使用 Jackson 进行 Map 序列化和反序列化

17 Mar 2025 | 6 分钟阅读

就像 Java 对象一样，我们可以使用 Jackson 序列化和反序列化 Java Map。我们可以使用 Jackson 轻松地将 Map<Object, Object>, Map<Object, String>, 和 Map<String, String> 序列化为 JSON 格式的字符串，或者从 JSON 格式的字符串反序列化它们。

Java Map 的序列化

将 Java 对象转换为字节流的过程称为序列化。根据需要，可以将字节流持久化或共享。 Java Map 是最不直观的序列化对象之一。

让我们举一些例子来了解如何使用 Jackson 序列化 Map，例如 Map<Object, Object>, Map<Object, String> 或 Map<String, String>。

SerializeMapExample1.java

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

//create class SerializeMapExample1 to serialize Java Map
public class SerializeMapExample1 {  

	//main() methods start  
	public static void main(String args[]) {  
		
		int n;
		Map<String, String> student = new HashMap<String, String>();
		
		// create an instance of Scanner class
		Scanner sc = new Scanner(System.in);
		
		System.out.println("Enter total number of students: ");
		n = Integer.valueOf(sc.nextLine());
		
		for(int i = 0; i < n; i++) {
			String id, name;
			System.out.println("Enter student id of "+(i+1)+" student:");
			id = sc.nextLine();
			System.out.println("Enter student name of "+(i+1)+" student:");
			name = sc.nextLine();
			
			// fill student map
			student.put(id, name);
		}
		
		// create an instance of the ObjectMapper class
		ObjectMapper mapper = new ObjectMapper();
		
		try {
			// serialize student Map by using Jackson
			String jsonString = mapper
					.writerWithDefaultPrettyPrinter()
					.writeValueAsString(student);
			System.out.println("Java Map after serialize using Jackson: "+jsonString);
		} catch (JsonProcessingException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		// close Scanner class object
		sc.close();
		
	}  
}

输出

Map Serialization and Deserialization with Jackson

我们还可以在之前的示例中添加更多代码行来序列化一个以 Java 对象作为值的 Map。

SerializeMapExample2.java

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

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

//create class SerializeMapExample2 to serialize Java Map
public class SerializeMapExample2 {  

	//main() methods start  
	public static void main(String args[]) {  
		
		int n;
		
		Map<String, Student> student = new HashMap<String, Student>();
		
		// create an instance of Scanner class
		Scanner sc = new Scanner(System.in);
		
		System.out.println("Enter total number of students: ");
		n = Integer.valueOf(sc.nextLine());
		
		for(int i = 0; i < n; i++) {
			String id, fName, lName, course;
			
			Student std = new Student();
			
			System.out.println("Enter id of "+(i+1)+" student:");
			id = sc.nextLine();
			
			System.out.println("Enter first name of "+(i+1)+" student:");
			fName = sc.nextLine();
			std.setFName(fName);
			
			System.out.println("Enter last name of "+(i+1)+" student:");
			lName = sc.nextLine();
			std.setLName(lName);
			
			System.out.println("Enter course of "+(i+1)+" student:");
			course = sc.nextLine();
			std.setCourse(course);
			
			// fill student map
			student.put(id, std);
		}
		
		// create an instance of the ObjectMapper class
		ObjectMapper mapper = new ObjectMapper();
		
		// enable SerializationFeature in ObjectMapper
		mapper.enable(SerializationFeature.INDENT_OUTPUT);
		
		String jsonString;
		try {
			
			jsonString = mapper.writeValueAsString(student);
			System.out.println("After converting Map to JSON :");
	        System.out.println(jsonString);
		} catch (JsonProcessingException e) {
			e.printStackTrace();
		}
		
		// close Scanner class object
		sc.close();
		
