Spring boot Jackson Json Serialization for List 实现类（PagedList）

Question

有没有办法为具有自定义属性的列表实现类添加序列化？

我正在使用 Spring-boot 1.3 开发 Rest 服务。我必须以 Paged-List 或 Normal-List 的形式返回 JSON 响应，具体取决于 Controller 上的请求。所以，我必须将控制器方法的返回类型保持为通用public List<Employee> getEmployees(int departmentId)

我正在实现如下列表（使用泛型用于不同的对象列表）

public class PagedList<E> implements List<E>  {
  private List<E> list;
  private long totalRecords; //Getter-setters are added

  public PagedList(List<E> list) {
    super();
    this.list = list;
  }

  public PagedList(List<E> list, long totalRecords) {
    super();
    this.list = list;
    this.totalRecords = totalRecords;
  }

  @Override
  public boolean add(E element) {
     return this.list.add(element);
  }
  //All other List abstract methods implemented same as above using this.list
}

为相同添加了JsonSerializer：public class PagedListSerializer extends JsonSerializer<PagedList>，serialize() 方法中的序列化逻辑。使用spring-boot jackson customization注册：

@Bean
public Jackson2ObjectMapperBuilder objectMapperBuilder() {
    Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
    builder.serializerByType(PagedList.class, new PagedListSerializer());
    return builder;
}

当我尝试return PagedList<Employee>(list, 1000) 时，我无法得到以下回复。它的返回与普通列表相同。不执行自定义序列化。如何获得以下分页响应？

{ 
  list : [{employeeId: "1", name: "John" }, ... ],
  totalRecords : 1000
}

Answer 1

您可能不需要自定义反序列化器来获取此 json。只需在您的课程中添加 @JsonFormat(shape = JsonFormat.Shape.OBJECT) 注释即可：

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
public static class PagedList<E> implements List<E>  {
    @JsonProperty
    private List<E> list;

    @JsonProperty // no need for this if you have getter-setters 
    private long totalRecords; 

    @JsonIgnore
    @Override
    public boolean isEmpty() {
        return false;
    }

...

这里是完整的演示：https://gist.github.com/varren/35c4ede769499b1290f98e39a2f85589

cmets 后更新：

我认为 Spring 使用 Jacksons return mapper.writerFor(List.class).writeValueAsString(new MyList()); 这是演示：

@RestController
@RequestMapping(value="/")
public static class MyRestController  {
    private static final ObjectMapper mapper = new ObjectMapper();

    //returns [] for both 0 and 1
    @RequestMapping(value="test", method = RequestMethod.GET)
    public List test(@RequestParam int user) {
        return user == 0 ? new ArrayList(): new MyList();
    }

    //returns [] for 0 and expected custom {"empty": true} for 1
    @RequestMapping(value="testObj", method = RequestMethod.GET)
    public Object testObj(@RequestParam int user) {
        return user == 0 ? new ArrayList(): new MyList();
    }

    // returns expected custom {"empty": true}
    @RequestMapping(value="testMyList", method = RequestMethod.GET)
    public MyList testMyList() {
        return new MyList();
    }

    // returns expected custom {"empty": true}
    @RequestMapping(value="testMyListMapper", method = RequestMethod.GET)
    public String testMyListMapper() throws JsonProcessingException {
        return mapper.writeValueAsString(new MyList());
    }

    // returns []
    @RequestMapping(value="testMyListMapperListWriter", method = RequestMethod.GET)
    public String testMyListMapperListWriter() throws JsonProcessingException {
        return mapper.writerFor(List.class).writeValueAsString(new MyList());
    }
}
@JsonFormat(shape = JsonFormat.Shape.OBJECT)
public static class MyList extends ArrayList {}

所以您必须选项 1）返回 Object 而不是 List 或 选项 2）为 List 注册自定义序列化程序（而不是 PageList）builder.serializerByType(List.class, new PagedListSerializer()); 像这样：

public class PagedListSerializer extends JsonSerializer<List> {
  @Override
  public void serialize(List valueObj, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
    if (valueObj instanceof PagedList) {
      PagedList value = (PagedList) valueObj;
      gen.writeStartObject();
      gen.writeNumberField("totalRecords", value.getTotalRecords());
      gen.writeObjectField("list", value.getList());
      gen.writeEndObject();
    }else{
      gen.writeStartArray();
      for(Object obj : valueObj)
        gen.writeObject(obj);
      gen.writeEndArray();
    }
  }
}

Answer 2

您可以创建您的 customObject Mapper 并在那里使用您的序列化程序。

  <mvc:annotation-driven>
    <mvc:message-converters>
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="objectMapper">
                <bean class="custom.CustomObjectMapper"/>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>