How to convert JSON model to JAVA model?

You can’t because it’s not the same file structure and writing (one is not ”real” code, and the second is a java code), but I can suggest you import your JSON file into BlockBench as a custom block, and after convert in a JAVA model. You will surely have some problems after, but it’s the only solution I have.

If you love to change JSON to JAVA, it's impossible, but you could always cheat the system, you make a block with your JSON model and equip it on a head of an invisible entity (either zombie or biped model). However, your mob will not have any animation at all but trust me, it just work, I do it in my mod and it work quite well! :D

edit: You can change the model from JSON to JAVA but with problems about angle rotation and texture in Blockbench as Goldorion  said above