	}  
}
class Student {

    private String fName;
    private String lName;
    private String course;
    
    public String toString() {
        return "Name = "+fName + " " +lName+" Course = "+course;
    }
    
    public String getFName() {
    	return fName;
    }
    public void setFName(String fName) {
    	this.fName = fName;
    }
    
    public String getLName() {
    	return lName;
    }
    public void setLName(String lName) {
    	this.lName = lName;
    }
    
    public String getCourse() {
    	return course;
    }
    public void setCourse(String course) {
    	this.course = course;
    }
    
}

输出

让我们再举一个例子，了解如何将 Map<Object, Object> 序列化为 JSON 字符串。

SerializeMapExample3.java

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

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

//create class SerializeMapExample3 to serialize Java Map
public class SerializeMapExample3 {  

	//main() methods start  
	public static void main(String args[]) {  
		
		int n;
		
		Map<Course, Student> student = new HashMap<Course, Student>();
		
		// create an instance of Scanner class
		Scanner sc = new Scanner(System.in);
		
		System.out.println("Enter total number of students: ");
		n = Integer.valueOf(sc.nextLine());
		
		for(int i = 0; i < n; i++) {
			String id, fName, lName;
			
			Student std = new Student();
			Course course = new Course();
			
			System.out.println("Enter id of "+(i+1)+" student:");
			id = sc.nextLine();
			std.setId(id);
			
			System.out.println("Enter first name of "+(i+1)+" student:");
			fName = sc.nextLine();
			std.setFName(fName);
			
			System.out.println("Enter last name of "+(i+1)+" student:");
			lName = sc.nextLine();
			std.setLName(lName);
			
			System.out.println("Enter course of "+(i+1)+" student:");
			course.setName(sc.nextLine());
			
			System.out.println("Enter Semester: ");
			course.setSemester(sc.nextLine());
			
			// fill student map
			student.put(course, std);
		}
		
		// create an instance of the ObjectMapper class
		ObjectMapper mapper = new ObjectMapper();
		
		// enable SerializationFeature in ObjectMapper
		mapper.enable(SerializationFeature.INDENT_OUTPUT);
		
		String jsonString;
		try {
			
			jsonString = mapper.writeValueAsString(student);
			System.out.println("After converting Map to JSON :");
	        System.out.println(jsonString);
		} catch (JsonProcessingException e) {
			e.printStackTrace();
		}
		
		// close Scanner class object
		sc.close();
		
	}  
}
class Student {

	private String id;
	private String fName;
    private String lName;    
    public String toString() {
        return "Student Id = "+id+" Name = "+fName + " " +lName;
    }
    public String getId() {
    	return id;
    }
    public void setId(String id) {
    	this.id = id;
    }
    
    public String getFName() {
    	return fName;
    }
    public void setFName(String fName) {
    	this.fName = fName;
    }
    
    public String getLName() {
    	return lName;
    }
    public void setLName(String lName) {
    	this.lName = lName;
    }
    
}
class Course {
	private String name;
	private String semester;
	
	public String toString() {
        return "Course Name = "+name+" Semester = "+semester;
    }
    
    public String getName() {
    	return name;
    }
    public void setName(String name) {
    	this.name = name;
    }
    
    public String getSemester() {
    	return semester;
    }
    public void setSemester(String semester) {
    	this.semester = semester;
    }
}

输出

将 JSON 字符串反序列化为 Java Map

将字节流转换为 Java 对象的过程称为反序列化。我们可以将 JSON 数据反序列化为不同签名的 Map（Map<String, String>, Map<String, Object> 和 Map<Object, Object>）。

让我们举一个例子来了解如何将 JSON 字符串反序列化为 Map<String, String>。

DeserializeMapExample1.java

import java.util.HashMap;
import java.util.Map;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
public class DeserializeJsonToMapExample1 {
	//main() methods start  
	public static void main(String args[]) throws JsonMappingException, JsonProcessingException {
		// create an instance of ObjectMapper class
		ObjectMapper mapper = new ObjectMapper();
		
		// create json string
		String studentString = "{\"id\": \"001\", \"fName\":\"Nicholas\", \"lName\":\"Clemens\"}";
		
