使用 GSON 将复杂对象（反）序列化为 JSON

Question

I have a problem with serializing and deserializing my object structure with GSON. In order to describe the problem i'll have to describe my class structure a bit:

I have a java abstract class, let's name it "A". There are also classes "BA", "CA", "DA" that are abstract too and they extend class "A". Each of them has it's own constructor, non of which is non-arg. Finally there are several (many!) classes those extend "BC", or "CA" or "DA". Instances of those "bottom" classes are kept in "ArrayList" list.

Now, i'm trying to "jsonize" that array list. For creating Json string I'm using this code:

Gson gs = new Gson();
Type listOfTestObject = new TypeToken<List<A>>(){}.getType();
String rez = gs.toJson(getListOfAs(), listOfTestObject);

And i'm trying to deserialize that json using this (in another class):

Type listOfTestObject = new TypeToken<ArrayList<A>>(){}.getType();
ArrayList<A> listOfAs = gs.fromJson(jsonREZString, listOfTestObject);

but above code throws this:

Unable to invoke no-args constructor for class packagename.A. Register an InstanceCreator with Gson for this type may fix this problem.

Now, I have create a non-args constructor in the class "A", but no luck. I have read about "InstanceCreator" but it looks like I would have to create an "InstanceCreator" for each concrete class that extends "A"! Right? I can't do it, because I have many (many!) classes that extend "A" through "BA", "CA" or "DA".

What am I missing? How can I simply deserialize (serialization seems fine) this complex structure without adding custom deserialializastion code for each type?

Answer 1

In fact you might have here 2 distinct problems.

1) You have polymorphic types thus you probably want to serialize objects as their concrete type and not A. 2) You want to deserialize to the concrete types that do not provide no arg ctrs.

Gson does not support 1 & 2, there is an extension for 1 but I never used it.

Maybe Genson solves your problem, it supports both 1 & 2.

Genson genson = new Genson.Builder()
                            // enables polymorphic types support
                            .setWithClassMetadata(true)
                            // enables no arg support
                            .setWithDebugInfoPropertyNameResolver(true)
                          .create();

// will look like: [{"@class": "com.xxx.SomeConcreteClass", ...}, {"@class": "com.XXX.OtherClass"}]
String json = genson.serialize(getListOfAs());
List<A> listOfA = genson.deserialize(json, new GenericType<List<A>>() {});

You don't need to specify the type during serialization, except if you want that only parent fields be present in the output.

ex: genson.serialize(getListOfAs(), GenericType>() {}) will serialize only attributes from A, you can also force genson to always use runtime types by setting setUseRuntimeTypeForSerialization(true) on the builder.

Also if you don't want impl details to leak in the json representation, you can define aliases (builder.addAlias("someAlias", SomeClass.class) for your types, they will be used instead of full package+classname.