Polymorphism or Inheritance in JSON with Java and Ruby

Question

For context, we are storing most of our data as JSON strings. This works very well with Hadoop on the backend and is easy to handle in Ruby on the front end. My data types fit the natural pattern for inheritance.

For simplicity, lets say I have a class Pet and a process FeedPet that feeds a pet. I also have a process WalkDog that only applies to Dog, which is a kind of pet. My data is organized such that I never need to worry about trying to walk a pet that isn't a dog.

What I would like to do is have Pet and Dog extends Pet, with Dog having an additional method "getLeash()", but I can't figure out how to map this to JSON.

Instead, I have a class Pet with a species data hashmap, so the WalkDog process would call pet.getSpeciesData("leash") instead of dog.getLeash().

I can create a Dog extends Pet, and I can serialize that to JSON using the Jackson library. The JSON object will have a leash field. But assume that I want to feed all the pets. All pets have a getFood() method. So the FeedPet process deserializes the objects to Pet. But this loses the leash field.

The WalkDog process can do this because it knows all of its input is going to be Dogs, so it can read it as a Dog and write it back out as a Dog.

Is there any way to serialize java objects to JSON such that I can preserve their type? I'm thinking something like Rails single table inheritance, but it would have to be something that the JSON libraries understand.

Answer 1

To make it work you must both embed some object type information in data (where it is only useful for deserializing) and usually use external schema definition which otherwise would not be needed (like XML Schema for xml; since that's basically a generic type system). This has same problems as ORM has: Hibernate has to use ugly work-arounds (n+1 - way joins, "super tables" or discriminator fields).

Or another way to put it: data mapping/binding is not quite the same as object serialization/deserialization (latter tries to preserve more of object identity).

Here I am assuming that what you want is basically something like:

Pet pet = mapper.readValue(jsonString, Pet.class);
// (and possibly get an exception if Pet is an abstract class...)
Leash l = ((Dog) pet).getLeash();

If this is not the case, you could just simply bind to Dog

Dog dog = mapper.readValue(jsonString, Dog.class);

So anyway: Jackson project has feature request for doing just this, which allow you to do what (I think) you want.

However, this solution will mostly work for Java, as there is no standard way of passing Object type info within JSON. With XML this can sort of be done with XML Schema defined "xsi:type" attribute; which identifies Schema type, which is then mapped to class (yes, rather complicated way, but it does work).

UPDATE: Jackson 1.5 added support for this (i.e. implemented JACKSON-91 feature request), so it can be used for generating type identifiers, to allow proper handling of polymorphic types. It should work with non-Java systems too, given that you can fully configure details of how type information is to be included; and is NOT limited to using Java class names.