领域模型
让我们考虑一下我们的数据库中有以下post和post_comment表：
JPASqlResultSetMapping
SqlResultSetMappingJPA 注释如下所示：
@Repeatable(SqlResultSetMappings.class)
@Target({TYPE})
@Retention(RUNTIME)
public @interface SqlResultSetMapping {
String name();
EntityResult[] entities() default {};
ConstructorResult[] classes() default {};
ColumnResult[] columns() default {};
}
注释是可重复的SqlResultSetMapping ，并应用于实体类级别。除了采用 Hibernate 用来注册映射的唯一名称之外，还有三个映射选项：
EntityResultConstructorResultColumnResult
接下来，我们将了解这三个映射选项是如何工作的，以及您需要使用它们的用例。
JPA SqlResultSetMapping - EntityResult
该EntityResult选项允许您将 JDBCResultSet列映射到一个或多个 JPA 实体。
假设我们要获取前 5 个实体以及与给定模式匹配的Post所有关联PostComment实体。title
正如我在本文中所解释的，我们可以使用DENSE_RANK SQL 窗口函数来了解如何过滤post和post_comment连接记录，如下面的 SQL 查询所示：
SELECT *
FROM (
SELECT
*,
DENSE_RANK() OVER (
ORDER BY
"p.created_on",
"p.id"
) rank
FROM (
SELECT
p.id AS "p.id", p.created_on AS "p.created_on",
p.title AS "p.title", pc.post_id AS "pc.post_id",
pc.id as "pc.id", pc.created_on AS "pc.created_on",
pc.review AS "pc.review"
FROM post p
LEFT JOIN post_comment pc ON p.id = pc.post_id
WHERE p.title LIKE :titlePattern
ORDER BY p.created_on
) p_pc
) p_pc_r
WHERE p_pc_r.rank <= :rank
但是，我们不想返回标量列值的列表。我们想从这个查询中返回 JPA 实体，所以我们需要配置注解的entities属性，如下所示：@SqlResultSetMapping
@NamedNativeQuery(
name = "PostWithCommentByRank",
query = """
SELECT *
FROM (
SELECT
*,
DENSE_RANK() OVER (
ORDER BY
"p.created_on",
"p.id"
) rank
FROM (
SELECT
p.id AS "p.id", p.created_on AS "p.created_on",
p.title AS "p.title", pc.post_id AS "pc.post_id",
pc.id as "pc.id", pc.created_on AS "pc.created_on",
pc.review AS "pc.review"
FROM post p
LEFT JOIN post_comment pc ON p.id = pc.post_id
WHERE p.title LIKE :titlePattern
ORDER BY p.created_on
) p_pc
) p_pc_r
WHERE p_pc_r.rank <= :rank
""",
resultSetMapping = "PostWithCommentByRankMapping"
)
@SqlResultSetMapping(
name = "PostWithCommentByRankMapping",
entities = {
@EntityResult(
entityClass = Post.class,
fields = {
@FieldResult(name = "id", column = "p.id"),
@FieldResult(name = "createdOn", column = "p.created_on"),
@FieldResult(name = "title", column = "p.title"),
}
),
@EntityResult(
entityClass = PostComment.class,
fields = {
@FieldResult(name = "id", column = "pc.id"),
@FieldResult(name = "createdOn", column = "pc.created_on"),
@FieldResult(name = "review", column = "pc.review"),
@FieldResult(name = "post", column = "pc.post_id"),
}
)
}
)
有了SqlResultSetMapping就位，我们可以像这样获取Post和PostComment实体：
List<Object[]> postAndCommentList = entityManager
.createNamedQuery("PostWithCommentByRank")
.setParameter("titlePattern", "High-Performance Java Persistence %")
.setParameter("rank", POST_RESULT_COUNT)
.getResultList();
而且，我们可以验证实体是否被正确获取：
assertEquals(
POST_RESULT_COUNT * COMMENT_COUNT,
postAndCommentList.size()
);
for (int i = 0; i < COMMENT_COUNT; i++) {
Post post = (Post) postAndCommentList.get(i)[0];
PostComment comment = (PostComment) postAndCommentList.get(i)[1];
assertTrue(entityManager.contains(post));
assertTrue(entityManager.contains(comment));
assertEquals(
"High-Performance Java Persistence - Chapter 1",
post.getTitle()
);
assertEquals(
String.format(
"Comment nr. %d - A must read!",
i + 1
),
comment.getReview()
);
}
这@EntityResult在通过 SQL 存储过程获取 JPA 实体时也很有用。查看这篇文章了解更多详情。
JPA SqlResultSetMapping - 构造函数结果
假设我们要执行一个聚合查询，计算post_coment每个记录的数量post并返回post title用于报告目的。我们可以使用下面的 SQL 查询来实现这个目标：
SELECT
p.id AS "p.id",
p.title AS "p.title",
COUNT(pc.*) AS "comment_count"
FROM post_comment pc
LEFT JOIN post p ON p.id = pc.post_id
GROUP BY p.id, p.title
ORDER BY p.id
