杰克逊动态属性名称

Question

我想序列化一个对象,以便根据字段的类型对其中一个字段进行不同的命名.例如:

public class Response {
    private Status status;
    private String error;
    private Object data;
        [ getters, setters ]
    }

在这里,我希望将字段data序列化为类似的东西,data.getClass.getName()而不是始终data根据情况调用包含不同类型的字段.

我如何使用Jackson实现这样的技巧？

Answer 1

使用自定义JsonSerializer.

public class Response {
  private String status;
  private String error;

  @JsonProperty("p")
  @JsonSerialize(using = CustomSerializer.class)
  private Object data;

  // ...
}

public class CustomSerializer extends JsonSerializer<Object> {
  public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
    jgen.writeStartObject();
    jgen.writeObjectField(value.getClass().getName(), value);
    jgen.writeEndObject();
  }
}

然后,假设您要序列化以下两个对象:

public static void main(String... args) throws Exception {
  ObjectMapper mapper = new ObjectMapper();
  Response r1 = new Response("Error", "Some error", 20);
  System.out.println(mapper.writeValueAsString(r1));
  Response r2 = new Response("Error", "Some error", "some string");
  System.out.println(mapper.writeValueAsString(r2));
}

第一个将打印:

{"status":"Error","error":"Some error","p":{"java.lang.Integer":20}}

第二个:

{"status":"Error","error":"Some error","p":{"java.lang.String":"some string"}}

我已经使用了p包装器对象的名称,因为它只是用作placeholder.如果要删除它,则必须为整个类编写自定义序列化程序,即a JsonSerializer<Response>.

Answer 2

我有一个使用@JsonAnyGetter注释的简单解决方案,它就像一个魅力.

import java.util.Collections;
import java.util.Map;

public class Response {
    private Status status;
    private String error;

    @JsonIgnore
    private Object data;

    [getters, setters]

    @JsonAnyGetter
    public Map<String, Object> any() {
        //add the custom name here
        //use full HashMap if you need more than one property
        return Collections.singletonMap(data.getClass().getName(), data);
    }
}

不需要包装器,不需要自定义序列化器.

Answer 3

我自己的解决方案。

@Data
@EqualsAndHashCode
@ToString
@JsonSerialize(using = ElementsListBean.CustomSerializer.class)
public class ElementsListBean<T> {

    public ElementsListBean()
    {
    }

    public ElementsListBean(final String fieldName, final List<T> elements)
    {
        this.fieldName = fieldName;
        this.elements = elements;
    }

    private String fieldName;

    private List<T> elements;

    public int length()
    {
        return (this.elements != null) ? this.elements.size() : 0;
    }

    private static class CustomSerializer extends JsonSerializer<Object> {
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException,
                JsonProcessingException
        {
            if (value instanceof ElementsListBean) {
                final ElementsListBean<?> o = (ElementsListBean<?>) value;
                jgen.writeStartObject();
                jgen.writeArrayFieldStart(o.getFieldName());
                for (Object e : o.getElements()) {
                    jgen.writeObject(e);
                }
                jgen.writeEndArray();
                jgen.writeNumberField("length", o.length());
                jgen.writeEndObject();
            }
        }
    }
}