Jackson 中的通用注释

2025年03月17日 | 阅读 9 分钟

通用注释包括我们在序列化和反序列化中使用的几个基本注释。 JsonProperty 和 JsonView 是序列化和反序列化时常用的两个注释。

让我们通过一个例子详细了解它们中的每一个

@JsonProperty

这是一个重要的通用注释，用于标记非标准的 getter/setter 方法，即与 Json 属性一起使用。让我们举个例子来理解它是如何工作的

JsonPropertyExample.java

// import required classes and package
import java.io.IOException;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;

// create class JsonPropertyExample to understand how it works 
public class JsonPropertyExample {
    
    // main() method start
    public static void main(String args[]) throws IOException {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        String json = "{\"name\":\"Sharon\",\"id\":101, \"course\":\"MCA\"}";
        
        // de-serializing Json into Java object
        Student std = mapper.readerFor(Student.class).readValue(json);
        
        // print id, name and course of student
        System.out.println(std.getStdId());
        System.out.println(std.getStdName());
        System.out.println(std.getCourse());
    }
}
class Student {
    private int id;
    private String name;
    private String course;
    
    // default constructor
    Student(){
        
    }
    // parameterized constructor 
    Student(int id, String name, String course){
        this.id = id;
        this.name = name;
        this.course = course;
    }
    @JsonProperty("")
    public int getStdId() {
        return id;
    }
    @JsonProperty("id")
    public void setStdId(int id) {
        this.id = id;
    }
    @JsonProperty("name")
    public String getStdName() {
        return name;
    }
    @JsonProperty("name")
    public void setStdName(String name) {
        this.name = name;
    }  
    @JsonProperty("course")
    public String getCourse() {
        return course;
    }
    @JsonProperty("course")
    public void setCourse(String course) {
        this.course = course;
    }  
}

输出

@JsonFormat

这是另一个重要的注释，用于在序列化和反序列化时指定格式。它主要用于指定日期格式。

让我们举一个例子来理解如何使用注释来指定日期字段的格式。

JsonFormatExample.java

//import required classes and package

import java.io.IOException;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class JsonFormatExample to understand how it works 
public class JsonFormatExample {
    
    // main() method start
    public static void main(String args[]) throws IOException, ParseException  {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);
        
        try{
            
            Student std = new Student();
            
            String stdId, stdName, stdCourse, stdAddmission;
            
            System.out.println("Enter Student Id:");
            stdId = sc.nextLine();
            std.setStdId(stdId);
            
            System.out.println("Enter Student Name:");
            stdName = sc.nextLine();
            std.setStdName(stdName);
            
            System.out.println("Enter Course Name:");
            stdCourse = sc.nextLine();
            std.setCourse(stdCourse);
            
            System.out.println("Enter Addmission Date(dd-MM-yyyy):");
            stdAddmission = sc.nextLine();
            
            SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");
            Date date = sdf.parse(stdAddmission);
            std.setAddmission(date);
            
            // convert Java type into Json string
            String jsonString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(std); 
            System.out.println(jsonString); 
        } 
        catch (IOException e) { 
            e.printStackTrace(); 
        }
        
        // close Scanner class object
        sc.close();
    }
}
class Student {
    private String id;
    private String name;
    public String course;
    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "dd-MM-yyyy")
    public Date addmission;
    
    // getter and setter
    @JsonProperty("id")
    public String getStdId() {
        return id;
    }
    @JsonProperty("id")
    public void setStdId(String id) {
        this.id = id;
    }
    @JsonProperty("name")
    public String getStdName() {
        return name;
    }
    @JsonProperty("name")
    public void setStdName(String name) {
        this.name = name;
    }  
    public String getCourse() {
        return course;
    }
    public void setCourse(String course) {
        this.course = course;
    }
    public Date getAddmission() {
        return addmission;
    }
    public void setAddmission(Date addmission) {
        this.addmission = addmission;
    } 
}

