Ejemplo que muestra 2 maneras de obtener recursos del classpath.
From ClassLoader, all paths are "absolute" already - there's no context from which they could be relative. Therefore you don't need a leading slash.
InputStream in = this.getClass().getClassLoader() .getResourceAsStream("SomeTextFile.txt");
From Class, the path is relative to the package of the class unless you include a leading slash, so if you don't want to use the current package, include a slash like this:.
InputStream in = this.getClass().getResourceAsStream("/SomeTextFile.txt");