The following testcase generates the output:

service:jmx:rmi://jndi/rmi://myhost:9999/jmxri
/rmi://myhost:9999/jmxri << the /jndi part has been swallowed
service:jmx:rmi:///jndi/rmi://myhost:9999/jmxri

I also believe that "service:jmx:rmi://jndi/rmi://myhost:9999/jmxri"
is incorrect (missing a third / in rmi://jndi.

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import javax.management.remote.*;

public class url {
    public static void main(String args[]) {
try {
System.out.println(new JMXServiceURL("service:jmx:rmi://jndi/rmi://myhost:9999/jmxri").toString());
System.out.println(new JMXServiceURL("service:jmx:rmi://jndi/rmi://myhost:9999/jmxri").getURLPath());
System.out.println(new JMXServiceURL("rmi","",0,"/jndi/rmi://myhost:9999/jmxri").toString());
} catch (Exception x) {
x.printStackTrace();
System.exit(1);
}
    }
}