输出

@JsonUnwrapped

这是另一个重要的注释，它包含在通用注释中。它用于在序列化和反序列化时解包对象的值。

让我们举个例子来理解如何使用注释来包装值。

JsonUnwrapperdExample.java

//import required classes and package
import java.io.IOException;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonUnwrapped;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class JsonUnwrapperdExample to understand the working of it
public class JsonUnwrapperdExample {
    
    // main() method start
    public static void main(String args[]) throws IOException, ParseException  {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);
        
        try{
            
            Student std = new Student();
            
            String stdId, firstName, lastName, stdCourse, stdAddmission;
            
            System.out.println("Enter Student Id:");
            stdId = sc.nextLine();
            std.setStdId(stdId);
            
            System.out.println("Enter First Name:");
            firstName = sc.nextLine();
            
            System.out.println("Enter Last Name:");
            lastName = sc.nextLine();
            
            std.setStdName(new Student.Name(firstName, lastName));
            
            System.out.println("Enter Course Name:");
            stdCourse = sc.nextLine();
            std.setCourse(stdCourse);
            
            System.out.println("Enter Addmission Date(dd-MM-yyyy):");
            stdAddmission = sc.nextLine();
            
            SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");
            Date date = sdf.parse(stdAddmission);
            std.setAddmission(date);
            
            // convert Java type into Json string
            String jsonString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(std); 
            System.out.println(jsonString); 
        } 
        catch (IOException e) { 
            e.printStackTrace(); 
        }
        
        // close Scanner class object
        sc.close();
    }
}
class Student {
    private String id;
    @JsonUnwrapped
    public Name name;
    public String course;
    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "dd-MM-yyyy")
    public Date addmission;
    
    // getter and setter
    @JsonProperty("id")
    public String getStdId() {
        return id;
    }
    @JsonProperty("id")
    public void setStdId(String id) {
        this.id = id;
    }
    @JsonProperty("name")
    public Name getStdName() {
        return name;
    }
    @JsonProperty("name")
    public void setStdName(Name name) {
        this.name = name;
    }  
    public String getCourse() {
        return course;
    }
    public void setCourse(String course) {
        this.course = course;
    }
    public Date getAddmission() {
        return addmission;
    }
    public void setAddmission(Date addmission) {
        this.addmission = addmission;
    } 
    static class Name {
        public String firstName;
        public String lastName;
        
        // parameterized constructor
        public Name(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }
    }
}

输出

@JsonUnwrapped

这是另一个重要的注释，用于控制要序列化还是不序列化的值。让我们举一个例子来理解如何使用注释来控制这些值。

JsonViewExample.java

//import required classes and package
import java.io.IOException;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonView;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class JsonViewExample to understand how it works 
public class JsonViewExample {
    
    // main() method start
    public static void main(String args[]) throws IOException, ParseException  {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);
        
        try{
            
            Student std = new Student();
            
            String stdId, stdName, stdCourse, stdAddmission;
            
            System.out.println("Enter Student Id:");
            stdId = sc.nextLine();
            std.setStdId(stdId);
            
            System.out.println("Enter Student Name:");
            stdName = sc.nextLine();
            std.setStdName(stdName);
            
            System.out.println("Enter Course Name:");
            stdCourse = sc.nextLine();
            std.setCourse(stdCourse);
            
            System.out.println("Enter Addmission Date(dd-MM-yyyy):");
            stdAddmission = sc.nextLine();
            
            SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy");
            Date date = sdf.parse(stdAddmission);
            std.setAddmission(date);
            
            // convert Java type into Json string
            String jsonString = mapper.writerWithDefaultPrettyPrinter().withView(Views.Public.class).writeValueAsString(std); 
            System.out.println(jsonString); 
        } 
        catch (IOException e) { 
            e.printStackTrace(); 
        }
        
