Miscellaneous Annotations in Jackson

Custom annotation and MixIn annotation come in miscellaneous annotations. Miscellaneous annotations are used at the time of serializing and de-serializing Json.

Let's understand each one of them one by one with an example.

Custom Annotation

@JacksonAnnotationsInside is one of the most important annotations that help developers implement custom annotation. Let's take an example to understand how to create and use custom annotation.

CustomAnnotationExample.java

//import required classes and package if any
import java.io.IOException;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.util.Scanner;

import com.fasterxml.jackson.annotation.JacksonAnnotationsInside;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.annotation.JsonPropertyOrder;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class CustomAnnotationExample to understand the working of annotation
public class CustomAnnotationExample {
    
    //main() methods start
    public static void main(String args[]) {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);		
        
        // using try-catch for serializing Product
        try {
            
            Product product;
            
            String proId, proName, proPrice, proExpiry;
            
            System.out.println("Enter Product Id:");
            proId = sc.nextLine();
            
            System.out.println("Enter Product Name");
            proName = sc.nextLine();
            
            System.out.println("Enter Product Price");
            proPrice = sc.nextLine();
            
            System.out.println("Enter Product Expiry in dd-mm-yyyy format");
            proExpiry = sc.nextLine();
            
            // set values to Student object by using constructor
            product = new Product(proId, proName, proPrice, proExpiry);   
            
            String jsonString = mapper
                .writerWithDefaultPrettyPrinter()
                .writeValueAsString(product);
            
            System.out.println(jsonString);
        }
        catch (IOException e) { 
            e.printStackTrace();
        }
        
        //close Scanner class object
        sc.close();
        
    }
}
//create class Product by using @CustomAnnotation
@CustomAnnotation
class Product {  
    
    //Creating properties of Product class  
    public String proId;    
    public String name;    
    public String price;
    public String expire;
    public String seller;
    
    // constructor
    Product(String proId, String name, String price, String expire){
        this.proId = proId;
        this.name = name;
        this.price = price;
        this.expire = expire;
    }
    
}
@JacksonAnnotationsInside
@JsonInclude(value = Include.NON_NULL)
@Retention(RetentionPolicy.RUNTIME)
@JsonPropertyOrder({ "name", "proId", "expire", "price" })
@interface CustomAnnotation {
    
}

Output:

MixIn Annotation

If we want to associate annotations without modifying the target class, we use mixin annotation. Let's take an example to understand how it is used for associating annotations.

MixInAnnotationExample.java

//import required classes and package if any
import java.io.IOException;
import java.util.Scanner;

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

//create class MixInAnnotationExample to understand the working of annotation
public class MixInAnnotationExample {
    
    //main() methods start
    public static void main(String args[]) {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        ObjectMapper mapper1 = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);		
        
        // using try-catch for serializing Product
        try {
            
            Product product;
            
            String proId, proName, proPrice, proExpiry, proCode;
            
            System.out.println("Enter Product Id:");
            proId = sc.nextLine();
            
            System.out.println("Enter Product Name");
            proName = sc.nextLine();
            
            System.out.println("Enter Product Code");
            proCode = sc.nextLine();
            
            System.out.println("Enter Product Price");
            proPrice = sc.nextLine();
            
            System.out.println("Enter Product Expiry in dd-mm-yyyy format");
            proExpiry = sc.nextLine();
            
            // set values to Student object by using constructor
            product = new Product(proId, proName, proCode, proPrice, proExpiry);   
            
            String str = mapper
                .writerWithDefaultPrettyPrinter()
                .writeValueAsString(product);
            System.out.println(str);
            
            mapper1.addMixIn(Name.class, MixInForIgnoreType.class);
            str = mapper1
                .writerWithDefaultPrettyPrinter()
                .writeValueAsString(product);
            System.out.println(str);
        }
        catch (IOException e) { 
            e.printStackTrace();
        }
        
        //close Scanner class object
        sc.close();
        
    }
}
//create class Product
class Product {  
    
    //Creating properties of Product class  
    public String proId;    
    public Name proName;    
    public String price;
    public String expire;
    public String seller;
    
    // constructor
    Product(String proId, String name, String code, String price, String expire){
        this.proId = proId;
        proName = new Name(name, code);
        this.price = price;
        this.expire = expire;
    }
    
}
// create class Name
class Name {
    public String name;
    Name(String fName, String code){
        name = fName+" ("+code+")";
    }       
}
@JsonIgnoreType
class MixInForIgnoreType {}

Output: