如何将 JSON 映射到 Java 模型类

how to map a JSON to a java model class

提问by Thilina Dinith Fonseka

I need to map JSONobj to a class and its arrays to ArrayListin Androidand it should have all the children data as well. (with nested arraylists too) and i need to convert updated data list again to jsonobject

我需要将JSONobj映射到一个类，并将其数组映射到ArrayListin Android，它也应该包含所有子数据。（也有嵌套的数组列表），我需要再次将更新的数据列表转换为jsonobject

my json string is

我的 json 字符串是

  {
    "type": "already_planted",
    "crops": [
      {
        "crop_id": 1,
        "crop_name": "apple",
        "crop_details": [
          {
            "created_id": "2017-01-17",
            "questions": [
              {
                "plants": "10"
              },
              {
                "planted_by": "A person"
              }
            ]
          },
          {
            "created_id": "2017-01-30",
            "questions": [
              {
                "plants": "15"
              },
              {
                "planted_by": "B person"
              }
            ]
          }
        ]
      },
      {
        "crop_id": 2,
        "crop_name": "Cashew",
        "crop_details": [
          {
            "created_id": "2017-01-17",
            "questions": [
              {
                "plants": "11"
              },
              {
                "planted_by": "c person"
              }
            ]
          }
        ]
      }
    ]
  }

回答by Bishoy Abd

First of all,you need to create the class that you are going to map JSON inside.

首先，您需要创建要在其中映射 JSON 的类。

Fortunately, there is a website that can do it for you here

幸运的是，有一个网站，可以为你做它在这里

---

---

secondly, you can use google Gsonlibrary for easy mapping

其次，您可以使用谷歌Gson库进行轻松映射

1. add the dependency.

1.添加依赖。

        dependencies {
          implementation 'com.google.code.gson:gson:2.8.6'
         }  

2. from your object to JSON.

2. 从你的对象到 JSON。

        MyData data =new MyData() ; //initialize the constructor 
        Gson gson = new Gson();  
        String Json = gson.toJson(data );  //see firstly above above
        //now you have the json string do whatever.

3. from JSON to object .

3.从JSON到对象。

        String  jsonString =doSthToGetJson(); //http request 
        MyData data =new MyData() ; 
        Gson gson = new Gson();  
        data= gson.fromJson(jsonString,MyData.class); 
        //now you have Pojo do whatever

for more information about gson see this tutorial.

有关 gson 的更多信息，请参阅本教程。

回答by yu wang

If you use JsonObject, you can define your entity classas this:

如果您使用JsonObject，您可以将您的定义entity class为：

public class Entity {
    String type;
    List<Crops> crops;
}

public class Crops {
    long crop_id;
    String crop_name;
    List<CropDetail> crop_details;
}

public class CropDetail {
    String created_id;
    List<Question> questions;
}

public class Question {
    int plants;
    String planted_by;
}

public void convert(String json){
    JsonObject jsonObject = new JsonObject(jsonstring);
    Entity entity = new Entity();
    entity.type = jsonObject.optString("type");
    entity.crops = new ArrayList<>();
    JsonArray arr = jsonObject.optJSONArray("crops");
    for (int i = 0; i < arr.length(); i++) {
        JSONObject crops = arr.optJSONObject(i);
        Crops cps = new Crops();
        cps.crop_id = crops.optLong("crop_id");
        cps.crop_name = crops.optString("crop_name");
        cps.crop_details = new ArrayList<>();
        JsonArray details = crops.optJsonArray("crop_details");
        // some other serialize codes
        ..........
     }
}

So you can nested to convert your json string to an entity class.