        // close Scanner class object
        sc.close();
    }
}
class Student {
    @JsonView(Views.Public.class)
    private String id;
    @JsonView(Views.Public.class)
    private String name;
    @JsonView(Views.Internal.class)
    public String course;
    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "dd-MM-yyyy")@JsonView(Views.Public.class)
    public Date addmission;
    
    // getter and setter
    @JsonProperty("id")
    public String getStdId() {
        return id;
    }
    @JsonProperty("id")
    public void setStdId(String id) {
        this.id = id;
    }
    @JsonProperty("name")
    public String getStdName() {
        return name;
    }
    @JsonProperty("name")
    public void setStdName(String name) {
        this.name = name;
    }  
    public String getCourse() {
        return course;
    }
    public void setCourse(String course) {
        this.course = course;
    }
    public Date getAddmission() {
        return addmission;
    }
    public void setAddmission(Date addmission) {
        this.addmission = addmission;
    } 
}
class Views {
    
    static class Public {
        
    }
    static class Internal extends Public {
        
    }
}

输出

@JsonManagedReferences/@JsonBackReferences

@JsonManagedReference 和 @JsonBackReferences 是两个重要的注释，用于显示与父子关系的关系。

@JsonManagedReference 用于引用父对象，而 @JsonBackReferences 用于标记子对象。

让我们举个例子来理解如何使用这两个注释来显示对象之间的关系。

JsonReferenceExample.java

//import required classes and package
import java.io.IOException;
import java.text.ParseException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

import com.fasterxml.jackson.annotation.JsonBackReference;
import com.fasterxml.jackson.annotation.JsonManagedReference;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class JsonReferenceExample to understand how it works 
public class JsonReferenceExample {
    
    // main() method start
    public static void main(String args[]) throws IOException, ParseException  {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);
        
        try{
            
            Student std = new Student();
            
            String stdId, stdName, stdCourse;
            
            System.out.println("Enter Student Id:");
            stdId = sc.nextLine();
            std.setStdId(stdId);
            
            System.out.println("Enter Student Name:");
            stdName = sc.nextLine();
            std.setStdName(stdName);
            
            System.out.println("Enter Course Name:");
            stdCourse = sc.nextLine();
            std.setCourse(stdCourse);
            
            Book obj1 = new Book(121,"Learn C Programming", std);
            Book obj2 = new Book(131,"Learn Advance Java", std);
            
            std.addBook(obj1);
            std.addBook(obj2);
            
            // convert Java type into Json string
            String jsonString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(obj1); 
            System.out.println(jsonString); 
        } 
        catch (IOException e) { 
            e.printStackTrace(); 
        }
        
        // close Scanner class object
        sc.close();
    }
}
class Student {
    private String id;
    private String name;
    public String course;
    
    @JsonBackReference
    public List<Book> books;
    
    // default constructor
    public Student() {
        this.books = new ArrayList<Book>();
    }
    // getter and setter
    @JsonProperty("id")
    public String getStdId() {
        return id;
    }
    @JsonProperty("id")
    public void setStdId(String id) {
        this.id = id;
    }
    @JsonProperty("name")
    public String getStdName() {
        return name;
    }
    @JsonProperty("name")
    public void setStdName(String name) {
        this.name = name;
    }  
    public String getCourse() {
        return course;
    }
    public void setCourse(String course) {
        this.course = course;
    }
    public void addBook(Book book){
        books.add(book);
    }
}
class Book {
    public int bookId;
    public String bookName;
    
    Book(int bookId, String bookName, Student bookOwner){
        this.bookId = bookId;
        this.bookName = bookName;
        this.bookOwner = bookOwner;
    }
    @JsonManagedReference
    public Student bookOwner;
}

输出

@JsonIdentityInfo

当对象具有父子关系时，使用 @JsonIdentityInfo 表明在序列化/反序列化期间将使用对象标识。

让我们举个例子来理解 @JsonIdentityInfo 注释的工作原理。

JsonIdentityInfoExample.java

//import required classes and package
import java.io.IOException;
import java.text.ParseException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

import com.fasterxml.jackson.annotation.JsonIdentityInfo;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.ObjectIdGenerators;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class JsonIdentityInfoExample to understand how it works 
public class JsonIdentityInfoExample {
    
