获取比你实际所需要的更多数据并不好，此外，当您不打算修改实体时，获取实体（通过在持久化上下文中加入的方式获取实体）是最常见的错误之一，它隐含性能损失。
因此，使用DTO可允许我们仅提取所需的数据。在这个应用程序中，我们依赖 @SqlResultSetMapping和EntityManager来实现。
假设三个实体TopCategory和MiddleCategory是一对多关系，MiddleCategory和BottomCategory 是一对多关系，现在需要分别从TopCategory、MiddleCategory和BottomCategory 中提取各自的namet、namem和nameb字段组成CategoryDto：
TopCategory代码如下，我们使用了注释@SqlResultSetMapping构造获得CategoryDto的字段，TopCategory是父级根实体(DDD聚合中的实体根)。
@SqlResultSetMapping(name = "CategoryDtoMapping",
classes = {
@ConstructorResult(
targetClass = CategoryDto.class,
columns = {
@ColumnResult(name = "namet"),
@ColumnResult(name = "namem"),
@ColumnResult(name = "nameb")
}
)}
)
@Entity
@Table(name = "top_category")
public class TopCategory implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(cascade = CascadeType.ALL,
mappedBy = "topCategory", orphanRemoval = true)
private List<MiddleCategory> middleCategories = new ArrayList<>();
// helper methods
public void addMiddleCategory(MiddleCategory middleCategory) {
middleCategories.add(middleCategory);
middleCategory.setTopCategory(this);
}
public void removeMiddleCategory(MiddleCategory middleCategory) {
middleCategory.setTopCategory(null);
middleCategories.remove(middleCategory);
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<MiddleCategory> getMiddleCategories() {
return middleCategories;
}
public void setMiddleCategories(List<MiddleCategory> middleCategories) {
this.middleCategories = middleCategories;
}
}
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然后实现自己的DAO仓储，在其中调用EntityManager的createNativeQuery，其他方式点击标签#DTO可看到更多EntityManager调用方式。
@Repository
@Transactional
public class Dao<T, ID extends Serializable> implements GenericDao<T, ID> {
@PersistenceContext
private EntityManager entityManager;
@Override
public <S extends T> S persist(S entity) {
Objects.requireNonNull(entity, "Cannot persist a null entity");
entityManager.persist(entity);
return entity;
}
@Transactional(readOnly=true)
public List<CategoryDto> fetchCategories() {
Query query = entityManager.createNativeQuery(
"SELECT t.name AS namet, m.name AS namem, b.name AS nameb "
+ "FROM middle_category m "
+ "INNER JOIN top_category t ON t.id=m.top_category_id "
+ "INNER JOIN bottom_category b ON m.id=b.middle_category_id", "CategoryDtoMapping");
List<CategoryDto> result = query.getResultList();
return result;
}
protected EntityManager getEntityManager() {
return entityManager;
}
}
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下面是Dao客户端Service的调用方式：
@Service
public class CategoryService {
private final Dao dao;
public CategoryService(Dao dao) {
this.dao = dao;
}
public List<CategoryDto> fetchCategories() {
return dao.fetchCategories();
}
}
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源代码可以在这里找到