how to use gson handle dynamic response data? (json to java object)

Question

Here is Json list of users:

{
    'response': {
        'result': {
            'users': [{
                name: "ABC",
                age: "20",
                gender: "F"
            },
            {
                name: "cbd",
                age: "23",
                gender: "M"
            }]
        }
    }
}

And sometimes return like this

{
    'response': {
        'result': {
            'users': {
                name: "ABC",
                age: "20",
                gender: "F"
            }
        }
    }
}

When it is return single user I can use following java class to covert json to java

class Data{
    private response response;
    class response{
        private result result;
        class result{
            private users users;
            class users{
                private String name;
                private String age;
                private String gender;

            }

        }
    }

}

but if returns lists of users I get Exception. How can I handle it?

Answer 1

To do what you want, you have to change users to a List<User> and write a (simple) custom deserializer. Without custom deserializer Gson won't parse, unless you switch users to Object (but in that case it will return a list of maps).

Before showing you a full example that you can copy and run in your IDE, two notes. User " instead of ' as string delimiter and follow Java naming conventions. Classes must have at least first letter in upper case. Putting lowercase won't change Gson behavior since it looks at class fields name not to class names.

package stackoverflow.questions.q19124387;

import java.lang.reflect.Type;
import java.util.*;



import com.google.gson.*;
import com.google.gson.reflect.TypeToken;

public class Q19124387 {

   public static class User {
      private String name;
      private String age;
      private String gender;
      @Override
      public String toString() {
         return "User [name=" + name + ", age=" + age + ", gender=" + gender + "]";
      }



   }

   public static class Result {
      private List<User> users = new ArrayList<User>();

   }

   public static class Response {
      private Result result;

   }

   public static class Data {
      private Response response;

   }

   public static class CustomDeserializer implements JsonDeserializer<Result> {

      public Result deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {

         Result r = null;

         JsonElement usersField = json.getAsJsonObject().get("users");
         if (usersField.isJsonNull() || usersField.isJsonPrimitive())
            ; // if is null or is a primitive type will return an empty result
         else if (usersField.isJsonObject()) {
            r = new Result();
            r.users.add((User) context.deserialize(usersField, User.class));

         } else if (usersField.isJsonArray()) {
            Type listOfUserType = new TypeToken<List<User>>() {}.getType();
            r = new Result();

            r.users.addAll((Collection<? extends User>) context.deserialize(usersField, listOfUserType));

         }
         return r;

      }

   }

   /**
    * @param args
    */
   public static void main(String[] args) {
      String json1 = " {\"response\": {  \"result\": { \"users\": [{ name: \"ABC\",  age: \"20\", gender: \"F\"  },  { name: \"cbd\",  age: \"23\", gender: \"M\"  }] }  }  }";

      String json2 = "{\"response\": { \"result\": {\"users\": { name: \"ABC\", age: \"20\",gender: \"F\" } } } }  ";

      Gson g = new Gson();
      try {
         Data d1 = g.fromJson(json1, Data.class);
         System.out.println("Parsed json1, list of users");
      } catch (Exception e) {
         System.out.println("Cannot parse json1, list of users");
      }
      try {
         Data d2 = g.fromJson(json2, Data.class);
         System.out.println("Parsed json2, single user");
      } catch (Exception e) {
         System.out.println("Cannot parse json2, single user");
      }

      GsonBuilder gb = new GsonBuilder();
      gb.registerTypeAdapter(Result.class, new CustomDeserializer());
      Gson g2 = gb.create();

      Data d1 = g2.fromJson(json1, Data.class);
      Data d2 = g2.fromJson(json2, Data.class);

      System.out.println("Second user of json1 is " + d1.response.result.users.get(1));
      System.out.println("First user of json2 is " + d2.response.result.users.get(0));

   }

}

this is the execution of code:

Parsed json1, list of user
Cannot parse json2, single user
Second user of json1 is User [name=cbd, age=23, gender=M]
First user of json2 is User [name=ABC, age=20, gender=F]