassGraph;
import io.github.classgraph.ResourceList;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;

/**
 * This work in (JUnit, Jar, etc.)
 * It's better to create a new instance each time
 * 
 * @author Aleksei Vlasov (github.com/adideas)
 */
public class IntellijIdeaResourceLoader {
    String dir = null;
    public IntellijIdeaResourceLoader() {
        if (Thread.currentThread().getContextClassLoader() == null) {
            // without jar
            dir = System.getProperty("user.dir");
            if (dir == null) {
                File currentDirFile = new File(".");
                String helper = currentDirFile.getAbsolutePath();
                try {
                    dir = helper.substring(0, helper.length() - currentDirFile.getCanonicalPath().length());
                } catch (Exception ignore) {}
            }
            dir = new File(dir, "resource").getAbsolutePath();
        }
    }

    public URL getResource(Class<?> anyClassFromYouProject, String filePath) {
        if (dir != null) {
            try {
                return new URL("file://" + new File(dir, filePath).getAbsolutePath());
            } catch (Exception ignore) {}
        }
        URL url = null;
        url = anyClassFromYouProject.getClassLoader().getResource(filePath);
        if (url != null) {return url;}

        Thread.currentThread().setContextClassLoader(anyClassFromYouProject.getClassLoader());
        url = Thread.currentThread().getContextClassLoader().getResource(filePath);
        if (url != null) {return url;}

        ResourceList resources = new ClassGraph().acceptPaths(filePath).scan().getAllResources();
        if (!resources.isEmpty()) {
            resources.get(0).getURI();
        }
        return null;
    }

    public InputStream getResourceAsStream(Class<?> anyClassFromYouProject, String filePath) {
        URL url = getResource(anyClassFromYouProject, filePath);
        if (url != null) {
            try {
                return url.openStream();
            } catch (IOException ignore) {}
        }

        return null;
    }
}

Example error
Example error2
Example error 3
Example error 4
Example error 5
Example error 6
Example error 7
Example error 8

This is an old problem, and everyone seems to solve it in their own way. I hope this helps someone.

Thank you. I just had to change line 17 to not equal; currently, I am not worried about building an actual Jar for my application.