我们还希望将帖子标题和评论数封装在以下 DTO 中：
public class PostTitleWithCommentCount {
private final String postTitle;
private final int commentCount;
public PostTitleWithCommentCount(
String postTitle,
int commentCount) {
this.postTitle = postTitle;
this.commentCount = commentCount;
}
public String getPostTitle() {
return postTitle;
}
public int getCommentCount() {
return commentCount;
}
}
要将上述 SQL 查询的结果集映射到PostTitleWithCommentCountDTO，我们可以使用注解的classes属性，如下所示：@SqlResultSetMapping
@NamedNativeQuery(
name = "PostTitleWithCommentCount",
query = """
SELECT
p.id AS "p.id",
p.title AS "p.title",
COUNT(pc.*) AS "comment_count"
FROM post_comment pc
LEFT JOIN post p ON p.id = pc.post_id
GROUP BY p.id, p.title
ORDER BY p.id
""",
resultSetMapping = "PostTitleWithCommentCountMapping"
)
@SqlResultSetMapping(
name = "PostTitleWithCommentCountMapping",
classes = {
@ConstructorResult(
columns = {
@ColumnResult(name = "p.title"),
@ColumnResult(name = "comment_count", type = int.class)
},
targetClass = PostTitleWithCommentCount.class
)
}
)
注解允许我们指示 Hibernate在ConstructorResult实例化 DTO 对象时使用哪个 DTO 类以及调用哪个构造函数。
请注意，我们使用注解的type属性@ColumnResult来指定comment_count应该将其转换为 Java int。这是必需的，因为某些 JDBC 驱动程序使用Long或BigInteger用于 SQL 聚合函数结果。
这是PostTitleWithCommentCount使用 JPA 调用命名本机查询的方式：
List<PostTitleWithCommentCount> postTitleAndCommentCountList = entityManager
.createNamedQuery("PostTitleWithCommentCount")
.setMaxResults(POST_RESULT_COUNT)
.getResultList();
而且，我们可以看到返回的PostTitleWithCommentCountDTO 已正确获取：
assertEquals(POST_RESULT_COUNT, postTitleAndCommentCountList.size());
for (int i = 0; i < POST_RESULT_COUNT; i++) {
PostTitleWithCommentCount postTitleWithCommentCount =
postTitleAndCommentCountList.get(i);
assertEquals(
String.format(
"High-Performance Java Persistence - Chapter %d",
i + 1
),
postTitleWithCommentCount.getPostTitle()
);
assertEquals(COMMENT_COUNT, postTitleWithCommentCount.getCommentCount());
}
有关使用 JPA 和 Hibernate 获取 DTO 投影的最佳方法的更多详细信息，请查看这篇文章。
JPA SqlResultSetMapping - 列结果
前面的示例展示了如何将 SQL 聚合结果集映射到 DTO。但是，如果我们想要返回我们正在计算评论的 JPA 实体怎么办？
为了实现这个目标，我们可以使用entities属性来定义Post我们正在获取的实体，并使用注释的classes属性@SqlResultSetMapping来映射标量值，在我们的例子中是关联post_comment记录的数量：
@NamedNativeQuery(
name = "PostWithCommentCount",
query = """
SELECT
p.id AS "p.id",
p.title AS "p.title",
p.created_on AS "p.created_on",
COUNT(pc.*) AS "comment_count"
FROM post_comment pc
LEFT JOIN post p ON p.id = pc.post_id
GROUP BY p.id, p.title
ORDER BY p.id
""",
resultSetMapping = "PostWithCommentCountMapping"
)
@SqlResultSetMapping(
name = "PostWithCommentCountMapping",
entities = @EntityResult(
entityClass = Post.class,
fields = {
@FieldResult(name = "id", column = "p.id"),
@FieldResult(name = "createdOn", column = "p.created_on"),
@FieldResult(name = "title", column = "p.title"),
}
),
columns = @ColumnResult(
name = "comment_count",
type = int.class
)
)
执行PostWithCommentCount命名的本机查询时：
List<Object[]> postWithCommentCountList = entityManager
.createNamedQuery("PostWithCommentCount")
.setMaxResults(POST_RESULT_COUNT)
.getResultList();
我们将获得Post实体和commentCount标量列值：
assertEquals(POST_RESULT_COUNT, postWithCommentCountList.size());
for (int i = 0; i < POST_RESULT_COUNT; i++) {
Post post = (Post) postWithCommentCountList.get(i)[0];
int commentCount = (int) postWithCommentCountList.get(i)[1];
assertTrue(entityManager.contains(post));
assertEquals(i + 1, post.getId().intValue());
assertEquals(
String.format(
"High-Performance Java Persistence - Chapter %d",
i + 1
),
post.getTitle()
);
assertEquals(COMMENT_COUNT, commentCount);
}