Jackson 忽略空字段

17 Mar 2025 | 4 分钟阅读

在本节中，我们将了解如何通过使用 Jackson 在序列化 Java 类时忽略空字段。

我们可以在类级别、字段级别或全局范围内忽略空字段。让我们逐一了解它们。

在类级别忽略空字段

为了在类级别忽略空字段，我们使用带有 include.NON_NULL 的 @JsonInclude 注解。让我们举个例子来了解如何使用 @JsonInclude 注解来忽略类级别的空字段。

IgnoreNullFieldExample1.java

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

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

//create class IgnoreNullFieldExample1 to understand how we can ignore the null field on marshalling
public class IgnoreNullFieldExample1 {  

	//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
		try {  
     
			College colg;  
     
			String name, university, street, state, city, rank;  
     
			System.out.println("Enter College Name:");  
			name = sc.nextLine();  
     
			System.out.println("Enter University Name:");  
			university = sc.nextLine();  
     
			System.out.println("Enter Street:");  
			street = sc.nextLine();  
			
			System.out.println("Enter City:");  
			city = sc.nextLine();
			
			System.out.println("Enter State:");  
			state = sc.nextLine();
			
			System.out.println("Enter College Rank:");  
			rank = sc.nextLine();  
     
			// set values to College  object by using constructor  
			colg = new College(name, university, new Address(street, state, null), null);
			
			// serialize College using mapper  
			String jsonString = mapper  
					.writerWithDefaultPrettyPrinter()  
					.writeValueAsString(colg);  
			System.out.println(jsonString);  
	            
		} catch (IOException e) {   
			e.printStackTrace();  
		}  
 
		//close Scanner class object  
		sc.close();  
 
	}  
}  
//create class College by using @JsonInclude(JsonInclude.Include.NON_NULL)
@JsonInclude(JsonInclude.Include.NON_NULL)
class College {    

	//Creating properties of College class    
	public String name;      
	public String university;   
	public Address address;  
	public String rank;   

	// constructor  
	College(String name, String university, Address address, String rank){  
		this.name = name;  
		this.university = university;  
		this.address = address;  
		this.rank = rank;  
	}  

}
//create class Address 
@JsonInclude(JsonInclude.Include.NON_NULL)
class Address {  
	public String street;
	public String state;
	public String city;
	
	Address(String street, String state, String city){  
		  this.street = street;
		  this.state = state;
		  this.city = city;
	}         
}

输出

忽略特定的空字段

在这里，我们使用带有 include.NON_NULL 的 @JsonInclude 注解在字段级别忽略特定的空字段。让我们举个例子来了解如何使用 @JsonInclude 注解来忽略特定的空字段。

IgnoreNullFieldExample2.java

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

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

//create class IgnoreNullFieldExample2 to understand how we can ignore specific null fields on marshalling
public class IgnoreNullFieldExample2 {  

	//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
		try {  
   
			College colg;  
   
			String name, university, street, state, city, rank;  
   
			System.out.println("Enter College Name:");  
			name = sc.nextLine();  
   
			System.out.println("Enter University Name:");  
			university = sc.nextLine();  
   
			System.out.println("Enter Street:");  
			street = sc.nextLine();  
			
			System.out.println("Enter City:");  
			city = sc.nextLine();
			
			System.out.println("Enter State:");  
			state = sc.nextLine();
			
			System.out.println("Enter College Rank:");  
			rank = sc.nextLine();  
   
			// set values to College  object by using constructor  
			colg = new College(name, university, new Address(street, state, city), null);
			
			// serialize College using mapper  
			String jsonString = mapper  
					.writerWithDefaultPrettyPrinter()  
					.writeValueAsString(colg);  
			System.out.println(jsonString);  
	            
		} catch (IOException e) {   
			e.printStackTrace();  
		}  

		//close Scanner class object  
		sc.close();  

	}  
}  
//create class College by using @JsonInclude(JsonInclude.Include.NON_NULL)
class College {    

	//Creating properties of College class    
	public String name;      
	public String university;   
	public Address address; 
	
	@JsonInclude(JsonInclude.Include.NON_NULL)
	public String rank;   

	// constructor  
	College(String name, String university, Address address, String rank){  
		this.name = name;  
		this.university = university;  
		this.address = address;  
		this.rank = rank;  
	}  

}
//create class Address 
class Address {  
	public String street;
	public String state;
	public String city;
	
	Address(String street, String state, String city){  
		  this.street = street;
		  this.state = state;
		  this.city = city;
	}         
}

输出

全局忽略空字段

在这里，我们在 ObjectMapper 类上使用 JsonInclude.Include.NON_NULL 来配置全局忽略空字段的行为。我们使用 setSerializationInclusion() 方法并使用 Include.NON_NULL，如下所示

让我们举个例子来了解如何通过使用 setSerializationInclusion() 方法全局忽略空字段。

IgnoreNullFieldExample3.java

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

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

//create class IgnoreNullFieldExample3 to understand how we can ignore null fields globally
public class IgnoreNullFieldExample3 {  

	//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
		try {  
 
			College colg;  
 
			String name, university, street, state, city, rank;  
 
			System.out.println("Enter College Name:");  
			name = sc.nextLine();  
 
			System.out.println("Enter University Name:");  
			university = sc.nextLine();  
 
			System.out.println("Enter Street:");  
			street = sc.nextLine();  
			
			System.out.println("Enter City:");  
			city = sc.nextLine();
			
			System.out.println("Enter State:");  
			state = sc.nextLine();
			
			System.out.println("Enter College Rank:");  
			rank = sc.nextLine();  
 
			// set values to College  object by using constructor  
			colg = new College(name, university, new Address(street, state, null), null);
			
			// serialize College using mapper  
			String jsonString = mapper
					.setSerializationInclusion(JsonInclude.Include.NON_NULL)  
					.writerWithDefaultPrettyPrinter()  
					.writeValueAsString(colg);  
			System.out.println(jsonString);  
	            
		} catch (IOException e) {   
			e.printStackTrace();  
		}  

		//close Scanner class object  
		sc.close();  

	}  
}  
//create class College
class College {    

	//Creating properties of College class    
	public String name;      
	public String university;   
	public Address address; 
	public String rank;   

	// constructor  
	College(String name, String university, Address address, String rank){  
		this.name = name;  
		this.university = university;  
		this.address = address;  
		this.rank = rank;  
	}  

}
//create class Address 
class Address {  
	public String street;
	public String state;
	public String city;
	
	Address(String street, String state, String city){  
		  this.street = street;
		  this.state = state;
		  this.city = city;
	}         
}

输出