Jackson 中的杂项注释

2025年3月17日 | 阅读 3 分钟

自定义注释和 MixIn 注释属于杂项注释。杂项注释用于序列化和反序列化 Json。

让我们通过一个例子来逐一理解它们。

自定义注释

@JacksonAnnotationsInside 是最重要的注释之一，它可以帮助开发人员实现自定义注释。让我们举一个例子来理解如何创建和使用自定义注释。

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 {
    
}

输出

MixIn 注释

如果我们想在不修改目标类的情况下关联注释，我们使用 mixin 注释。让我们举一个例子来理解它如何用于关联注释。

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 {}

输出