How to Write JSON into File Using GSON in Java

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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.