我有一个实体,希望在加载时能够实现多对一的懒加载关系。
@Entity
public class Product {
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name = "atc_code_id")
private ATCCode atcCode;
}
@Entity
public class ATCCode {
@OneToMany(mappedBy = "atcCode")
private Set<Product> products;
}
我正在使用以下代码来加载产品:
Query q = entityManager.createQuery("select p from Product as p");
List<Product> products = q.getResultList();
从Hibernate日志的倒数第二行可以看到,发出了第二个select语句来初始化atcCode代理。为什么会这样呢?
17925 07 Aug 2015 11:45:20 SQL DEBUG 352066 kb - select product0_.id ...
17926 07 Aug 2015 11:45:20 Loader DEBUG 348515 kb - Result set row: 0
17927 07 Aug 2015 11:45:20 Loader DEBUG 348515 kb - Result row: EntityKey[com.galexis.search.importer.search.searchdb.model.Product#1]
17928 07 Aug 2015 11:45:20 Loader DEBUG 348515 kb - Result set row: 1
17929 07 Aug 2015 11:45:20 Loader DEBUG 348515 kb - Result row: EntityKey[com.galexis.search.importer.search.searchdb.model.Product#2]
17929 07 Aug 2015 11:45:20 TwoPhaseLoad DEBUG 348515 kb - Resolving associations for [com.galexis.search.importer.search.searchdb.model.Product#1]
17931 07 Aug 2015 11:45:20 TwoPhaseLoad DEBUG 348515 kb - Done materializing entity [com.galexis.search.importer.search.searchdb.model.Product#1]
17931 07 Aug 2015 11:45:20 TwoPhaseLoad DEBUG 348515 kb - Resolving associations for [com.galexis.search.importer.search.searchdb.model.Product#2]
17931 07 Aug 2015 11:45:20 TwoPhaseLoad DEBUG 348515 kb - Done materializing entity [com.galexis.search.importer.search.searchdb.model.Product#2]
17931 07 Aug 2015 11:45:20 SessionImpl DEBUG 348515 kb - Initializing proxy: [com.galexis.search.importer.search.searchdb.model.ATCCode#100]
17931 07 Aug 2015 11:45:20 SQL DEBUG 348515 kb - select atccode0_.id ...
我希望返回一个代理以替代atcCode。只要我不访问atcCode,它就不应该从数据库中加载。
你有任何想法为什么Hibernate会初始化代理对象吗?
这是问题的简化版本。实际上,我的产品(product)有很多多对一(many-to-one)关系,它们都被声明为懒加载(lazy)。但由于它们实际上被急加载了(eagerly),所以我会遭受到大量的N+1查询性能损失。