How to Read and Write JSON using GSON

GSON is an open source JSON parser developed by Google to serialize and deserialize JSON in Java. In my previous article, we looked at reading and writing JSON to a file using JSON.simple. In this article, we will learn how to use GSON to Read and Write JSON in Java  (parse JSON file to Java objects and vice-versa Java object to JSON)

Read and Write JSON – GSON

Dependency

As a pre-requisite, you are required to download the gson2.7.jar (or) if you are running on maven add the below dependency to your pom.xml

<dependency>
  <groupId>com.google.code.gson</groupId>
  <artifactId>gson</artifactId>
  <version>2.7</version>
</dependency>

JSON file content(employee.json)

{
    "EmployeeName": "www.javainterviewpoint.com",
    "EmployeeId": 999,
    "Department": [
        "Engineering",
        "Automobile",
        "Manufacturing",
        "Quality Assurance"
    ]
}

Employee.java

A simple POJO for holding the Employee details.

package com.javainterviewpoint;

import java.util.List;

public class Employee
{
    private String EmployeeName;
    private int EmployeeId;
    private List<String> Department;
    
    public Employee()
    {
        super();
    }
    public Employee(String employeeName, int employeeId, List department)
    {
        super();
        EmployeeName = employeeName;
        EmployeeId = employeeId;
        Department = department;
    }

    public String getEmployeeName()
    {
        return EmployeeName;
    }
    public void setEmployeeName(String employeeName)
    {
        EmployeeName = employeeName;
    }
    public int getEmployeeId()
    {
        return EmployeeId;
    }
    public void setEmployeeId(int employeeId)
    {
        EmployeeId = employeeId;
    }
    public List<String> getDepartment()
    {
        return Department;
    }
    public void setDepartment(List<String> department)
    {
        Department = department;
    }
}

JSON Reader

package com.javainterviewpoint;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

import com.google.gson.Gson;

public class JSON_Reader_GSON
{
    public static void main(String[] args)
    {
        try
        {
            //Create a new Gson object
            Gson gson = new Gson();

            //Read the employee.json file
            BufferedReader br = new BufferedReader(  
                    new FileReader("D:\\employee.json"));
            
            //convert the json to  Java object (Employee)
            Employee employee = gson.fromJson(br, Employee.class);

            //Printing the Employee Details
            System.out.println("****Employee Details****");
            System.out.println("Employee Name    : "+employee.getEmployeeName());
            System.out.println("Employee ID      : "+employee.getEmployeeId());
            System.out.print("Employee Department: ");
            for(String department : employee.getDepartment())
            {
                System.out.print(department+" | ");
            }
        }
        catch (IOException e) 
        {  
            e.printStackTrace();  
        }  
    }
}

We will be performing the below steps to parse a JSON File in Java

  • Create a new Gson object
Gson gson = new Gson();
  • Read the employee.json using BufferedReader
BufferedReader br = new BufferedReader(  
                    new FileReader("D:\\employee.json"));
  • Using the fromJson() method of the gson object, parse the JSON to Java Object (Employee.java) and it automatically maps the values to the corresponding property.
Employee employee = gson.fromJson(br, Employee.class);
  • Finally, print the Employee details
System.out.println("****Employee Details****");
System.out.println("Employee Name    : "+employee.getEmployeeName());
System.out.println("Employee ID      : "+employee.getEmployeeId());
System.out.print("Employee Department: ");
for(String department : employee.getDepartment())
{
    System.out.print(department+" | ");
}

Output :

Read and Write JSON

JSON Writer

package com.javainterviewpoint;

import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import com.google.gson.Gson;

public class JSON_Writer_GSON
{
    public static void main(String[] args)
    {
        try
        {
            //Create a new Employee object
            Employee employee = new Employee();
            //Set values to its properties
            employee.setEmployeeName("JavaInterviewPoint");
            employee.setEmployeeId(100);

            List departmentList = new ArrayList();
            departmentList.add("Chemical");
            departmentList.add("Mechanical");
            departmentList.add("Automobile");
            departmentList.add("Computer");

            employee.setDepartment(departmentList);

            // Create a new Gson object
            Gson gson = new Gson();

            //convert the Java object to json
            String jsonString = gson.toJson(employee);
            //Write JSON String to file        
            FileWriter fileWriter = new FileWriter("D:\\employee.json");
            fileWriter.write(jsonString);
            fileWriter.close();
            
        } catch (IOException e)
        {
            e.printStackTrace();
        }
    }
}
  • Create a new Employee object
Employee employee = new Employee();
  • Set values to the properties of the Employee class.
       employee.setEmployeeName("JavaInterviewPoint");
       employee.setEmployeeId(100);
       
       List departmentList = new ArrayList();
       departmentList.add("Chemical");
       departmentList.add("Mechanical");
       departmentList.add("Automobile");
       departmentList.add("Computer");
       
       employee.setDepartment(departmentList);
  • Convert Java Object to JSON, using the toJson() method of Gson.
String jsonString = gson.toJson(employee);
  • Finally, using the FileWriter write the json string to the employee.json

Output :

employee.json will have the below content

{
	"EmployeeName":"JavaInterviewPoint",
	"EmployeeId":100,
	"Department":
		["Chemical","Mechanical","Automobile","Computer"]
}
Note : You should be having the properties of the Java class exactly the same name as the JSON Key,if not the value will not be automatically mapped. Say for Example if i have JSON has the key as “EmployeeId” and the Java class has the property as “Employeeid”, then while JSON parsing this fields will not be automatically mapped.

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