Jackson Annotations - Mixin Annotation

Overview

Mixin Annotation is a way to associate annotations without modifying the target class.

Create a class to be used as Mixin

Create a class where required annotation is applied.

@JsonIgnoreType
class MixInForIgnoreType {}

Create a class where annotation is to be applied indirectly.

class Name {
   public String name;
   Name(String name){
      this.name = name;
   }       
}

Apply the Mixin

ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(Name.class, MixInForIgnoreType.class);

Example - Serialization without Mixin Annotation

JacksonTester.java

package com.tutorialspoint;

import java.io.IOException;

import com.fasterxml.jackson.databind.ObjectMapper;

public class JacksonTester {
   public static void main(String args[]) {
      try {
	     ObjectMapper mapper = new ObjectMapper();
         Student student = new Student(1,11,"1ab","Mark");       
         String jsonString = mapper
            .writerWithDefaultPrettyPrinter()
            .writeValueAsString(student);
         System.out.println(jsonString);
      }
      catch (IOException e) { 
         e.printStackTrace();
      }     
   }
}
class Student {
   public int id;
   public String systemId;
   public int rollNo;
   public Name nameObj;

   Student(int id, int rollNo, String systemId, String name) {
      this.id = id;
      this.systemId = systemId;
      this.rollNo = rollNo;
      nameObj = new Name(name);
   }
}
class Name {
   public String name;
   Name(String name){
      this.name = name;
   }       
}

Output

Run the JacksonTester and verify the output −

{
  "id" : 1,
  "systemId" : "1ab",
  "rollNo" : 11,
  "nameObj" : {
    "name" : "Mark"
  }
}

Example - Serialization with Mixin Annotation

JacksonTester.java

package com.tutorialspoint;

import java.io.IOException;

import com.fasterxml.jackson.annotation.JsonIgnoreType;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JacksonTester {
   public static void main(String args[]) {
      try {
    	 Student student = new Student(1,11,"1ab","Mark"); 
         ObjectMapper mapper = new ObjectMapper();
         mapper.addMixIn(Name.class, MixInForIgnoreType.class);
         String jsonString = mapper
            .writerWithDefaultPrettyPrinter()
            .writeValueAsString(student);
         System.out.println(jsonString);
      }
      catch (IOException e) { 
         e.printStackTrace();
      }     
   }
}
class Student {
   public int id;
   public String systemId;
   public int rollNo;
   public Name nameObj;

   Student(int id, int rollNo, String systemId, String name) {
      this.id = id;
      this.systemId = systemId;
      this.rollNo = rollNo;
      nameObj = new Name(name);
   }
}
class Name {
   public String name;
   Name(String name){
      this.name = name;
   }       
}
@JsonIgnoreType
class MixInForIgnoreType {}

Output

Run the JacksonTester and verify the output −

{
  "id" : 1,
  "systemId" : "1ab",
  "rollNo" : 11
}

Here we can see, using addMixin() method, we're able to apply annotations without modify the original class.