JPA hibernate FetchType.EAGER LAZY 一对多，多对一用对象导航查询

最新推荐文章于 2024-12-17 02:06:10 发布

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最新推荐文章于 2024-12-17 02:06:10 发布

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本文深入探讨ORM中一查多与多查一场景下，EAGER与LAZY加载策略的区别及其实现原理。通过具体案例，分析不同场景下加载策略的选择对性能的影响，以及如何根据实际需求调整。

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坑的地方，2个默认不一样，仔细想想符合实际，记法：OL,ME

OneToMany.class:
FetchType fetch() default FetchType.LAZY; 因为多方数据量大，先find 一 get多用懒加载根据实际应用场景，完全有必要懒加载

ManyToOne.class
FetchType fetch() default FetchType.EAGER; 因为多查一，只关联一条主表数据，直接用立即加载，根据实际应用场景，无所谓

一查多（观察sql日志的from 后是否join
   LAZY:
       One的get方法时执行sql获取多的集合 from 多方数据表 where 外键=
   EAGER
       findOne一句sql获取全部 from cst_customer customer0_ left outer join

   多查一
   One：默认LAZY
       MANY EAGER：一条sql, join
       MANY LAZY：两条sql 1sql.查M表，带外键字段    2sql.用外键查主表

   One：EAGER
       MANY EAGER： get前2条(1.join 已经查了全部数据 ,2多表又去查了自己——感觉多余)
       MANY LAZY： 2条sql，1多表查自己，2join查主表

视频案例 https://www.bilibili.com/video/av53910024?p=85

多查一时，双 FetchType.EAGER 多表多余sql

Hibernate: select linkman0_.lkm_id as lkm_id1_1_0_,
 linkman0_.lkm_cust_id as lkm_cust9_1_0_,
 linkman0_.lkm_email as lkm_emai2_1_0_,
 linkman0_.lkm_gender as lkm_gend3_1_0_,
 linkman0_.lkm_memo as lkm_memo4_1_0_,
 linkman0_.lkm_mobile as lkm_mobi5_1_0_,
 linkman0_.lkm_name as lkm_name6_1_0_,
 linkman0_.lkm_phone as lkm_phon7_1_0_,
 linkman0_.lkm_position as lkm_posi8_1_0_,
 customer1_.cust_id as cust_id1_0_1_,
 customer1_.cust_address as cust_add2_0_1_,
 customer1_.cust_industry as cust_ind3_0_1_,
 customer1_.cust_level as cust_lev4_0_1_,
 customer1_.cust_name as cust_nam5_0_1_,
 customer1_.cust_phone as cust_pho6_0_1_,
 customer1_.cust_source as cust_sou7_0_1_ from cst_linkman linkman0_ left outer join cst_customer customer1_ on linkman0_.lkm_cust_id=customer1_.cust_id where linkman0_.lkm_id=?

 Hibernate: select linkmans0_.lkm_cust_id as lkm_cust9_1_0_,
 linkmans0_.lkm_id as lkm_id1_1_0_,
 linkmans0_.lkm_id as lkm_id1_1_1_,
 linkmans0_.lkm_cust_id as lkm_cust9_1_1_,
 linkmans0_.lkm_email as lkm_emai2_1_1_,
 linkmans0_.lkm_gender as lkm_gend3_1_1_,
 linkmans0_.lkm_memo as lkm_memo4_1_1_,
 linkmans0_.lkm_mobile as lkm_mobi5_1_1_,
 linkmans0_.lkm_name as lkm_name6_1_1_,
 linkmans0_.lkm_phone as lkm_phon7_1_1_,
 linkmans0_.lkm_position as lkm_posi8_1_1_ from cst_linkman linkmans0_ where linkmans0_.lkm_cust_id=?