I've got an application that runs on JBoss server. Also I've got nginx as reverse proxy server for this application. Now I want to configure ssl. Do I have to configure it on both servers? JKS for JBoss and crt, key for nginx?
It depends. I'm assuming, that nginx serves (and proxies) ports 80 and 443 to JBoss. For this alone, setting um SSL in nginx is sufficient.
If JBoss needs to serve a different port via SSL directly, without nginx proxying, to handle your application needs, (e.g. 8443) then you need to setup SSL in JBoss too.