Try:

InputStream is = MyTest.class.getResourceAsStream("/test.csv");

IIRC getResourceAsStream() by default is relative to the class's package.

As @Terran noted, don't forget to add the / at the starting of the filename

Finally, this is what I did:

private File getFileFromURL() {
    URL url = this.getClass().getClassLoader().getResource("/sql");
    File file = null;
    try {
        file = new File(url.toURI());
    } catch (URISyntaxException e) {
        file = new File(url.getPath());
    } finally {
        return file;
    }
}

...

File folder = getFileFromURL();
File[] listOfFiles = folder.listFiles();

To answer your question posted in the title of this topic...

Step 1--> Right Click on Java Project, Select the option "Properties"

Step 2--> Select "Java Build Path" from the left side menu, make sure you are on "Source" tab, click "Add Folder"

Step 3--> Click the option "Create New Folder..." available at the bottom of the window

Step 4--> Enter the name of the new folder as "resources" and then click "Finish"

Step 5--> Now you'll be able to see the newly created folder "resources" under your java project, Click "Ok", again Click "Ok"

Final Step --> Now you should be able to see the new folder "resources" under your java project

You can determine what files are in a folder in resources (even if its in a jar) using the FileSystem class.

public static void doSomethingWithResourcesFolder(String inResourcesPath) throws URISyntaxException {
    URI uri = ResourcesFolderUts.class.getResource(inResourcesPath).toURI();
    try( FileSystem fileSystem = FileSystems.newFileSystem(uri, Collections.emptyMap() ) ){
        Path folderRootPath = fileSystem.getPath(inResourcesPath);
        Stream<Path> walk = Files.walk(folderRootPath, 1);
        walk.forEach(childFileOrFolder -> {
            //do something with the childFileOrFolder
        });
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

inResourcesPath should be something like "/images_resultats"

Note that the childFileOrFolder paths can only be used while the FileSystem remains open, if you try to (for example) return the paths then use them later you've get a file system closed exception.

Change ResourcesFolderUts for one of your own classes