It seems that URLClassLoader became Mr.Jar-friendly (JEP#238). At least this could be easily checked via findResource() method for example.
===
   URLClassLoader urlCL = new URLClassLoader(new URL[]{new URL("file:///D:/mr_jar_1.jar")});
   System.out.println("resource = " + urlCL.findResource("text.txt"));
===

When running with JDK9 the output is:

resource = jar:file:/D:/mr_jar_1.jar!/META-INF/versions/9/text.txt

However its spec http://download.java.net/java/jdk9/docs/api/java/net/URLClassLoader.html
 seems to be completely silent about JEP#238 support.