GSON example – Read and write JSON

Table of Contents

Java JSON Tutorial Content:

In this post,we will see how can we read and write JSON using GSON.

Java JSON Tutorial Content:

reading and writing JSON using json-simple.We will use another way(i.e. GSON) of reading JSON.
Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.
There are various goals or advantages of using GSON(taken from GSON offcial page)

Provide simple toJson() and fromJson() methods to convert Java objects to JSON and vice-versa
Allow pre-existing unmodifiable objects to be converted to and from JSON
Extensive support of Java Generics
Allow custom representations for objects
Support arbitrarily complex objects (with deep inheritance hierarchies and extensive use of generic types)

Reading and writing JSON using GSON is very easy.You can use these two methods:
toJSON() : It will convert simple pojo object to JSON string.
FromJSON() : It will convert JSON string to pojo object.
In this post,we will read and write JSON using GSON.
Download gson-2.2.4 jar from here.
Create a java project named “GSONExample”
Create a folder jars and paste downloaded jar gson-2.2.4.jar.
right click on project->Property->java build path->Add jars and then go to src->jars->gson-2.2.4.jar.

Click on ok.
Then you will see json-simple-1.1 jar added to your libraries and then click on ok

First create a pojo named “Country.java”


package org.arpit.java2blog.pojo;

import java.util.ArrayList;
import java.util.List;

public class Country {

    String name;
    int population;
    private List listOfStates;

    //getter and setter methods

    public String getName() {
        return name;
    }

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

    public int getPopulation() {
        return population;
    }

    public void setPopulation(int population) {
        this.population = population;
    }

    public List getListOfStates() {
        return listOfStates;
    }

    public void setListOfStates(List listOfStates) {
        this.listOfStates = listOfStates;
    }

}

package org.arpit.java2blog.pojo;

import java.util.ArrayList;

import java.util.List;

public class Country {

String name;

int population;

private List listOfStates;

//getter and setter methods

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getPopulation() {

return population;

}

public void setPopulation(int population) {

this.population = population;

}

public List getListOfStates() {

return listOfStates;

}

public void setListOfStates(List listOfStates) {

this.listOfStates = listOfStates;

}

Write JSON to file:

Create a new class named “GSONWritingToFileExample.java” in src->org.arpit.java2blog


package org.arpit.java2blog;

import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.arpit.java2blog.pojo.Country;
import com.google.gson.Gson;

/*
 * @Author : Arpit Mandliya
 */
public class GSONWritingToFileExample {

 public static void main(String[] args) {

  Country countryObj=new Country();
  countryObj.setName("India");
  countryObj.setPopulation(1000000);
  List listOfStates=new ArrayList();
  listOfStates.add("Madhya Pradesh");
  listOfStates.add("Maharastra");
  listOfStates.add("Rajasthan");

  countryObj.setListOfStates(listOfStates);
  Gson gson = new Gson();

  // convert java object to JSON format,
  // and returned as JSON formatted string
  String json = gson.toJson(countryObj);

  try {
   //write converted json data to a file named "CountryGSON.json"
   FileWriter writer = new FileWriter("E:CountryGSON.json");
   writer.write(json);
   writer.close();

  } catch (IOException e) {
   e.printStackTrace();
  }

  System.out.println(json);

     }
}

package org.arpit.java2blog;

import java.io.FileWriter;

import java.io.IOException;

import java.util.ArrayList;

import java.util.List;

import org.arpit.java2blog.pojo.Country;

import com.google.gson.Gson;

* @Author : Arpit Mandliya

public class GSONWritingToFileExample {

public static void main(String[] args) {

Country countryObj=new Country();

countryObj.setName("India");

countryObj.setPopulation(1000000);

List listOfStates=new ArrayList();

listOfStates.add("Madhya Pradesh");

listOfStates.add("Maharastra");

listOfStates.add("Rajasthan");

countryObj.setListOfStates(listOfStates);

Gson gson = new Gson();

// convert java object to JSON format,

// and returned as JSON formatted string

String json = gson.toJson(countryObj);

try {

//write converted json data to a file named "CountryGSON.json"

