?? JSON Handling in Java with Jackson

Here we will help you to understand JSON processing in Java, including parsing, serialization, deserialization, and converting data between formats (e.g. JSON ? XML). The focus is on Real World use cases with Jackson, one of the most widely used JSON libraries in Java.

?? 1. What is JSON and Why is it Used?

? JSON (JavaScript Object Notation) is a lightweight data format used for data exchange between systems.

? It is human-readable, easy to parse, and widely used in REST APIs.

? Java Application often consume or produce JSON when interacting with APIs, databases or message queues.

?? Example of JSON Data:

{
 "id": 1000,
  "name": "Marcelo Santos.",
  "email": "[email protected]",
  "roles": ["USER", "ADMIN"]
}        

??? 2. Working with JSON in Java

? How to convert JSON to Java Objects (Deserialization)?

?? Example: Converting JSON to Java Object using Jackson

import com.fasterxml.jackson.databind.ObjectMapper;
class User {
    public int id;
    public String name;
    public String email;
    public String[] roles;
}
public class JsonToJava {
    public static void main(String[] args) throws Exception {
        String json = "{\"id\":1000, \"name\":\"Marcelo Santos\", \"email\":\"[email protected]\", \"roles\":[\"USER\", \"ADMIN\"]}";
        ObjectMapper objectMapper = new ObjectMapper();
        User user = objectMapper.readValue(json, User.class);
        System.out.println("User Name: " + user.name);
    }
}        

? Output:

User Name: Marcelo Santos        

?? Use Case: Converting API responses to Java objects for further processing.

? How to Convert Java Objects to JSON (Serialization)

?? Example: Converting Java Object to JSON using Jackson

import com.fasterxml.jackson.databind.ObjectMapper;
class Product {
    public int id;
    public String name;
    public double price;
    public Product(int id, String name, double price) {
        this.id = id;
        this.name = name;
        this.price = price;
    }
}
public class JavaToJson {
    public static void main(String[] args) throws Exception {
        Product product = new Product(1, "Laptop", 2500.99);
        ObjectMapper objectMapper = new ObjectMapper();
        String jsonString = objectMapper.writeValueAsString(product);
        System.out.println(jsonString);
    }
}        

? Output:

{"id":1,"name":"Laptop","price": 2500.99}        

?? Use Case: Sending JSON responses in REST APIs.

?? 3. Customizing JSON with Jackson

? Ignoring Fields in JSON Serialization

?? Example: Using @JsonIgnore to Hide Fields

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.databind.ObjectMapper;
class Employee {
    public String name;
    
    @JsonIgnore
    public String ssn;  // This field will not be serialized
    public Employee(String name, String ssn) {
        this.name = name;
        this.ssn = ssn;
    }
}
public class IgnoreFieldExample {
    public static void main(String[] args) throws Exception {
        Employee employee = new Employee("Marcelo Santos", "123-456-789");
        ObjectMapper objectMapper = new ObjectMapper();
        String jsonString = objectMapper.writeValueAsString(employee);
        System.out.println(jsonString);
    }
}        

? Output:

{"name":"Marcelo Santos"}        

?? Use Case: Hiding sensitive fields (e.g., passwords, SSNs) in API responses.

? Changing JSON Property Names

?? Example: Using @JsonProperty to Rename Fields

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;
class Customer {
    @JsonProperty("full_name")
    public String name;
    public Customer(String name) {
        this.name = name;
    }
}
public class RenameJsonField {
    public static void main(String[] args) throws Exception {
        Customer customer = new Customer("Marcelo Santos");
        ObjectMapper objectMapper = new ObjectMapper();
        String jsonString = objectMapper.writeValueAsString(customer);
        System.out.println(jsonString);
    }
}        

? Output:

{"full_name":"Marcelo Santos"}        

?? Use Case: Adapting Java fields to match API contract requirements.

