如何在Spring Data REST项目中使用DTO？

Question

Spring Data REST仅自动公开域对象。但是大多数情况下，我们必须处理数据传输对象。那么如何以SDR方式做到这一点呢？

Answer 1

Entities

实体必须实现Identifiable接口。例如：

@Entity
public class Category implements Identifiable<Integer> {

    @Id
    @GeneratedValue
    private final Integer id;

    private final String name;

    @OneToMany
    private final Set<Product> products = new HashSet<>();

    // skipped
}

@Entity
public class Product implements Identifiable<Integer> {

    @Id
    @GeneratedValue
    private final Integer id;

    private final String name;

    // skipped
}

Projections

创建一个投影接口，存储库查询方法将返回：

public interface CategoryProjection {

    Category getCategory();
    Long getQuantity();
}

这将是DTO的基础。在此示例中，DTO将代表a Category，而Products 的数量属于它。

Repository methods

Create方法将返回投影：一个投影，一个DTO列表和一个DTO页面列表。

@RepositoryRestResource
public interface CategoryRepo extends JpaRepository<Category, Integer> {

    @RestResource(exported = false)
    @Query("select c as category, count(p) as quantity from Category c join c.products p where c.id = ?1 group by c")
    CategoryProjection getDto(Integer categoryId);

    @RestResource(exported = false)
    @Query("select c as category, count(p) as quantity from Category c join c.products p group by c")
    List<CategoryProjection> getDtos();

    @RestResource(exported = false)
    @Query("select c as category, count(p) as quantity from Category c join c.products p group by c")
    Page<CategoryProjection> getDtos(Pageable pageable);
}

DTO

从其接口实施DTO：

@Relation(value = "category", collectionRelation = "categories")
public class CategoryDto implements CategoryProjection {

    private final Category category;
    private final Long quantity;

    // skipped
}

Relation当Spring Data REST渲染对象时使用注释。

控制者

向RepositoryRestController其中添加自定义方法将满足DTO的请求：

@RepositoryRestController
@RequestMapping("/categories")
public class CategoryController {

    @Autowired private CategoryRepo repo;
    @Autowired private RepositoryEntityLinks links;
    @Autowired private PagedResourcesAssembler<CategoryProjection> assembler;

    /**
    * Single DTO
    */
    @GetMapping("/{id}/dto")
    public ResponseEntity<?> getDto(@PathVariable("id") Integer categoryId) {
        CategoryProjection dto = repo.getDto(categoryId);

        return ResponseEntity.ok(toResource(dto));
    }

    /**
    * List of DTO
    */
    @GetMapping("/dto")
    public ResponseEntity<?> getDtos() {
        List<CategoryProjection> dtos = repo.getDtos();

        Link listSelfLink = links.linkFor(Category.class).slash("/dto").withSelfRel();
        List<?> resources = dtos.stream().map(this::toResource).collect(toList());

        return ResponseEntity.ok(new Resources<>(resources, listSelfLink));
    }

    /**
    * Paged list of DTO
    */
    @GetMapping("/dtoPaged")
    public ResponseEntity<?> getDtosPaged(Pageable pageable) {
        Page<CategoryProjection> dtos = repo.getDtos(pageable);

        Link pageSelfLink = links.linkFor(Category.class).slash("/dtoPaged").withSelfRel();
        PagedResources<?> resources = assembler.toResource(dtos, this::toResource, pageSelfLink);

        return ResponseEntity.ok(resources);
    }

    private ResourceSupport toResource(CategoryProjection projection) {
        CategoryDto dto = new CategoryDto(projection.getCategory(), projection.getQuantity());

        Link categoryLink = links.linkForSingleResource(projection.getCategory()).withRel("category");
        Link selfLink = links.linkForSingleResource(projection.getCategory()).slash("/dto").withSelfRel();

        return new Resource<>(dto, categoryLink, selfLink);
    }
}

从存储库收到Projections时，我们必须先完成从Projection到DTO的转换，然后将其“包装”到ResourceSupport对象，然后再发送给客户端。为此，我们使用辅助方法toResource：创建一个新的DTO，为此对象创建必要的链接，然后Resource使用该对象及其链接创建一个新的。

Result

请参阅Postman网站上的API文档

Singe DTO

GET http://localhost:8080/api/categories/6/dto

{
    "category": {
        "name": "category1"
    },
    "quantity": 3,
    "_links": {
        "category": {
            "href": "http://localhost:8080/api/categories/6"
        },
        "self": {
            "href": "http://localhost:8080/api/categories/6/dto"
        }
    }
}

List of DTO

GET http://localhost:8080/api/categories/dto

{
    "_embedded": {
        "categories": [
            {
                "category": {
                    "name": "category1"
                },
                "quantity": 3,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/6"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/6/dto"
                    }
                }
            },
            {
                "category": {
                    "name": "category2"
                },
                "quantity": 2,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/7"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/7/dto"
                    }
                }
            }
        ]
    },
    "_links": {
        "self": {
            "href": "http://localhost:8080/api/categories/dto"
        }
    }
}

Paged list of DTO

GET http://localhost:8080/api/categories/dtoPaged

{
    "_embedded": {
        "categories": [
            {
                "category": {
                    "name": "category1"
                },
                "quantity": 3,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/6"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/6/dto"
                    }
                }
            },
            {
                "category": {
                    "name": "category2"
                },
                "quantity": 2,
                "_links": {
                    "category": {
                        "href": "http://localhost:8080/api/categories/7"
                    },
                    "self": {
                        "href": "http://localhost:8080/api/categories/7/dto"
                    }
                }
            }
        ]
    },
    "_links": {
        "self": {
            "href": "http://localhost:8080/api/categories/dtoPaged"
        }
    },
    "page": {
        "size": 20,
        "totalElements": 2,
        "totalPages": 1,
        "number": 0
    }
}