使用Gson如何转换Json对象，该对象包含由许多类实现的接口的对象-Java

Question

I have an object of UserMsg that I want to send using JSON

public class UserMsg {
private String Type; 
private UserData userData; 

//Getters & Setters }

UserData is an Interface

there are many classes implementing UserData .. and everytime I send the Json Object Its userData could be an object of any of these classes.

so on the Receiving side I used this code:

        Gson gson = new Gson(); 
        UserMsg usermsg = gson.fromJson(JsonObject,UserMsg.class);

and got this error:

Unable to invoke no-args constructor for interface UserData. Register an InstanceCreator with Gson for this type may fix this problem.

I tried to Register an InstanceCreator for UserData but It doesn't work so I need the right way to make it run.

Answer 1

You may want to create a custom Adapter and inside the read/write methods you may set the parsing methodology in order to handle different classes implementing your UserData interface. Something like this:

public class UserMsgAdapter extends TypeAdapter {

  @Override
  public UserMsg read(final JsonReader in) throws IOException {
    final UserMsg data = new UserMsg();

    in.beginObject();
    while (in.hasNext()) {

      in.nextName();
      if(in.nextString().equals(someType))
         // create one instance of a particular implementation

      }
    }
    in.endObject();

    return data;
  }

  @Override
  public void write(final JsonWriter out, final UserMsg msg) throws IOException {
    out.beginObject();
    // write according to the instance type
    out.endObject();
  }
}

And dont forget to register this custom adapter:

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(UserMsg.class, new UserMsgAdapter());
final Gson gson = gsonBuilder.create();