☕ JSON with Java – Encode, Decode, and Work with JSON (2025 Guide)

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🧲 Introduction – Why Use JSON with Java?

JSON (JavaScript Object Notation) is a lightweight, human-readable format commonly used for data exchange between servers and clients. Java provides multiple libraries to work with JSON, allowing developers to parse, generate, and manipulate JSON data easily within REST APIs, desktop apps, or enterprise systems.

🎯 In this guide, you’ll learn:

• The most popular JSON libraries in Java
• How to encode and decode JSON with real examples
• Using JSON in RESTful APIs
• Best practices and tips for error-free data handling

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📚 Popular JSON Libraries in Java

Library Name	Description
org.json	Simple and minimalistic native implementation
Jackson	Powerful, fast, supports POJO mapping
Gson (by Google)	Lightweight, easy-to-use, great for Android
JSON-B	Standardized API (Jakarta EE)

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✅ Working with JSON using org.json

🧪 Encode – Create JSON Object from Java Map

import org.json.JSONObject;
import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    HashMap<String, Object> user = new HashMap<>();
    user.put("name", "Alice");
    user.put("age", 30);

    JSONObject json = new JSONObject(user);
    System.out.println(json.toString());
  }
}

🔍 Output:

{"name":"Alice","age":30}

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🧪 Decode – Parse JSON String to Java Object

import org.json.JSONObject;

public class Main {
  public static void main(String[] args) {
    String jsonString = "{\"name\":\"Bob\", \"age\":25}";
    JSONObject obj = new JSONObject(jsonString);

    String name = obj.getString("name");
    int age = obj.getInt("age");

    System.out.println("Name: " + name + ", Age: " + age);
  }
}

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🌳 Using JSON with Jackson (POJO Mapping)

Jackson is ideal for converting between Java objects and JSON.

🧪 Encode – Object to JSON String

import com.fasterxml.jackson.databind.ObjectMapper;

class User {
  public String name;
  public int age;

  User(String name, int age) {
    this.name = name;
    this.age = age;
  }
}

public class Main {
  public static void main(String[] args) throws Exception {
    User user = new User("Carol", 22);
    ObjectMapper mapper = new ObjectMapper();
    String json = mapper.writeValueAsString(user);
    System.out.println(json);
  }
}

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🧪 Decode – JSON String to Object

String jsonStr = "{\"name\":\"Dan\",\"age\":40}";
User user = mapper.readValue(jsonStr, User.class);
System.out.println(user.name + " is " + user.age + " years old.");

✔️ Great for working with REST APIs and data models
✔️ Automatically maps fields between JSON and Java objects

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📡 JSON in Java REST API (Spring Boot Example)

@RestController
public class UserController {

  @PostMapping("/api/user")
  public String receiveJson(@RequestBody User user) {
    return "Received user: " + user.name;
  }

  @GetMapping("/api/user")
  public User sendJson() {
    return new User("Eva", 28);
  }
}

✔️ Spring Boot automatically handles JSON serialization/deserialization
✔️ Uses Jackson under the hood

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🛠️ Best Practices for JSON in Java

Best Practice	Benefit
Use @JsonProperty for custom names	Handles snake_case → camelCase conversions
Always handle exceptions (try/catch)	Prevents crashes from malformed JSON
Validate JSON input	Ensures security and correct structure
Use POJOs for complex structures	Improves maintainability and readability

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📌 Summary – Recap & Next Steps

JSON is essential in modern Java development, especially for building APIs, mobile apps, and data services. Libraries like Jackson, Gson, and org.json make handling JSON easy and flexible.

🔍 Key Takeaways:

• Use org.json for quick JSON parsing and generation
• Use Jackson or Gson for working with POJOs and APIs
• Spring Boot auto-handles JSON in REST controllers
• Validate and format JSON for clean data communication

⚙️ Real-world use:

Used in REST APIs, Spring Boot apps, Android apps, enterprise systems, and web services.

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❓ FAQ – JSON with Java

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❓ Which Java library is best for JSON?
✅ Jackson is the most powerful and commonly used for enterprise apps. Gson is great for lightweight or Android projects.

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❓ Can I convert a JSON string to a Java object?
✅ Yes. Use Jackson’s ObjectMapper.readValue() or Gson’s fromJson().

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❓ How do I return JSON from a Java REST API?
✅ Use Spring Boot’s @RestController with @RequestBody and @ResponseBody.

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❓ Do I need to install org.json separately?
✅ In some cases, yes. It may need to be added as a dependency if not bundled with your environment.

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