Jackson ObjectMapper setSerializationInclusion() 不起作用

Question

我刚刚熟悉杰克逊绑定。但是，当我测试 setSerializationInclusion(JsonInclude.Include.NON_NULL) 时，我发现它有时不起作用。

这是我的代码

package com.blithe.main;

import com.blithe.model.Student;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Jackson_2_NullValue {
    public static void main(String[] args) throws JsonProcessingException {

        ObjectMapper mapper = new ObjectMapper();


        Student s = new Student();
        String stundetString = mapper.writeValueAsString(s);
        System.out.println(stundetString);


        // exclude null fields
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

        s.setName("ss");
        stundetString = mapper.writeValueAsString(s);
        System.out.println(stundetString);
    }
}

和 POJO

package com.blithe.model;

import java.util.Date;
import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;

// @JsonIgnoreProperties(ignoreUnknown = true)
// exclude null fields for the whole class
// @JsonInclude(Include.NON_NULL)
public class Student {

    // exclude the field whe it's empty ("")
    // @JsonInclude(value=Include.NON_EMPTY)
    private String name;

    private Integer age;

    private Date birth;

    // Jackson ignores it
    @JsonIgnore
    private String nickName;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Date getBirth() {
        return birth;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }

    public String getNickName() {
        return nickName;
    }

    public void setNickName(String nickName) {
        this.nickName = nickName;
    }
}

输出是

{"name":null,"age":null,"birth":null}
{"name":"ss","age":null,"birth":null}

后一个应该排除空值，但事实并非如此。

但是，当我这样放置代码时。

package com.blithe.main;

import com.blithe.model.Student;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Jackson_2_NullValue {
    public static void main(String[] args) throws JsonProcessingException {

        ObjectMapper mapper = new ObjectMapper();
        mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

        Student s = new Student();
        String stundetString = mapper.writeValueAsString(s);
        System.out.println(stundetString);


        // exclude null fields
        // mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

        s.setName("ss");
        stundetString = mapper.writeValueAsString(s);
        System.out.println(stundetString);
    }
}

它适用于下面的输出

{}
{"name":"ss"}

这是正常现象还是只是某种错误？我想念什么吗？唯一的 maven 依赖项是 jackson-databind 2.7.4。欢迎任何讨论。谢谢！

Answer 1

ObjectMappers使用时请勿更改设置。一旦使用映射器，由于序列化器和反序列化器的缓存，并非所有设置都会生效。

配置实例一次，首次使用后请勿更改设置。这样做是为了线程安全和性能。

更新：无效链接替换为 archive.org 链接

• http://wiki.fasterxml.com/JacksonFAQThreadSafety
• http://wiki.fasterxml.com/JacksonBestPracticesPerformance