Java Jackson ObjectMapper setSerializationInclusion（）不工作

我刚开始熟悉杰克逊的装帧。然而，当我测试setSerializationInclusion（JsonInclude.Include.NON_NULL）时，我发现它有时不起作用

这是我的密码

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);
    }
}

还有波乔

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}

后面的一个应该是null值，但它不是

然而，当我这样写代码时

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);
    }
}

它与下面的输出一起工作

{}
{"name":"ss"}

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这是正常的还是只是某种错误？我错过什么了吗？maven唯一的依赖项是jackson databind 2.7.4。欢迎任何讨论。谢谢

使用对象映射器时，请勿更改设置。映射器使用后，由于序列化程序和反序列化程序的缓存，并非所有设置都会生效

配置实例一次，首次使用后不要更改设置。这样做是为了线程安全和性能

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因此，关键是如果您在多个位置使用ObjectMapper，请不要一次又一次地创建对象。它采用首次初始化的配置

如果你在全球范围内不断地改变，那就行不通了