java - 将 JPA 或 Hibernate 投影查询映射到 DTO(数据传输对象)

Question

在我的 DAO 层中，我有一个像这样的 Find 函数

public List<?> findCategoryWithSentenceNumber(int offset, int maxRec) {
  Criteria crit = getSession().createCriteria(Category.class, "cate");
    crit.createAlias("cate.sentences", "sent");

    crit.setProjection(Projections.projectionList().
    add(Projections.property("title"), "title").
    add(Projections.count("sent.id"), "numberOfSentence").
    add(Projections.groupProperty("title"))
  );

  crit.setFirstResult(offset);
  crit.setMaxResults(maxRec);

  return crit.list();
}

所以，为了读取数据，我必须使用循环(带有 Iterator)

List<?> result = categoryDAO.findCategoryWithSentenceNumber(0, 10);
// List<DQCategoryDTO> dtoList = new ArrayList<>(); 

for (Iterator<?> it = result.iterator(); it.hasNext(); ) {
  Object[] myResult = (Object[]) it.next();

  String  title = (String) myResult[0];
  Long count = (Long) myResult[1];


  assertEquals("test", title); 
  assertEquals(1, count.intValue()); 

  // dQCategoryDTO = new DQCategoryDTO();
  // dQCategoryDTO.setTitle(title);
  // dQCategoryDTO.setNumberOfSentence(count);
  // dtoList.add(dQCategoryDTO);

}

我的问题是:是否有任何 api、框架可以轻松转换 List<?> result进入 DTO 的列表对象(例如，DQCategoryDTO)不使用任何循环、迭代器和调用 setter/getter 来填充值？

Answer 1

您有很多选项可以将您的投影映射到 DTO 结果集:

使用元组和 JPQL 的 DTO 投影

List<Tuple> postDTOs = entityManager.createQuery("""
    select
           p.id as id,
           p.title as title
    from Post p
    where p.createdOn > :fromTimestamp
    """, Tuple.class)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of(2016, 1, 1, 0, 0, 0)
        .toInstant(ZoneOffset.UTC )))
.getResultList();

assertFalse(postDTOs.isEmpty());
 
Tuple postDTO = postDTOs.get(0);
assertEquals( 
    1L, 
    postDTO.get("id") 
);

使用构造函数表达式和 JPQL 的 DTO 投影

List<PostDTO> postDTOs = entityManager.createQuery("""
    select new com.vladmihalcea.book.hpjp.hibernate.query.dto.projection.jpa.PostDTO(
        p.id,
        p.title
    )
    from Post p
    where p.createdOn > :fromTimestamp
    """, PostDTO.class)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

You can also omit the DTO package name from the JPA constructor expression, and reference the DTO by its simple Java class name (e.g., PostDTO).

List<PostDTO> postDTOs = entityManager.createQuery("""
    select new PostDTO(
        p.id,
        p.title
    )
    from Post p
    where p.createdOn > :fromTimestamp

      """, PostDTO.class)

.setParameter( "fromTimestamp", Timestamp.from(
  LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
      .toInstant( ZoneOffset.UTC ) ))
.getResultList();

使用元组和原生 SQL 查询的 DTO 投影

这个可以从 Hibernate 5.2.11 开始，所以还有一个升级的理由。

List<Tuple> postDTOs = entityManager.createNativeQuery("""
    SELECT
           p.id AS id,
           p.title AS title
    FROM Post p
    WHERE p.created_on > :fromTimestamp
    """, Tuple.class)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

使用 ConstructorResult 的 DTO 投影

如果我们使用之前介绍的相同的PostDTO类类型，我们必须提供以下@SqlResultSetMapping:

@NamedNativeQuery(
    name = "PostDTO",
    query = """
        SELECT
               p.id AS id,
               p.title AS title
        FROM Post p
        WHERE p.created_on > :fromTimestamp
        """,
    resultSetMapping = "PostDTO"
)
@SqlResultSetMapping(
    name = "PostDTO",
    classes = @ConstructorResult(
        targetClass = PostDTO.class,
        columns = {
            @ColumnResult(name = "id"),
            @ColumnResult(name = "title")
        }
    )
)

现在，名为 native query 的 SQL 投影执行如下:

List<PostDTO> postDTOs = entityManager.createNamedQuery("PostDTO")
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

使用 ResultTransformer 和 JPQL 进行 DTO 投影

这一次，您的 DTO 需要为您需要 Hibernate 从底层 JDBC ResultSet 填充的属性设置 setter 。

DTO 投影如下所示:

List<PostDTO> postDTOs = entityManager.createQuery("""
    select
           p.id as id,
           p.title as title
    from Post p
    where p.createdOn > :fromTimestamp
    """)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 ).toInstant( ZoneOffset.UTC ) ))
.unwrap( org.hibernate.query.Query.class )
.setResultTransformer( Transformers.aliasToBean( PostDTO.class ) )
.getResultList();

使用 ResultTransformer 和原生 SQL 查询的 DTO 投影

List postDTOs = entityManager.createNativeQuery("""
    select
           p.id as \"id\",
           p.title as \"title\"
    from Post p
    where p.created_on > :fromTimestamp
    """)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 ).toInstant( ZoneOffset.UTC ) ))
.unwrap( org.hibernate.query.NativeQuery.class )
.setResultTransformer( Transformers.aliasToBean( PostDTO.class ) )
.getResultList();