loading class with getResource() gives different results

Question

I cannot understand how Class.getResource() and ClassLoader.getResource() methods works. I wrote this:

public class Prova {
    
    public static void main(String[] args) {
        System.out.println(Prova.class.getResource("/java/lang/String.class"));
        System.out.println(String.class.getResource("/java/lang/String.class"));
        System.out.println(Prova.class.getClassLoader().getResource("/java/lang/String.class"));
    }
}

Here is the output:

jar:file:/C:/Program%20Files/Java/jre1.8.0_301/lib/rt.jar!/java/lang/String.class
jar:file:/C:/Program%20Files/Java/jre1.8.0_301/lib/rt.jar!/java/lang/String.class
null

So, why is the third null? I would expect to get the same result as before.

Please notice that this is a simple case. Inside a webserver I have noticed much more weird situations; null appears in many more cases.

Answer 1

Class.getResource expects a path relative to its package subdirectory. Absolute (=from the top directory) paths start with a /.

ClassLoader only knows absolute paths which are not allowed to start with a /.