How to convert Java objects to JSON?

To convert a Java object or POJO  (Plain Old Java Object) to JSON, we can use JSONObjectone of the constructors that takes an object as a parameter. In the example below, we convert the StudentPOJO to a JSON string. StudentThe class must provide getter methods, JSONObjectby calling these methods to create the JSON string.

In this code snippet, we do the following:

• Use setter methods to create Studentobjects and set their properties.
• Create a JSONObjectcall objectand use Studentthe object as an argument to its constructor.
• JSONObjectUse the getter method to generate the JSON string.
• Call object.toString()the method to get the JSON string.

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.google.gson.Gson;
import org.json.JSONObject;

import java.util.Arrays;

public class PojoToJSON {

    public static void main(String[] args) throws JsonProcessingException {
        Student student = new Student();
        student.setId(1L);
        student.setName("Alice");
        student.setAge(20);
        student.setCourses(Arrays.asList("Engineering", "Finance", "Chemistry"));

        JSONObject object = new JSONObject(student);
        String json = object.toString();
        System.out.println(json);
        System.out.println(new Gson().toJson(student));
        // Creating Object of ObjectMapper define in Jackson API
        ObjectMapper Obj = new ObjectMapper();

        // Converting the Java object into a JSON string
        String jsonStr = Obj.writeValueAsString(student);
        // Displaying Java object into a JSON string
        System.out.println(jsonStr);

    }
}

Running this code produces the following results:

{"courses":["Engineering","Finance","Chemistry"],"name":"Alice","id":1,"age":20}
{"id":1,"name":"Alice","age":20,"courses":["Engineering","Finance","Chemistry"]}
{"id":1,"name":"Alice","age":20,"courses":["Engineering","Finance","Chemistry"]}

The class used in the code above Student:

import java.util.List;

public class Student {

    private Long id;
    private String name;
    private int age;
    private List<String> courses;

    public Student(Long id, String name, int age, List<String> courses) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.courses = courses;
    }

    Student() {

    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public List<String> getCourses() {
        return courses;
    }

    public void setCourses(List<String> courses) {
        this.courses = courses;
    }

}

Maven dependencies

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example.javaobjectjson</groupId>
    <artifactId>JavaObjectJSON</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>

        <!-- https://mvnrepository.com/artifact/org.json/json -->
        <dependency>
            <groupId>org.json</groupId>
            <artifactId>json</artifactId>
            <version>20211205</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/com.google.code.gson/gson -->
        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
            <version>2.9.0</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.12.1</version>
        </dependency>  

    </dependencies>
</project>