如何在没有注释的情况下序列化 object 中的列表？

Question

我有 JSON 看起来像这样（完整的 JSON 更大）：

{
    "legalLastName": "lastName",
    "legalFirstName": "firstName",
    "terminationDate": null,
    "collegeEducation": [{
        "major": "Finance",
        "school": "Towson University",
        "quarter": null,
        "degreeType": "B.S.",
        "yearEarned": "1990",
        "degreeLevel": "Undergraduate"
    }]
}

如何使用 ObjectMapper 将自定义序列化应用于collegeEducation ？ 我不能使用注释JsonSerializer ，因为生成了 POJO，并且该库不提供应用该注释的方法。 所以我想对除列表之外的所有字段使用默认的 ObjectMapper 序列化。

Answer 1

Objective: Education 中一个名为degreeType的字段将被序列化为 json field degree

可以使用 Mixin 或自定义序列化程序

学生和教育 class

让我们定义学生和教育 class 如下

static class Education {
  String major;
  String degreeType;
}

static class Student {
  String legalLastName;
  String legalFirstName;
  List<Education> educationList;
}

米信

使用 mixin 定义具有更新字段规范的并行 class

Mixin class 教育用

1. 在实际object String degreeType;
2. 将要序列化的自定义字段名称添加到@JsonProperty("degree")

abstract class EducationMixin {
  @JsonProperty("degree")
  String degreeType;
}

使用 Mixin 配置 ObjectMapper

1. 启用字段可见性（因为 class 没有 getter/setter）
2. 注册教育 mixin

static void withMixin(Student student) throws Exception {
  ObjectMapper studentMapper = new ObjectMapper();
  studentMapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
  studentMapper = studentMapper.addMixIn(Education.class, EducationMixin.class);
  System.out.println(studentMapper.writeValueAsString(student));
}

或自定义序列化器

自定义序列化程序可用于序列化 java 对象。

为教育添加自定义序列化程序

将自定义序列化器 class 添加到序列化教育

static class EducationSerializer extends JsonSerializer<Education> {
  @Override
  public void serialize(Education education, JsonGenerator gen, SerializerProvider serializers)
    throws IOException {
    gen.writeStartObject();
    gen.writeStringField("major", education.major);
    gen.writeStringField("degree", education.degreeType);
    gen.writeEndObject();
  }
}

使用必要的配置初始化 ObjectMapper

1. 启用字段可见性（因为 class 没有 getter/setter）
2. 注册教育序列化程序

static void withCustom(Student student) throws Exception {
  ObjectMapper studentMapper = new ObjectMapper();
  studentMapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
  SimpleModule studentModule = new SimpleModule();
  studentModule.addSerializer(Education.class, new EducationSerializer());
  studentMapper.registerModule(studentModule);
  System.out.println(studentMapper.writeValueAsString(student));
}

完整的工作代码

import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.PropertyAccessor;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.module.SimpleModule;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class MixinTest {

    public static void main(String[] args) throws Exception {
        Student student = new Student();
        student.legalFirstName = "first";
        student.legalLastName = "last";

        Education education = new Education();
        education.degreeType = "degreetypevalue";
        education.major = "majorvalue";
        student.educationList = new ArrayList<>();
        student.educationList.add(education);

        withMixin(student);
        withCustom(student);
    }

    static void withMixin(Student student) throws Exception {
        ObjectMapper studentMapper = new ObjectMapper();
        studentMapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
        studentMapper = studentMapper.addMixIn(Education.class, EducationMixin.class);
        System.out.println(studentMapper.writeValueAsString(student));
    }

    static void withCustom(Student student) throws Exception {
        ObjectMapper studentMapper = new ObjectMapper();
        studentMapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY);
        SimpleModule studentModule = new SimpleModule();
        studentModule.addSerializer(Education.class, new EducationSerializer());
        studentMapper.registerModule(studentModule);
        System.out.println(studentMapper.writeValueAsString(student));
    }

    static class Education {
        String major;
        String degreeType;
    }

    static class Student {
        String legalLastName;
        String legalFirstName;
        List<Education> educationList;
    }

    abstract class EducationMixin {
        @JsonProperty("degree")
        String degreeType;
    }

    static class EducationSerializer extends JsonSerializer<Education> {
        @Override
        public void serialize(Education education, JsonGenerator gen, SerializerProvider serializers)
            throws IOException {
            gen.writeStartObject();
            gen.writeStringField("major", education.major);
            gen.writeStringField("degree", education.degreeType);
            gen.writeEndObject();
        }
    }
}