How can I serialize a nested object using Gson @SerializedName in Java?

Question

I have twitter data. There are some nested objects in this data. I want to gather these fields into a single object in Java. I'm using @SerializedName anotation to import some of the nested fields into my java object.

My sample json looks like this:

{
  "created_at": "Sat Jun 15 19:21:29 +0000 2019",
  "text": "RT @BuzzTechy: [BEST] Udemy Course - Create a Python Powered Chatbot in Under 60 Minutes  \n\nhttps:\/\/t.co\/jMIW38FmmZ \n\n#AI #Python #Chatbot\u2026",
  "source": "\u003ca href=\"https:\/\/allentowngroup.com\" rel=\"nofollow\"\u003ebobbidigi\u003c\/a\u003e",
  "truncated": false,
  "in_reply_to_screen_name": "asdsf"
  "user": {
    "id": 1724601306,
    "name": "Rob's Coding News In The Hood"
  }
}

And my java object:

public class TweetEntity implements Serializable {

private static long serialVersionUID = 1L;

@SerializedName("created_at")
private Date createdAt;

private String text;

private String source;

private Boolean truncated;

@SerializedName("in_reply_to_screen_name")
private String inReplyToScreenName;

@SerializedName("user.name")
private String userName;

}

But this does not work. Does anyone have any idea or knowledge about it?

Sam · Accepted Answer · 2020-05-09 08:39:43Z

7

Got a little simpler version of the previous answer working


public class TweetEntity {

  ...

  @SerializedName("user")
  @JsonAdapter(UsernameDeserializer.class)
  private String userName;
}

public static class UsernameDeserializer implements JsonDeserializer<String> {
    @Override
    public String deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) {
        return json.getAsJsonObject().get("name").getAsString();
    }
}

answered May 9, 2020 at 8:39

Sam

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Stephan Schlecht · Accepted Answer · 2019-06-16 11:03:25Z

If you don't want to have a separate user object but instead having userName as a property in TweetEntity you can do the following:

write a custom TweetEntity Deserializer
extract the user name via calling getAsJsonObject twice (on user and on name)
deserialize a TweetEntity the normal way (using correct date format)
set user name on TweetEntity

Test

To be able to test we need to access some properties in TweetEntity:

Date getCreatedAt() {
    return createdAt;
}

void setUserName(String userName) {
    this.userName = userName;
}
String getUserName() {
    return userName;
}

TweetEntity Deserializer

The date string used in your example looks like this:

Sat Jun 15 19:21:29 +0000 2019

For such a date, you must specify a custom format for deserialization.

The corresponding date format is:

E MMM dd hh:mm:ss Z yyyy

So we a first extracting the user JsonElement and from there the name JsonElement. After deserializing a TweetEntity instance with the custom date format (and missing userName), we can then set the userName property.

In code it looks like this:

import com.google.gson.*;
import java.lang.reflect.Type;


public class TweetEntityDeserializer implements JsonDeserializer<TweetEntity> {

    @Override
    public TweetEntity deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        JsonElement user = json.getAsJsonObject().get("user");
        JsonElement userName = user.getAsJsonObject().get("name");

        Gson g = new GsonBuilder().setDateFormat("E MMM dd hh:mm:ss Z yyyy").create();
        TweetEntity entity = g.fromJson(json, TweetEntity.class);
        entity.setUserName(userName.getAsString());
        return entity;
    }

}

Test

Let's try it with a small, self-contained Java program.

import com.google.gson.*;

public class Main {

    public static void main(String[] args) {
        String json = "{\"created_at\":\"Sat Jun 15 19:21:29 +0000 2019\",\"text\":\"RT @BuzzTechy: [BEST] Udemy Course - Create a Python Powered Chatbot in Under 60 Minutes  \\n\\nsomeUrl \\n\\n#AI #Python #Chatbot?\",\"source\":\"<a href=\\\"https://allentowngroup.com\\\" rel=\\\"nofollow\\\">bobbidigi</a>\",\"truncated\":false,\"in_reply_to_screen_name\":\"asdsf\",\"user\":{\"id\":1724601306,\"name\":\"Rob's Coding News In The Hood\"}}";
        Gson g = new GsonBuilder()
                .registerTypeAdapter(TweetEntity.class, new TweetEntityDeserializer())
                .create();
        TweetEntity entity = g.fromJson(json, TweetEntity.class);
        System.out.println("created at: " + entity.getCreatedAt());
        System.out.println("userName: " + entity.getUserName());
    }

}

Output to Console

created at: Sat Jun 15 21:21:29 CEST 2019
userName: Rob's Coding News In The Hood

So the userName field is gathered without nesting into the TweetEntity object as well as the custom date format is observed. So it works!

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