Jackson 自定义序列化器：将对象更改为包含对象的简单属性和值

Question

我遇到的问题是我有一个在很多地方都使用的公共对象，在这个例子中，公共对象是学生。 Student 是另一个名为 Enrollment 的对象的属性（但是它可能是许多其他类的属性）。现在，如果没有对 Student 应用自定义序列化，我会得到这样的结果：

{"session":"9am Wednesday","student":{"firstName":"Joe","lastName":"Bloggs"}}

我想做的是为 Student 应用一个自定义序列化程序，以便它出现在我的代码中的任何地方，例如在这个例子中它在 Enrollment 类中，我会得到这个：

{"session":"9am Wednesday","firstName":"Joe","lastName":"Bloggs"}

或者如果我这样选择：

{"session":"9am Wednesday","first":"Joe","last":"Bloggs"}

或者甚至是这样：

{"session":"9am Wednesday","name":"Joe Bloggs"}

这是我的示例代码：

public class Enrolment {

    private String session;
    private Student student;

    public Enrolment(String session, Student student) {
        this.session = session;
        this.student = student;
    }

    public String getSession() {
        return session;
    }

    public void setSession(String session) {
        this.session = session;
    }

    public Student getStudent() {
        return student;
    }

    public void setStudent(Student student) {
        this.student = student;
    }
}

@JsonSerialize(using = StudentSerializer.class)
public class Student {

    private String firstName;
    private String lastName;

    public Student(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}

public class StudentSerializer extends JsonSerializer<Student> {

    @Override
    public void serialize(Student value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        // ??????
    }
}

public class Launcher {

  public static void main(String[] args) throws Exception{

      ObjectMapper mapper = new ObjectMapper();
      Student student = new Student("Joe", "Bloggs");
      Enrolment enrolment = new Enrolment("9am Wednesday", student);

      System.out.println(mapper.writeValueAsString(enrolment));

  }
}

如果您也可以提供一个反序列化器，这样它就可以接受所需的 json 序列化示例之一并从中创建 Student 对象，那将是一个巨大的好处。同样，反序列化器就像序列化器一样必须附加到 Student 对象，以便它出现的任何地方都会执行相同的操作。

谢谢你:)

Answer 1

您希望 @JsonUnwrapped 注释从 1.9 版开始可用。您可以查看版本 2.0 here 的文档，但基本上您会：

public class Enrolment {
    ...
    @JsonUnwrapped
    public Student getStudent() {
        return student;
    }
    ...
}

你会得到你的第一个选择：

{"session":"9am Wednesday","firstName":"Joe","lastName":"Bloggs"}

注解依赖于默认的序列化器。一方面，您不能为 Student 自定义序列化程序，但另一方面，您可以使用其他注释，例如 @JsonProperty 并自定义 Student 以获得第二个选项。

您的第三个选项最好在Student 中添加自定义getter 和setter。在这种情况下，您将使用@JsonIgnore 来避免序列化其他属性。

Answer 2

这是我偶然发现的另一个选择：

public class Enrolment {

    private String session;
    private Student student;

    public Enrolment(String session, Student student) {
        this.session = session;
        this.student = student;
    }

    public String getSession() {
        return session;
    }

    public void setSession(String session) {
        this.session = session;
    }

    @JsonUnwrapped
    public Student getStudent() {
        return student;
    }

    public void setStudent(Student student) {
        this.student = student;
    }
}

public class StudentUnwrappingBeanSerializer extends UnwrappingBeanSerializer {

    public StudentUnwrappingBeanSerializer(BeanSerializerBase src, NameTransformer transformer) {
        super(src, transformer);
    }

    @Override
    public JsonSerializer<Object> unwrappingSerializer(NameTransformer transformer) {
        return new StudentUnwrappingBeanSerializer(this, transformer);
    }

    @Override
    protected void serializeFields(Object bean, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {

        Student student = (Student) bean;
        jgen.writeStringField("first", student.getFirstName());
        jgen.writeStringField("last", student.getLastName());
    }

    @Override
    public boolean isUnwrappingSerializer() {
        return true;
    }
}

public class Launcher {

  public static void main(String[] args) throws Exception{

      ObjectMapper mapper = new ObjectMapper();

      mapper.registerModule(new Module() {
          @Override
          public String getModuleName() {
              return "my.module";
          }

          @Override
          public Version version() {
              return Version.unknownVersion();
          }

          @Override
          public void setupModule(SetupContext context) {

              context.addBeanSerializerModifier(new BeanSerializerModifier() {
                  @Override
                  public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer) {
                      if(beanDesc.getBeanClass().equals(Student.class)) {
                          return new StudentUnwrappingBeanSerializer((BeanSerializerBase) serializer, NameTransformer.NOP);
                      }
                      return serializer;
                  }
              });

          }
      });

      Student student = new Student("Joe", "Bloggs");
      ExtendableOption<StudyType> studyType = new ExtendableOption<>(StudyType.DISTANCE);
      Enrolment enrolment = new Enrolment("9am Wednesday", student, studyType);

      System.out.println(mapper.writeValueAsString(enrolment));

  }
}