Question

最近，我开始了一份新工作（现在已经4个月了），我还有很多需要学习的东西。我们遇到的一个问题是，当我们从数据库中提取对象并将它们映射到（存储库 - ＆gt;服务 - ＆gt;控制器）JSON时，它会带来大量不必要的数据 - 例如（小例子）：

我们有一个Employee类，它有一个Department对象，它带来以下内容：

{
   'EmployeeId':1,
   'Name':'John',
   'Email':'email@moose.com'
   'Department': {
            'DepartmentId':1,
            'name':'Development',
            'location':'Somewhere',
   }
}

课程详情：

@Entity
@Table(name = "Employees")
@NamedQuery(name = "Employee.findAll", query = "SELECT e FROM Employees e")
public class Employee implements Serializable {
private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "EmployeeId")
    private long EmployeeId;

    @Column(name = "Email")
    private String email;

    @Column(name = "address")
    private String address;

    @ManyToOne
    @JoinColumn(name = "DepartmentId")
    @JsonBackReference(value = "department_employee")
    private Departament departament;

    // Other attributes, setters & getters

}

如果我想构建一个“自定义”映射器，这是最佳实践？或者我如何实现允许我创建一个JSON的东西，它带给我这样的东西：

{
   'EmployeeId':'1',
   'Name':'John',
   'Email':'email@moose.com'
}

我一直在做一些研究，我偶然发现了以下例子：

Jackson JSON Views

Jackson's @JsonView, @JsonFilter and Spring

此外，我一直在考虑使用包含HashMap的通用对象，因此我可以在Controller上包含我想要或需要特定屏幕的数据。

我真的很感激任何帮助 - 希望问题很清楚。

Answer 1

一个特别适合我的图书馆是Model Mapper。我使用它的方式是让你的Domain对象（就像你现在这样）和DTO。在您的情况下，Employee和EmployeeDTO。这样可以清楚地分离应用层，并且员工和employeeDto之间的映射非常简单，例如modelMapper(employee, EmployeeDTO.class)。对于简单的情况，您只需要在域类和dto类中命名具有相同名称的属性，并且库将负责复制值。对于更复杂的情况，它有几个替代方案仍然可以使您的代码清洁。

Answer 2

例如，假设您有一个JPA实体Comment（捕获用户发表的评论）并且您想要自定义序列化（从对象转换 - > JSON）

定义实体

//Comment Entity Attributes (This is just an example make sure you properly annotate the entity etc...):
private String id;
private final String userId;
private  String discussionId;
private final Date createdTime;
private final String comment;

创建自定义序列化程序

/*Serializer*/
import org.springframework.stereotype.Component;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;

@Component
@Slf4j
public class CommentSerializer extends JsonSerializer<Comment> {


    @Override
    public void serialize(Comment comment, JsonGenerator jgen, SerializerProvider provider)
            throws IOException,
            JsonProcessingException {
        jgen.writeStartObject();
        jgen.writeStringField("id", comment.getId());
        jgen.writeStringField("userId", comment.getUserId());
        jgen.writeStringField("firstName", redisRepo.getName(comment.getUserId(), false));
        jgen.writeStringField("discussionId", comment.getDiscussionId());
        jgen.writeStringField("createdTime", String.valueOf(comment.getCreatedTime().getTime()));
        jgen.writeStringField("comment", comment.getComment());
        jgen.writeEndObject();
    }


}

配置序列化程序

/*Config*/
import org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.boot.autoconfigure.web.HttpMessageConverters;

@Autowired
private CommentSerializer commentSerializer;

@Bean
public HttpMessageConverters httpMessageConverters() {
    return new HttpMessageConverters(mappingJackson2HttpMessageConverter());
}

   @Bean
    public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
        MappingJackson2HttpMessageConverter mappingJackson2JsonView = new MappingJackson2HttpMessageConverter();
        mappingJackson2JsonView.setObjectMapper(objectMapper());
        return mappingJackson2JsonView;
    }


   @Bean
    public ObjectMapper objectMapper() {



        Map<Class<?>, JsonSerializer<?>> serializerMap = new HashMap<>();
        serializerMap.put(Comment.class, commentSerializer);

        Jackson2ObjectMapperFactoryBean objectMapperFactoryBean = new Jackson2ObjectMapperFactoryBean();
        objectMapperFactoryBean.setIndentOutput(true);

        objectMapperFactoryBean.setSerializersByType(serializerMap);
        objectMapperFactoryBean.afterPropertiesSet();

        ObjectMapper objectMapper = objectMapperFactoryBean.getObject();
        objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
        objectMapper.findAndRegisterModules();
        objectMapper.addMixInAnnotations(OAuth2Exception.class, OAuth2ExceptionMixin.class);

        return objectMapper;
    }

现在每当调用相关的url / endpoint（如下所示）时，它总是按照CommentSerializer中的逻辑定义进行序列化。

    @RequestMapping(value = "/{id}", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE)
public Comment findOne(@PathVariable("id") String id)
        ....
    return comment;
}

JSON对象映射 - Spring Data

2 个答案: