How to convert Java object to / from JSON (Gson)

JSON is stand for JavaScript Object Notation, it is a lightweight data-interchange format. You can see many Java applications started to throw away XML format and start using json as a new s data-interchange format. Java is all about object, often times, you need to convert an object into json format for data-interchange or vice verse.

In this article, we show you how to use Gson, JSON library, to convert object to/from json.

Gson is easy to learn and implement, what we need to know are following two methods

1. toJson() – Convert Java object to JSON format
2. fromJson() – Convert JSON into Java object

1. Gson Dependency

For non-Maven user, get the Gson library from Gson official site, for Maven user, declares following dependency in your pom.xml.

<dependency><groupId>com.google.code.gson</groupId><artifactId>gson</artifactId><version>1.7.1</version>  </dependency>

2. POJO

A pojo, with initialized values. Later use Gson to convert this object to/from JSON formatted string.

package com.mkyong.core; import java.util.ArrayList;import java.util.List; public class DataObject { private int data1 = 100;private String data2 = "hello";private List<String> list = new ArrayList<String>() {  {add("String 1");add("String 2");add("String 3");  }}; //getter and setter methods @Overridepublic String toString() {   return "DataObject [data1=" + data1 + ", data2=" + data2 + ", list="+ list + "]";} }

3. toJson() example

Convert object to JSON string, and save it as “file.json“.

package com.mkyong.core; import java.io.FileWriter;import java.io.IOException;import com.google.gson.Gson; public class GsonExample {    public static void main(String[] args) { DataObject obj = new DataObject();Gson gson = new Gson(); // convert java object to JSON format,// and returned as JSON formatted stringString json = gson.toJson(obj); try {//write converted json data to a file named "file.json"FileWriter writer = new FileWriter("c:\\file.json");writer.write(json);writer.close(); } catch (IOException e) {e.printStackTrace();} System.out.println(json);     }}

Output

{"data1":100,"data2":"hello","list":["String 1","String 2","String 3"]}

4. fromJson() example

Read data from “file.json“, convert back to object and display it.

package com.mkyong.core; import java.io.BufferedReader;import java.io.FileReader;import java.io.IOException;import com.google.gson.Gson; public class GsonExample {    public static void main(String[] args) { Gson gson = new Gson(); try { BufferedReader br = new BufferedReader(new FileReader("c:\\file.json")); //convert the json string back to objectDataObject obj = gson.fromJson(br, DataObject.class); System.out.println(obj); } catch (IOException e) {e.printStackTrace();}     }}

Output

DataObject [data1=100, data2=hello, list=[String 1, String 2, String 3]]