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JPA Session 一勞永逸

tyheist / 2873人閱讀

摘要：引言再回顧一下問題場景教師班級學生在單元測試中跑這段代碼，是報錯的，，說明執(zhí)行完之后，就已經關閉了。如果是在單元測試中，就去想想是不是事務配錯了，導致掛掉了。茲可謂一勞而久逸。班固封燕然山銘一勞永逸，這是程序員最快樂的時候。

引言

再回顧一下問題場景：

Iterable teachers = teacherRepository.findAll();
for (Teacher teacher : teachers) {
    logger.debug("教師: " + teacher.getName());

    for (Klass klass : teacher.getKlasses()) {
        logger.debug("班級: " + klass.getName());

        for (Student student : klass.getStudents()) {
            logger.debug("學生: " + student.getName());
        }
    }
}

在單元測試中跑這段代碼，是報錯的，no Session，說明執(zhí)行完teacherRepository.findAll()之后，session就已經關閉了。繼續(xù)執(zhí)行，session已經關閉，再去數據庫查教師關聯的班級信息，就錯了。

然而呢？把這段代碼再放到Service里，寫一個接口，交給瀏覽器去調用，卻正常執(zhí)行，說明session還在。

然后就一直研究為什么不好使？如果能把這個原因分析明白，以后再遇到no session錯誤的時候就可以一勞永逸了。

探究 調試

調試最簡單的方法就是中斷，但是咱水平還不行，也不知道JPA內部去找Hibernate怎么調用的，中斷哪個方法呢？

后臺發(fā)現了另一種調試的方法，JPA的源碼中也是像我們開發(fā)時經常寫日志的，logger.debug()什么的。

slf4j中常用的日志級別就ERROR、WARN、INFO、DEBUG四種，我們可以將JPA的日志級別設置為DEBUG級別，這樣我們就可以根據日志推測到JPA內部到底是怎么執(zhí)行的了。

修改日志級別 
logging.level.org.springframework.orm.jpa=debug

修改配置文件，將JPA的日志級別設置為DEBUG。

在單元測試中執(zhí)行

完整日志：

2019-06-06 11:36:40.415 DEBUG 11391 --- [           main] o.s.orm.jpa.JpaTransactionManager        : Creating new transaction with name [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly
2019-06-06 11:36:40.416 DEBUG 11391 --- [           main] o.s.orm.jpa.JpaTransactionManager        : Opened new EntityManager [SessionImpl(1334204880)] for JPA transaction
2019-06-06 11:36:40.429 DEBUG 11391 --- [           main] o.s.orm.jpa.JpaTransactionManager        : Exposing JPA transaction as JDBC [org.springframework.orm.jpa.vendor.HibernateJpaDialect$HibernateConnectionHandle@73b74615]
2019-06-06 11:36:40.449  INFO 11391 --- [           main] o.h.h.i.QueryTranslatorFactoryInitiator  : HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select teacher0_.id as id1_2_, teacher0_.name as name2_2_ from teacher teacher0_
2019-06-06 11:36:40.598 DEBUG 11391 --- [           main] o.s.orm.jpa.JpaTransactionManager        : Initiating transaction commit
2019-06-06 11:36:40.598 DEBUG 11391 --- [           main] o.s.orm.jpa.JpaTransactionManager        : Committing JPA transaction on EntityManager [SessionImpl(1334204880)]
2019-06-06 11:36:40.601 DEBUG 11391 --- [           main] o.s.orm.jpa.JpaTransactionManager        : Closing JPA EntityManager [SessionImpl(1334204880)] after transaction
2019-06-06 11:36:40.602 DEBUG 11391 --- [           main] com.yunzhiclub.jpa.JpaApplicationTests   : 教師: 張三

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.yunzhiclub.jpa.entity.Teacher.klasses, could not initialize proxy - no Session
分析 
Creating new transaction with name [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly
Opened new EntityManager [SessionImpl(1334204880)] for JPA transaction
Exposing JPA transaction as JDBC [org.springframework.orm.jpa.vendor.HibernateJpaDialect$HibernateConnectionHandle@73b74615]
HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select teacher0_.id as id1_2_, teacher0_.name as name2_2_ from teacher teacher0_
Initiating transaction commit
Committing JPA transaction on EntityManager [SessionImpl(1334204880)]

