//create server-side function
db.system.js.save(
    {
        _id : "calculateAnimalDiet" ,
        value : function (animal, size, weight) { return ... }
    }
);

EntityManger em = ...;
StoredProcedureQuery sp = em.createStoredProcedureQuery("calculateAnimalDiet");

sp.registerStoredProcedureParameter("animal", String.class, ParameterMode.IN);
sp.registerStoredProcedureParameter("size", Integer.class, ParameterMode.IN);
sp.registerStoredProcedureParameter("weight", Double.class, ParameterMode.IN);

sp.setParameter("animal", "Giraffe");
sp.setParameter("size", 573);
sp.setParameter("weight", 1600.00);

sp.execute();

AnimalDiet = (AnimalDiet)sp.getSingleResult();