    // main() method start
    public static void main(String args[]) throws IOException, ParseException  {
        
        // create an instance of ObjectMapper class
        ObjectMapper mapper = new ObjectMapper();
        
        // create Scanner class object
        Scanner sc= new Scanner(System.in);
        
        try{
            
            Student std = new Student();
            
            String stdId, stdName, stdCourse;
            
            System.out.println("Enter Student Id:");
            stdId = sc.nextLine();
            std.setStdId(stdId);
            
            System.out.println("Enter Student Name:");
            stdName = sc.nextLine();
            std.setStdName(stdName);
            
            System.out.println("Enter Course Name:");
            stdCourse = sc.nextLine();
            std.setCourse(stdCourse);
            
            Book obj1 = new Book(121,"Learn C Programming", std);
            Book obj2 = new Book(131,"Learn Advance Java", std);
            
            std.addBook(obj1);
            std.addBook(obj2);
            
            // convert Java type into Json string
            String jsonString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(obj1); 
            System.out.println(jsonString); 
        } 
        catch (IOException e) { 
            e.printStackTrace(); 
        }
        
        // close Scanner class object
        sc.close();
    }
}
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
class Student {
    private String id;
    private String name;
    public String course;
    public List<Book> books;
    
    // default constructor
    public Student() {
        this.books = new ArrayList<Book>();
    }
    // getter and setter
    @JsonProperty("id")
    public String getStdId() {
        return id;
    }
    @JsonProperty("id")
    public void setStdId(String id) {
        this.id = id;
    }
    @JsonProperty("name")
    public String getStdName() {
        return name;
    }
    @JsonProperty("name")
    public void setStdName(String name) {
        this.name = name;
    }  
    public String getCourse() {
        return course;
    }
    public void setCourse(String course) {
        this.course = course;
    }
    public void addBook(Book book){
        books.add(book);
    }
}
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "bookId")
class Book {
    public int bookId;
    public String bookName;
    public Student bookOwner;
    
    Book(int bookId, String bookName, Student bookOwner){
        this.bookId = bookId;
        this.bookName = bookName;
        this.bookOwner = bookOwner;
    }
}

输出

@JsonFilter

@JsonFilter 是另一个有用的注释，我们在序列化/反序列化时使用它来应用过滤器。

让我们举个例子来理解如何使用 @JsonFilter 应用过滤器。

JsonFilterExample.java

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

import com.fasterxml.jackson.annotation.JsonFilter;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ser.FilterProvider;
import com.fasterxml.jackson.databind.ser.impl.SimpleBeanPropertyFilter;
import com.fasterxml.jackson.databind.ser.impl.SimpleFilterProvider;

//create class JsonFilterExample to understand the working of annotation
public class JsonFilterExample {
    
    //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);   
            
            FilterProvider filters = new SimpleFilterProvider() .addFilter(
                "nameFilter", SimpleBeanPropertyFilter.filterOutAllExcept("name"));
            
            // serialize Product using mapper
            String jsonString = mapper
                .writer(filters)
                .withDefaultPrettyPrinter()
                .writeValueAsString(product);
            System.out.println(jsonString);
        }
        catch (IOException e) { 
            e.printStackTrace();
        }
        
        //close Scanner class object
        sc.close();
        
    }
}
//create class Product by using @JsonFilter
@JsonFilter("nameFilter")
class Product {  
    
    //Creating properties of Product class  
    public String proId;    
    public String name;    
    public String price;
    public String expire; 
    
    // constructor
    Product(String proId, String name, String price, String expire){
        this.proId = proId;
        this.name = name;
        this.price = price;
        this.expire = expire;
    }
}

输出