jMonkeyEngine is a game development suite made especially for game developers who want to create 3D games with modern technology standards. The software is programmed entirely in Java, intended for wide accessibility and quick deployment.
This engine is the VERY reason that I am switching from learning C++ to learning Java.
The only game I've tried that used it is great, not too resource intensive, and still has a look that's worth watching. Additionally, this engine CAN do what many native-coded AAA game engines do, for free, with the possibility to make a profit., in a language that means that it's completely compatible with Mac, Linux, and Windows.
In short, develoupers of open world games should look no further.