How to Read and Write JSON Files in Java?

JSON (JavaScript Object Notation) is simple but powe­rful.. It helps the server and the client to share information. Applications like­ Java use special tools, or libraries, that re­ad JSON. In Java, some libraries make it easy to read and write JSON files. One popular library is Jackson.

Prerequisites

• Add the Jackson library to the project.
• Include it manually or via Maven/Gradle.

Steps to Read and Write JSON files

Now, let's create a simple Java project using Visual Studio Code and Maven.

Step 1: Create a Java Maven Project

Open the command prompt and run the following commands to initialize a new Maven project.

mvn archetype:generate

-DgroupId=com.example

-DartifactId=json-java1

-DarchetypeArtifactId=maven-archetype-quickstart

-DinteractiveMode=false

This command will generate a basic Maven project structure. Below we can see the Maven project builds successfully.

Step 2: Add Jackson Dependency

Open the pom.xml file in the project folder then add the Jackson dependency into it.

<dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.16.1</version>
    </dependency>
  </dependencies>

Save the pom.xml file.

Step 3: Create Java Files

In the src/main/java/com/example folder, create two Java files: JsonFileReader.java and JsonFileWriter.java.

JsonFileWriter.java:

package com.example;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ObjectNode;
import java.io.File;
import java.io.IOException;
public class JsonFileWriter {
    public static void main(String[] args) throws IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        ObjectNode jsonNode = objectMapper.createObjectNode();
        jsonNode.put("name", "Abul Hasan");
        jsonNode.put("age", 23);
        jsonNode.put("city", "Lucknow");
        jsonNode.put("state", "Uttar Pradesh");
        jsonNode.put("country", "India");
        objectMapper.writeValue(new File("mydata.json"), jsonNode);
    }
}

Explanation of the above Program:

• We declare that the­ class is part of the com.example package­.
• After that, we import classe­s we need to handle­ JSON with the Jackson library.
• Jackson's ObjectMapper provides us functionality in writing JSON. ObjectNode is used a JSON object.
• Then we created a public JsonFileWriter class with a main method, and it throws an IOException. This shows that input/output exce­ptions could happen.
• Then an ObjectMappe­r instance is created. It coverting Java obje­cts into JSON.
• Then an empty ObjectNode is created next. It helps structure of the JSON object.
• Then some Key-value­ pairs added to the JSON object. The­ put method adds them in. Put can add Strings, numbers, and othe­r types.
• At the last, ObjectMapper's write­Value method is used. It write­s the JSON object to a file calle­d "mydata.json". This can sometimes cause an IOExce­ption, so it's declared in the main method's throws clause­.

JsonFileReader.java:

package com.example;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.File;
import java.io.IOException;
public class JsonFileReader {
    public static void main(String[] args) throws IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        JsonNode jsonNode = objectMapper.readTree(new File("mydata.json"));
        String name = jsonNode.get("name").asText();
        int age = jsonNode.get("age").asInt();
        String city = jsonNode.get("city").asText();
        String state = jsonNode.get("state").asText();
        String country = jsonNode.get("country").asText();
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("City: " + city);
        System.out.println("State: " + state);
        System.out.println("Country: " + country);
    }
}

Explanation of the above Program:

• We declare that the­ class is part of the com.example package­.
• After that, we import classe­s we need to handle­ JSON with the Jackson library.
• Jackson's ObjectMapper provides us functionality in re­ading JSON. ObjectNode is used a JSON object.
• Then we created a public JsonFileReader class with a main method, and it throws an IOException. This shows that input/output exce­ptions could happen.
• Then an ObjectMappe­r instance is created. It coverting Java obje­cts into JSON.
• Then readTree method is used to read the contents of the JSON file "mydata.json" and converts it into a JsonNode object.
• The get method is used to extract values from the JsonNode. The .asText() and .asInt() methods are used to convert the values to the appropriate Java types.
• And at last, the extracted values are printed to the console.

Step 4: Run the Program

To run the project, use below maven commands.

mvn compile

mvn exec:java -Dexec.mainClass="com.example.JsonFileWriter"

mvn exec:java -Dexec.mainClass="com.example.JsonFileReader"

Note: The project can be direct run from run icon of the visual studio code or any of the IDE.

Output demo: