How to Write JSON into File Using GSON in Java

In this example, we use the Gson tree model API to write Java objects into JSON file.

Check out complete google GSON tutorial at https://www.javaguides.net/p/google-gson-tutorial.html.

Write JSON into a File Using GSON in Java

In the following example, we use the Gson tree model API to write Java objects into JSON.

package net.guides;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.Writer;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonElement;

class Student {

    private long studentId;
    private String studentName;

    public Student(long studentId, String studentName) {
        super();
        this.studentId = studentId;
        this.studentName = studentName;
    }

    public long getStudentId() {
        return studentId;
    }

    public void setStudentId(long studentId) {
        this.studentId = studentId;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }
}

public class GsonTreeModelWrite {

    public static void main(String[] args) throws FileNotFoundException, IOException {

        List < Student > students = new ArrayList < > ();

        Student student1 = new Student(100, "student1");
        Student student2 = new Student(200, "student2");
        Student student3 = new Student(300, "student3");
        Student student4 = new Student(400, "student4");
        Student student5 = new Student(500, "student5");

        students.add(student1);
        students.add(student2);
        students.add(student3);
        students.add(student4);
        students.add(student5);

        String fileName = "src/main/resources/students.json";

        Path path = Paths.get(fileName);

        try (Writer writer = Files.newBufferedWriter(path, StandardCharsets.UTF_8)) {

            Gson gson = new GsonBuilder().setPrettyPrinting().create();

            JsonElement tree = gson.toJsonTree(students);
            gson.toJson(tree, writer);
        }

        System.out.println("Students written to file");
    }
}

Here is a JSON wrote into a file - src/main/resources/students.json:

[
  {
    "studentId": 100,
    "studentName": "student1"
  },
  {
    "studentId": 200,
    "studentName": "student2"
  },
  {
    "studentId": 300,
    "studentName": "student3"
  },
  {
    "studentId": 400,
    "studentName": "student4"
  },
  {
    "studentId": 500,
    "studentName": "student5"
  }
]

A list of student objects is serialized into JSON format.

JsonElement tree = gson.toJsonTree(students);

The toJsonTree method serializes the specified object into its equivalent representation as a tree of JsonElements.

gson.toJson(tree, writer);

Finally, we write the tree object into the file.

Check out complete google GSON tutorial at https://www.javaguides.net/p/google-gson-tutorial.html.