While I don't use app_java at all, I did some work to at least get it to build on modern Fedora 27 systems without GCJ and with the following patch and spec: https://messinet.com/rpms/browser/kamailio/app_java.patch, https://messinet.com/rpms/browser/kamailio/kamailio.spec
Though even with that patch it probably my builds probably wouldn't work--the build is filled with lots of incompatible pointer type warnings like the following so the code probably needs some modernizing too. ``` BUILDSTDERR: java_msgobj.c: In function 'fill_sipmsg_object': BUILDSTDERR: java_msgobj.c:83:27: warning: passing argument 2 of '(*env)->SetIntField' from incompatible pointer type [-Wincompatible-pointer-types] BUILDSTDERR: (*env)->SetIntField(env, SipMsgInstance, fid, msg->id); ```