The Java role installs installs OpenJDK 8.
Note that on the NixOS platform it is not strictly necessary to activate a role to install software. You can install Java into the service-user:
ssh your-vm sudo -iu s-youserservice nix-env -iA nixos.jdk
To find out about which java packages are available, see the NixOS packages index. We also have some guidance for automatically installing Oracle Java.