4MLinux 8.1 (and later) supports a fully automatic installation of the vanilla version of Java Runtime Environment. This is one of so called "downloadable extensions" (i.e. optional software in 4MLinux, which is installed in the /opt directory by default).
1) Select Java RE from the Extensions menu:
2) Default security settings in Java are curiously high nowadays. This why I recommend to change them from "High" to "Medium":
3) Now you have everything needed to run JAR and JNLP files. Just double click on the file, which you want to open. Let's take JAP (privacy generator) as an examlpe:
Have fun :-)