Importance of @JsonRootName annotation using Jackson in Java?

In Java, Jackson is a library that is used to convert JSON to Java objects and vice versa. Jackson Annotations are used during serialization and deserialization. We use annotations with a particular field or method that is declared in Java. 

So, Annotations make the JSON output clearer as we required. In this Article, we will learn about one of its annotations @JsonRootName.

@JsonRootName Annotatation

The @JsonRootName annotation is a Jackson annotation that is used on a class if we want to do wrapping in our JSON attributes. It is a class-level annotation.

It wraps the object to be serialized with a top-level element. To make use of this annotation, you need to specify the name to be used as the wrapper element by passing it as a parameter, and you will have to enable wrapping in the ObjectMapper you are using.

For this, we can use the "WRAP_ROOT_VALUE" feature of the SerializationFeature enum that can be enabled to make the root value wrapped within a single property JSON object, where the key(wrapper element) is the root name. By default, Jackson does not wrap values.

Example

This program serializes an Employee object into JSON with a root wrapper named "user", using Jackson's @JsonRootName annotation. The output will be a JSON string with "user" as the root element encapsulating the employee data.

import com.fasterxml.jackson.annotation.JsonRootName;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.SerializationFeature;

public class JsonRootNameAnnotationTest {
   public static void main(String args[]) throws JsonProcessingException {
      ObjectMapper mapper = new ObjectMapper();
      String jsonString = mapper.enable(SerializationFeature.WRAP_ROOT_VALUE).writeValueAsString(new Employee());
      System.out.println(jsonString);
   }
}

@JsonRootName(value = "user")
class Employee {
   public int empId = 125;
   public String empName = "Raja Ramesh";
}

Following is the output of the above code -

{"user":{"empId":125,"empName":"Raja Ramesh"}}