?? 4. Converting JSON ? XML in Java

?? Example: Using Jackson for JSON → XML Conversion

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
public class JsonToXml {
    public static void main(String[] args) throws Exception {
        String json = "{\"id\":1000, \"name\":\"Marcelo Santos", \"email\":\"[email protected]\"}";
        ObjectMapper jsonMapper = new ObjectMapper();
        XmlMapper xmlMapper = new XmlMapper();
        Object obj = jsonMapper.readValue(json, Object.class);
        String xml = xmlMapper.writeValueAsString(obj);
        System.out.println(xml);
    }
}        

? Output:

<Object>
  <id>1000</id>
  <name>Marcelo Santos</name>
  <email>[email protected]</email>
</Object>        

?? Use Case: Converting JSON responses into XML format for legacy systems.

?? Example: Convert XML → JSON

public class XmlToJson {
    public static void main(String[] args) throws Exception {
        String xml = "<Object><id>1000</id><name>Marcelo Santos</name><email>[email protected]</email></Object>";
        XmlMapper xmlMapper = new XmlMapper();
        ObjectMapper jsonMapper = new ObjectMapper();
        Object obj = xmlMapper.readValue(xml, Object.class);
        String json = jsonMapper.writeValueAsString(obj);
        System.out.println(json);
    }
}        

? Output:

{"id":1000,"name":"Marcelo Santos","email":"[email protected]"}        

?? Use Case: Integrating modern JSON-based APIs with legacy XML-based systems.

?? 5. Jackson Core use

? ObjectMapper, used for converting Java objects to/from JSON, providing powerful features to serialize Java objects into JSON and deserialize JSON into Java Objects. Remember, JSON to/from Java, use ObjectMapper.

JSON to Java is Deserialization

Java to JSON is Serialization

? XmlMapper (Extends ObjectMapper), is responsible for handling XML serialization and deserialization, working similarly to ObjectMapper, but instead JSON it process XML.

??Key Methods in ObjectMapper and XmlMapper

? readValue(String content, Class<T> valueType), converts JSON (or XML in XMLMapper) into a JavaObject, used to deserialization

? writeValue(File resultFile, Object value), used to write Java Object as JSON or XML into a file.

? writeValueAsString(Object value), used to converts Java Objects into JSON or XML Strings

??Most Commonly Used Jackson Annotations

1?? @JsonProperty, renames a field in JSON

2?? @JsonIgnore, excludes a field from serialization

3?? @JsonInclude(JsonInclude.Include.NON_NULL), excludes null values from JSON output

4?? @JsonFormat(pattern = "yyyy-MM-dd"), format dates in JSON

5?? @JsonCreator, allows custom constructor JSON deserialization

6?? @JsonAnySetter, captures extra fields to mapped to properties

7?? @JsonIgnoreProperties(ignoreUnknown = true), ignores unknown JSON fields when deserializing

8?? @JsonUnwrapped, flattens nested objects in JSON

9?? @JsonAlias, supports multiple JSON field names

?? @JsonSetter, specifies a custom setter method for deserialization

?? 6. Real-World Use Cases for JSON Handling

? 1. REST API Communication, JSON is the most common data format for REST APIs

? 2. Data Storage, NoSQL Databases (MongoDB, Firebase) uses JSON for storing documents.

? 3. Configurations, Many applications store configurations in JSON Files (e.g. config.json)

? 4. Message Queues, JSON is widely used in Kafka, RabbitMQ for message-based communication

? 5. Logging & Monitoring, Many logging frameworks (e.g., Logstash, ELK stack) use JSON-formatted logs.

?? 7. Key Takeaways

? ?Jackson is the most popular library for JSON processing in Java

? ?ObjectMapper is the core class for converting JSON ? Java Objects

? Annotations like @JsonIgnore, @JsonProperty help control serialization behavior

? You can convert JSON ? XML easily using XMLMapper

? Mastering JSON handling is essential for working with APIs, databases and messaging systems.