FileWriter writer = new FileWriter("E:CountryGSON.json");

writer.write(json);

writer.close();

} catch (IOException e) {

e.printStackTrace();

}

System.out.println(json);

}

Run above program and content of file CountryGSON.json will be as below :


{"name":"India","population":1000000,"listOfStates":["Madhya Pradesh","Maharastra","Rajasthan"]}

{"name":"India","population":1000000,"listOfStates":["Madhya Pradesh","Maharastra","Rajasthan"]}

Read JSON to file:

Here we will read above created JSON file.
Create a new class named “JSONSimpleReadingFromFileExample.java” in src->org.arpit.java2blog


package org.arpit.java2blog;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.List;
import org.arpit.java2blog.pojo.Country;
import com.google.gson.Gson;

/*
 * @Author : Arpit Mandliya
 */
public class GSONReadingFromFileExample {

 public static void main(String[] args) {

  Gson gson = new Gson();

  try {

   System.out.println("Reading JSON from a file");
   System.out.println("----------------------------");

   BufferedReader br = new BufferedReader(
     new FileReader("E:file.json"));

    //convert the json string back to object
   Country countryObj = gson.fromJson(br, Country.class);

   System.out.println("Name Of Country: "+countryObj.getName());

   System.out.println("Population: "+countryObj.getPopulation());

   System.out.println("States are :");

   List listOfStates = countryObj.getListOfStates();
   for (int i = 0; i < listOfStates.size(); i++) {
    System.out.println(listOfStates.get(i));
   }

  } catch (IOException e) {
   e.printStackTrace();
  }
 }
}

package org.arpit.java2blog;

import java.io.BufferedReader;

import java.io.FileReader;

import java.io.IOException;

import java.util.List;

import org.arpit.java2blog.pojo.Country;

import com.google.gson.Gson;

* @Author : Arpit Mandliya

public class GSONReadingFromFileExample {

public static void main(String[] args) {

Gson gson = new Gson();

try {

System.out.println("Reading JSON from a file");

System.out.println("----------------------------");

BufferedReader br = new BufferedReader(

new FileReader("E:file.json"));

//convert the json string back to object

Country countryObj = gson.fromJson(br, Country.class);

System.out.println("Name Of Country: "+countryObj.getName());

System.out.println("Population: "+countryObj.getPopulation());

System.out.println("States are :");

List listOfStates = countryObj.getListOfStates();

for (int i = 0; i < listOfStates.size(); i++) {

System.out.println(listOfStates.get(i));

}

} catch (IOException e) {

e.printStackTrace();

}

Run above program and you will get following output:


Reading JSON from a file
----------------------------
Name Of Country: India
Population: 1000000
States are :
Madhya Pradesh
Maharastra
Rajasthan

Reading JSON from a file

----------------------------

Name Of Country: India

Population: 1000000

States are :

Madhya Pradesh

Maharastra

Rajasthan

Project Structure:

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Comments

Raghav says:

November 28, 2014 at 4:54 am

Thanks a lot for this POST.
This was very informative and useful.
Thanks a lot.

Kedar says:

January 20, 2015 at 5:26 am

nice article !!
Really Helpful

Jason says:

February 11, 2015 at 12:22 pm

could you please help me that how could i ad contents to an existing json file using gson

Muhammad Ali says:

March 7, 2015 at 12:30 pm

fantastic thank u very much keep it up

ankita says:

March 11, 2015 at 3:15 pm

really good one..

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April 23, 2015 at 2:58 pm

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John says:

September 24, 2015 at 11:57 pm

Is there an example w/simple json where you update records within a json object.

For example w/structure below how to add a key/value pair to each element?
{
“data”: [
{
“Atomic Symbol”: “H”,
“Atomic Number”: “1”,
},
{
“Atomic Symbol”: “He”,
“Atomic Number”: “2”,
}

alchemist.gamma says:

August 5, 2017 at 11:39 am

One of the simplest and best examples I have come across on the internet for GSON.. Great job!!