I'm not familiar with directjngine or any other Java stacks, but there hasn't been any breaking changes between Ext JS 4.x and 5.x, including the upcoming 5.1.0; which means that most of the server side implementations should work as they were if they're not doing something funky.
Support for new features is a totally different matter though, and most of the stacks lag behind the client side significantly. I try to keep my reference stack up to date but cannot vouch for other contributors; since the effort is community driven nobody can guarantee that particular implementation will support all available features and stay updated in the future.
Speaking as a fellow contributor though, I can surmise that directjngine author may be willing to accept pull requests. ;)