How to analyze class hierarchies using gson?

Question

I have a class hierarchy that I want to convert to and from GSON. I'm not sure how to approach this with GSON (I currently have a class Factorythat looks at JSONObject and is based on the presence or absence of keys, which it calls the correct constructor, which in turn delegates part of its work to a super class) . When I store these objects in a local SQLite DB, I use an integer to indicate my type, and the factory class uses this type to call the correct constructor. I don't have that type in JSON (which is not mine).

How do I tell GSON based on the contents of a JSON object for which an instance of the object is being created for me?

In the examples below, process the ...JSON inside the brackets, as there may or may not be more elements

Here's a breakdown of the class hierarchy:

There is an abstract abstract type: SuperTypewith a JSON representation{"ct":12345,"id":"abc123 ...}

There are two main abstract subtypes: TypeA(has json key "a") and TypeB(has json key "b")

Typea

Example: {"ct":12345,"id":"abc123, "a":{...}}

TypeAhas 15 children (call these TypeA_Aon TypeA_P). The JSON representation of these objects will be something like {"ct":12345,"id":"abc123, "a":{"aa":1 ...} ...}or{"ct":12345,"id":"abc123, "a":{"ag":"Yo dawg I head you like JSON" ...} ...}

Typeb

Example: {"ct":12345,"id":"abc123, "b":{...} ...}

TypeBhas another abstract subtype ( TypeB_A) and few children (let them name these TypeB_Bon TypeB_I). The JSON representation of these objects will be {"ct":12345,"id":"abc123, "b":{"ba":{...} ...} ...}either{"ct":12345,"id":"abc123, "b":{"bg":"Stayin alive" ...} ...}

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TypeAdapter TypeAdapterFactory, , , JSON.

GSON JSON, ?

.

Answer 1

, RTTAF , , , , . . , :

: Github gist

. () GSON Lombok .

Factory

public class CustomTypeAdapterFactory implements TypeAdapterFactory {
    @Override
    public <T> TypeAdapter<T> create (final Gson gson, final TypeToken<T> type) {
        if (type.getRawType () != SuperType.class)
            return null;

        final TypeAdapter<T> delegate = gson.getDelegateAdapter (this, type);

        return new TypeAdapter<T> () {
            @Override
            public void write (final JsonWriter jsonWriter, final T t) throws IOException {
                delegate.write (jsonWriter, t);
            }

            @Override
            public T read (final JsonReader jsonReader) throws IOException, JsonParseException {
                JsonElement tree = Streams.parse (jsonReader);
                JsonObject object = tree.getAsJsonObject ();

                if (object.has ("a"))
                    return (T) readTypeA (tree, object.getAsJsonObject ("a"));

                if (object.has ("b"))
                    return (T) readTypeB (tree, object.getAsJsonObject ("b"));

                throw new JsonParseException ("Cannot deserialize " + type + ". It is not a valid SuperType JSON.");
            }

            private TypeA readTypeA (final JsonElement tree, final JsonObject a) {
                if (a.has ("aa"))
                    return gson.getDelegateAdapter (CustomTypeAdapterFactory.this, TypeToken.get (TypeA_A.class)).fromJsonTree (tree);

                if (a.has ("ab"))
                    return gson.getDelegateAdapter (CustomTypeAdapterFactory.this, TypeToken.get (TypeA_B.class)).fromJsonTree (tree);

                if (a.has ("ac"))
                    return gson.getDelegateAdapter (CustomTypeAdapterFactory.this, TypeToken.get (TypeA_C.class)).fromJsonTree (tree);

                throw new JsonParseException ("Cannot deserialize " + type + ". It is not a valid TypeA JSON.");
            }

            private TypeB readTypeB (final JsonElement tree, final JsonObject b) {
                if (b.has ("ba"))
                    return gson.getDelegateAdapter (CustomTypeAdapterFactory.this, TypeToken.get (TypeB_A.class)).fromJsonTree (tree);

                if (b.has ("bb"))
                    return gson.getDelegateAdapter (CustomTypeAdapterFactory.this, TypeToken.get (TypeB_B.class)).fromJsonTree (tree);

                if (b.has ("bc"))
                    return gson.getDelegateAdapter (CustomTypeAdapterFactory.this, TypeToken.get (TypeB_C.class)).fromJsonTree (tree);

                throw new JsonParseException ("Cannot deserialize " + type + ". It is not a valid TypeB JSON.");
            }
        };
    }
}

SuperType.java

@Getter
@Setter
@EqualsAndHashCode
@ToString
public class SuperType {
    @SerializedName ("ct")
    protected long creationTime;
    @SerializedName ("id")
    protected String id;
}

Type_A , ( ).

TypeA.java

@Getter
@Setter
@EqualsAndHashCode (callSuper = true)
@ToString (callSuper = true)
public class TypeA extends SuperType {}

TypeA_A.java

@Getter
@Setter
@EqualsAndHashCode (callSuper = true)
@ToString(callSuper = true)
public class TypeA_A
  extends TypeA {

    @SerializedName ("a")
    protected AA aValue;

    @ToString
    private static class AA {
        @SerializedName ("aa")
        private String aaValue;
    }
}

Type_A TypeA_A.

Type_B , ( , ):

TypeB.java

@Getter
@Setter
@EqualsAndHashCode (callSuper = true)
@ToString (callSuper = true)
public class TypeB extends SuperType  {

// no member declared here

    protected static abstract class B {
        @SerializedName ("b1")
        protected String b1Value;
        @SerializedName ("b2")
        protected String b2Value;
    }
}

Type_BA.java

@Getter
@Setter
@EqualsAndHashCode (callSuper = true)
@ToString (callSuper = true)
public class TypeB_A
  extends TypeB {

    @SerializedName ("b")
    protected BA bValue;

    @ToString
    private static class BA extends B {
        @SerializedName ("ba")
        private String baValue;
    }
}

TypeB_B.java

@Getter
@Setter
@EqualsAndHashCode (callSuper = true)
@ToString (callSuper = true)
public class TypeB_B
  extends TypeB {

    @SerializedName ("b")
    protected BB bValue;

    @ToString
    private static class BB extends B {
        @SerializedName ("bb")
        private String bbValue;

        @SerializedName ("bb1")
        private String bb1Value;
    }
}

, , Java- Github.

@Jesse Wilson @Argyle , .

Answer 2

TypeAdapter, RuntimeTypeAdapterFactory, .

:

RuntimeTypeAdapterFactory<BillingInstrument> rta = RuntimeTypeAdapterFactory.of(
    BillingInstrument.class)
    .registerSubtype(CreditCard.class);
Gson gson = new GsonBuilder()
    .registerTypeAdapterFactory(rta)
    .create();

CreditCard original = new CreditCard("Jesse", 234);
assertEquals("{\"type\":\"CreditCard\",\"cvv\":234,\"ownerName\":\"Jesse\"}",
    gson.toJson(original, BillingInstrument.class));
BillingInstrument deserialized = gson.fromJson(
    "{type:'CreditCard',cvv:234,ownerName:'Jesse'}", BillingInstrument.class);
assertEquals("Jesse", deserialized.ownerName);
assertTrue(deserialized instanceof CreditCard);