Khronos does have some official OpenGL specs for Java. There are none from them for .NET.
Java runs on iOS via Codename One and Gluon AOT compilers.
Native interop in Java was made hard on purpose, as Sun was pushing for WORA. Oracle is in the process of improving the situation with Project Valhala.
I love both stacks, truth is that .NET still has a bit to catch up outside Windows, while Java lost a bit of its mojo on desktops.