我在服务器日志中收到警告“firstResult/maxResults specified with collection fetch;apply in memory!”。但是一切正常。但我不想要这个警告。

我的代码是

public employee find(int id) {
    return (employee) getEntityManager().createQuery(QUERY).setParameter("id", id).getSingleResult();
}

我的查询是

QUERY = "from employee as emp left join fetch emp.salary left join fetch emp.department where emp.id = :id"

最佳答案

虽然您获得了有效的结果，但 SQL 查询会获取所有数据，但效率并没有达到应有的水平。

所以，你有两个选择。

解决了两个可以在读写模式下获取实体的 SQL 查询的问题

解决此问题的最简单方法是执行两个查询:

。第一个查询将获取与提供的过滤条件匹配的根实体标识符。 .第二个查询将使用之前提取的根实体标识符来获取父实体和子实体。

这种方法很容易实现，如下所示:

List<Long> postIds = entityManager
.createQuery(
    "select p.id " +
    "from Post p " +
    "where p.title like :titlePattern " +
    "order by p.createdOn", Long.class)
.setParameter(
    "titlePattern",
    "High-Performance Java Persistence %"
)
.setMaxResults(5)
.getResultList();
 
List<Post> posts = entityManager
.createQuery(
    "select distinct p " +
    "from Post p " +
    "left join fetch p.comments " +
    "where p.id in (:postIds) "  +
    "order by p.createdOn", Post.class)
.setParameter("postIds", postIds)
.setHint(
    "hibernate.query.passDistinctThrough", 
    false
)
.getResultList();

解决了一个只能在只读模式下获取实体的 SQL 查询的问题

第二种方法是对符合我们过滤条件的父实体和子实体的结果集使用 SDENSE_RANK，并仅限制前 N 个帖子条目的输出。

SQL 查询如下所示:

@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.id as \"pc.id\", " +
        "               pc.created_on AS \"pc.created_on\", " +
        "               pc.review AS \"pc.review\", " +
        "               pc.post_id AS \"pc.post_id\" " +
        "        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"),
            }
        )
    }
)

@NamedNativeQuery 获取与提供的标题匹配的所有 Post 实体及其关联的 PostComment 子实体。 DENSE_RANK 窗口函数用于为每个 Post 和 PostComment 加入记录分配排名，以便我们稍后可以过滤我们有兴趣获取的 Post 记录的数量。

SqlResultSetMapping 提供 SQL 级列别名和需要填充的 JPA 实体属性之间的映射。

现在，我们可以像这样执行 PostWithCommentByRank @NamedNativeQuery:

List<Post> posts = entityManager
.createNamedQuery("PostWithCommentByRank")
.setParameter(
    "titlePattern",
    "High-Performance Java Persistence %"
)
.setParameter(
    "rank",
    5
)
.unwrap(NativeQuery.class)
.setResultTransformer(
    new DistinctPostResultTransformer(entityManager)
)
.getResultList();

现在，默认情况下，像 PostWithCommentByRank 这样的原生 SQL 查询会在同一 JDBC 行中获取 Post 和 PostComment，所以我们最终会得到一个Object[] 包含两个实体。

但是，我们希望将表格 Object[] 数组转换为父子实体树，因此，我们需要使用 Hibernate ResultTransformer .

DistinctPostResultTransformer 如下所示:

public class DistinctPostResultTransformer
        extends BasicTransformerAdapter {
 
    private final EntityManager entityManager;
 
    public DistinctPostResultTransformer(
            EntityManager entityManager) {
        this.entityManager = entityManager;
    }
 
    @Override
    public List transformList(
            List list) {
             
        Map<Serializable, Identifiable> identifiableMap =
            new LinkedHashMap<>(list.size());
             
        for (Object entityArray : list) {
            if (Object[].class.isAssignableFrom(entityArray.getClass())) {
                Post post = null;
                PostComment comment = null;
 
                Object[] tuples = (Object[]) entityArray;
 
                for (Object tuple : tuples) {
                    if(tuple instanceof Identifiable) {
                        entityManager.detach(tuple);
 
                        if (tuple instanceof Post) {
                            post = (Post) tuple;
                        }
                        else if (tuple instanceof PostComment) {
                            comment = (PostComment) tuple;
                        }
                        else {
                            throw new UnsupportedOperationException(
                                "Tuple " + tuple.getClass() + " is not supported!"
                            );
                        }
                    }
                }
 
                if (post != null) {
                    if (!identifiableMap.containsKey(post.getId())) {
                        identifiableMap.put(post.getId(), post);
                        post.setComments(new ArrayList<>());
                    }
                    if (comment != null) {
                        post.addComment(comment);
                    }
                }
            }
        }
        return new ArrayList<>(identifiableMap.values());
    }
}

DistinctPostResultTransformer必须分离正在获取的实体，因为我们正在覆盖子集合，并且我们不希望它作为实体状态转换传播:

post.setComments(new ArrayList<>());

关于java - 使用 Hibernate 时如何避免警告 "firstResult/maxResults specified with collection fetch; applying in memory!"？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/11431670/