This may be an issue with Java version. If you are double-clicking the JAR in Windows, it will use whatever version of Java happens to be the primary version on your machine. If this is not Java 8 (especially if it's Java 9 or later), that won't currently work well with Greenfoot's exported JARs, which currently only work with Java 8 (until we update to Java 9+ in future). Best work-around is to use a batch file as you describe -- you can try changing which Java is associated with .jar files but it's probably more trouble than it's worth, especially if other applications on your system need the newer Java.