I only wanted to post a solution for this problem that I had and I didn't found any solution or resources to solve it.

The problem is that if you are trying to print a report with the Jasper Reports module and it takes too long to print, and finally it prints a 0kb report, if you check the server logs you will find a "JNI error (class file version not supported)

The problem is that your Java version is too old or not compatible with the jasper reports tool or the jrxml file that you are using.

You have to uninstall your Java (in my case was Java 8) and check that the uninstallation was succesful by checking your current java version. If you are still seeing Java 8 on the prompt, you have to uninstall aswell the "AdoptOpenJDK".

In my case I had installed aswell Java 11, so when I uninstalled succesfully my Java 8, it appeared Java 11 on my prompt, and then, the report was printed.

NOTE: The Java path that you have to put on your enterprise settings on odoo is the java folder that comes with your Jasper Reports module.

I only wish this helps someone that finds or has this issue in the future.

Thanks for reading.