因此，您可以嵌套将 json 字符串转换为实体类。

回答by Blasanka

Here is how I do it without any packages, this do the work for me for small use cases:

这是我在没有任何包的情况下如何做到的，这对我来说适用于小型用例：

My modal class:

我的模态类：

package prog.com.quizapp.models;


import org.json.JSONException;
import org.json.JSONObject;

public class Question {
    private String question;
    private String correct_answer;
    private String answer_a;
    private String answer_b;
    private String answer_c;
    private String answer_d;

    public Question() {
    }

    public Question(String question, String answer_a, String answer_b, String answer_c, String answer_d, String correct_answer) {
        this.question = question;
        this.answer_a = answer_a;
        this.answer_b = answer_b;
        this.answer_c = answer_c;
        this.answer_d = answer_d;
        this.correct_answer = correct_answer;
    }

    public String getQuestion() {
        return question;
    }

    public void setQuestion(String question) {
        this.question = question;
    }

    public String getCorrect_answer() {
        return correct_answer;
    }

    public void setCorrect_answer(String correct_answer) {
        this.correct_answer = correct_answer;
    }

    public String getAnswer_a() {
        return answer_a;
    }

    public void setAnswer_a(String answer_a) {
        this.answer_a = answer_a;
    }

    public String getAnswer_b() {
        return answer_b;
    }

    public void setAnswer_b(String answer_b) {
        this.answer_b = answer_b;
    }

    public String getAnswer_c() {
        return answer_c;
    }

    public void setAnswer_c(String answer_c) {
        this.answer_c = answer_c;
    }

    public String getAnswer_d() {
        return answer_d;
    }

    public void setAnswer_d(String answer_d) {
        this.answer_d = answer_d;
    }

    @Override
    public String toString() {
        return "Question{" +
                "question='" + question + '\'' +
                ", correct_answer='" + correct_answer + '\'' +
                ", answer_a='" + answer_a + '\'' +
                ", answer_b='" + answer_b + '\'' +
                ", answer_c='" + answer_c + '\'' +
                ", answer_d='" + answer_d + '\'' +
                '}';
    }

    public static Question fromJson(JSONObject obj) throws JSONException {
        return new Question(
                obj.getString("question"),
                obj.getString("answer_a"),
                obj.getString("answer_b"),
                obj.getString("answer_c"),
                obj.getString("answer_d"),
                obj.getString("correct_answer"));
    }
}

And I have another class to get the json file from assets directory and mapped JsonObjectto my model class Question:

我还有另一个类可以从资产目录中获取 json 文件并映射JsonObject到我的模型类Question：

package prog.com.quizapp.utils;

import android.content.Context;
import android.util.Log;

import org.json.JSONObject;

import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Iterator;

import prog.com.quizapp.models.Question;

public class JsonSqlQueryMapper {
    private Context mContext;

    public JsonSqlQueryMapper(Context context) {
        this.mContext = context;
    }

    private static final String TAG = "JsonSqlQueryMapper";

    public JSONObject loadJSONFromAsset() {
        String json = null;
        try {
            InputStream is = mContext.getAssets().open("quiz_app.json");
            int size = is.available();
            byte[] buffer = new byte[size];
            is.read(buffer);
            is.close();
            json = new String(buffer, "UTF-8");
        } catch (IOException ex) {
            ex.printStackTrace();
            return null;
        }

        try {
            JSONObject quizObject = new JSONObject(json).getJSONObject("quiz");
            return quizObject;
        } catch (Exception e) {
            Log.d(TAG, "loadJSONFromAsset: " + e.getMessage());
            return null;
        }
    }

    public ArrayList<Question> generateInsertQueryForJsonObjects() {
        ArrayList<Question> questions = new ArrayList<>();
        JSONObject jsonObject = loadJSONFromAsset();
        try {
            Iterator<String> iter = jsonObject.keys();
            while (iter.hasNext()) {
                String key = iter.next();
                JSONObject value = jsonObject.getJSONObject(key);
                Question question = Question.fromJson(value.getJSONObject("question_two"));
                questions.add(question);
                Log.d(TAG, "generateInsertQueryForJsonObjects: " + question.getAnswer_a());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return questions;
    }
}

And in my MainActivity-> onCreate:

在我的MainActivity-> 中onCreate：

  JsonSqlQueryMapper mapper = new JsonSqlQueryMapper(MainActivity.this);
  mapper.generateInsertQueryForJsonObjects();

To check that everything working as I want. Here is the json file if you want to check https://github.com/Blasanka/android_quiz_app/blob/sqlite_db_app/app/src/main/assets/quiz_app.json

检查一切是否如我所愿。如果你想检查https://github.com/Blasanka/android_quiz_app/blob/sqlite_db_app/app/src/main/assets/quiz_app.json，这里是 json 文件

Regards!

问候！