		// create TypeReference of type HashMap<String, String>
		TypeReference<HashMap<String, String>> typeRef = new TypeReference<HashMap<String, String>>() {};
		
		// deserialize studentString into Map<String, String>
		Map<String, String> studentMap = mapper.readValue(studentString, typeRef);
		
		System.out.println("Student Map======>"+studentMap);
		
	}
}

输出

让我们再举一个例子来了解如何将 JSON 字符串反序列化为 Map<Object, String>。

DeserializeMapExample2.java

import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class DeserializeJsonToMapExample2 {
	//main() methods start  
	public static void main(String args[]) throws JsonMappingException, JsonProcessingException {
		// create an instance of ObjectMapper class
		ObjectMapper mapper = new ObjectMapper();
		
		// create json string
		String studentString = "{\"BCA\": \"001 and Nicholas and Clemens\", \"MCA\": \"002 and Sharon and Lorenz\"}";
		
		// create TypeReference of type HashMap<String, Student>
		TypeReference<HashMap<String, Student>> typeRef = new TypeReference<HashMap<String, Student>>() {};
		
		// deserialize studentString into Map<String, Student>
		Map<String, Student> studentMap = mapper.readValue(studentString, typeRef);
		
		System.out.println("Student Map======>"+studentMap);
		
	}
}
class Student {

	private String id;
	private String fName;
    	private String lName;
    
    	Student(String data){
    		String[] allData = data.split("and");
        	this.id = allData[0].trim();
        	this.fName = allData[1].trim();
        	this.lName = allData[2].trim();
    	}
    
    	public String toString() {
        	return "Student Id = "+id+" Name = "+fName + " " +lName;
    	}
    
    	public String getId() {
    		return id;
    	}
    	public String getFName() {
    		return fName;
    	}
    
    	public String getLName() {
    		return lName;
    	}
}

输出

让我们再举一个例子来了解如何将 JSON 字符串反序列化为 Map<Object, Object>。

DeserializeMapExample3.java

import java.util.HashMap;
import java.util.Map;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class DeserializeMapExample3 {
	//main() methods start  
	public static void main(String args[]) throws JsonMappingException, JsonProcessingException {
		// create an instance of ObjectMapper class
		ObjectMapper mapper = new ObjectMapper();
		
		// create json string
		String studentString = "{\"BCA and 2nd Semester\": \"001 and Nicholas and Clemens\", \"MCA and 4th Semester\": \"002 and Sharon and Lorenz\"}";
		
		// create TypeReference of type HashMap<Course, Student>
		TypeReference<HashMap<Course, Student>> typeRef = new TypeReference<HashMap<Course, Student>>() {};
		
		// deserialize studentString into Map<Course, Student>
		Map<Course, Student> studentMap = mapper.readValue(studentString, typeRef);
		
		System.out.println("Student Map======>"+studentMap);
		
	}
}
class Student {

	private String id;
	private String fName;
    	private String lName;
    
    	Student(String data){
    		String[] allData = data.split("and");
        	this.id = allData[0].trim();
        	this.fName = allData[1].trim();
        	this.lName = allData[2].trim();
    	}
    
    	public String toString() {
        	return "Student Id = "+id+" Name = "+fName + " " +lName;
    	}
    
    	public String getId() {
    		return id;
    	}
    	public String getFName() {
    		return fName;
    	}
    
    	public String getLName() {
    		return lName;
    	}
}
class Course {
	private String name;
	private String semester;
	
	public String toString() {
        	return "Course Name = "+name+" Semester = "+semester;
    	}
	
	Course(String data){
    		String[] allData = data.split("and");
        	this.name = allData[0].trim();
        	this.semester = allData[1].trim();
    	}
    
    	public String getName() {
    		return name;
    	}
    
    	public String getSemester() {
    		return semester;
    	}
}

输出

下一主题使用 Jackson 根据自定义条件序列化字段

使用 Jackson 进行 Map 序列化和反序列化

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将 JSON 字符串反序列化为 Java Map

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