How to Read a JSON file Using GSON in Java

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

Read a JSON file Using GSON in Java

Let's read below students.json file which contains an array of student objects.

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"
  }
]

Now, let's read above students.json file into Java objects and print to console:

package net.guides;

import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.Reader;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class GsonTreeModelRead {

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

        String fileName = "src/main/resources/students.json";
        Path path = Paths.get(fileName);

        try (Reader reader = Files.newBufferedReader(path,
            StandardCharsets.UTF_8)) {

            JsonParser parser = new JsonParser();
            JsonElement tree = parser.parse(reader);

            JsonArray array = tree.getAsJsonArray();

            for (JsonElement element: array) {

                if (element.isJsonObject()) {

                    JsonObject car = element.getAsJsonObject();

                    System.out.println("********************");
                    System.out.println(car.get("studentId").getAsLong());
                    System.out.println(car.get("studentName").getAsString());
                }
            }
        }
    }
}

Output:

********************
100
student1
********************
200
student2
********************
300
student3
********************
400
student4
********************
500
student5

In the example, we read JSON data from a file into a tree of JsonElements.

JsonParser parser = new JsonParser();
JsonElement tree = parser.parse(reader);

JsonParser parses JSON into a tree structure of JsonElements.

JsonArray array = tree.getAsJsonArray();

We get the tree as JsonArray.

 for (JsonElement element: array) {

                if (element.isJsonObject()) {

                    JsonObject car = element.getAsJsonObject();

                    System.out.println("********************");
                    System.out.println(car.get("studentId").getAsLong());
                    System.out.println(car.get("studentName").getAsString());
                }
}

We go through the JsonArray and print the contents of its elements.