RealmGenerator is a wbsite you can find under http://realmgenerator.eu, check http://realmgenerator.eu/about.html for more info.
You can also generate model files directly from command line. Example:
$ php processor-cli.php Android model.json
./data/realm_model585274f772158/You will get the name of the directory containing the generated files.
The main algorithm which does the conversion is written in PHP. Normally I'm an Android developer and I made this generataor becase I needed it. As I'm not an expert in PHP, there are probably many places where the code can be improved. I will be grateful for any feedback:).
I'm planning to add Objectiv C and React Native too.