上來是先執(zhí)行了一個事務，為什么會有事務呢？

JPA創(chuàng)建的倉庫實現是SimpleJpaRepository，我們看看源碼：

實現類上添加了事務注解，并采用了默認的REQUIRED傳播級別。

如果當前存在事務，則使用當前事務。如果不存在任何事務，則創(chuàng)建一個新的事務。

當前不存在事務，所以是teacherRepository.findAll()方法自己創(chuàng)建的事務。

Hibernate: select teacher0_.id as id1_2_, teacher0_.name as name2_2_ from teacher teacher0_
Initiating transaction commit
Committing JPA transaction on EntityManager [SessionImpl(1334204880)]
Closing JPA EntityManager [SessionImpl(1334204880)] after transaction
教師: 張三

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.yunzhiclub.jpa.entity.Teacher.klasses, could not initialize proxy - no Session

再接著看下面的日志，執(zhí)行了數據庫的查詢操作，提交了一個事務，然后Closing JPA EntityManager [SessionImpl(1334204880)] after transaction。

事務執(zhí)行之后，就關閉了EntityManager，也就是Hibernate中的Session。

Session is a hibernate-specific API, EntityManager is a standardized API for JPA.

EntityManager和Session還是有一些差別的，但是我們目前還未接觸到底層的實現，只需要把他們當成一個東西，只不過在不同領域叫法不同罷了。

在SpringMVC中執(zhí)行

執(zhí)行得很順利，完整日志如下：

2019-06-06 11:58:28.788 DEBUG 11443 --- [nio-8080-exec-1] o.j.s.OpenEntityManagerInViewInterceptor : Opening JPA EntityManager in OpenEntityManagerInViewInterceptor
2019-06-06 11:58:28.800 DEBUG 11443 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager        : Found thread-bound EntityManager [SessionImpl(193939447)] for JPA transaction
2019-06-06 11:58:28.800 DEBUG 11443 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager        : Creating new transaction with name [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly
2019-06-06 11:58:28.808 DEBUG 11443 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager        : Exposing JPA transaction as JDBC [org.springframework.orm.jpa.vendor.HibernateJpaDialect$HibernateConnectionHandle@56eb5438]
2019-06-06 11:58:28.820  INFO 11443 --- [nio-8080-exec-1] o.h.h.i.QueryTranslatorFactoryInitiator  : HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select teacher0_.id as id1_2_, teacher0_.name as name2_2_ from teacher teacher0_
2019-06-06 11:58:28.897 DEBUG 11443 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager        : Initiating transaction commit
2019-06-06 11:58:28.898 DEBUG 11443 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager        : Committing JPA transaction on EntityManager [SessionImpl(193939447)]
2019-06-06 11:58:28.901 DEBUG 11443 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager        : Not closing pre-bound JPA EntityManager after transaction
2019-06-06 11:58:28.902 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 教師: 張三
Hibernate: select klasses0_.teacher_id as teacher_3_0_0_, klasses0_.id as id1_0_0_, klasses0_.id as id1_0_1_, klasses0_.name as name2_0_1_, klasses0_.teacher_id as teacher_3_0_1_ from klass klasses0_ where klasses0_.teacher_id=?
2019-06-06 11:58:28.915 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 班級: 軟件工程
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
2019-06-06 11:58:28.917 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: Hello Kitty
2019-06-06 11:58:28.917 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 史努比
2019-06-06 11:58:28.917 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 班級: 網絡工程
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
2019-06-06 11:58:28.919 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 米老鼠
2019-06-06 11:58:28.919 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 唐老鴨
2019-06-06 11:58:28.919 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 教師: 李四
Hibernate: select klasses0_.teacher_id as teacher_3_0_0_, klasses0_.id as id1_0_0_, klasses0_.id as id1_0_1_, klasses0_.name as name2_0_1_, klasses0_.teacher_id as teacher_3_0_1_ from klass klasses0_ where klasses0_.teacher_id=?
2019-06-06 11:58:28.921 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 班級: 計算機科學與技術
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
2019-06-06 11:58:28.923 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 哪吒
2019-06-06 11:58:28.923 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 小竹熊
2019-06-06 11:58:28.923 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 班級: 物聯網
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
2019-06-06 11:58:28.925 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 喜羊羊
2019-06-06 11:58:28.925 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 學生: 灰太狼
2019-06-06 11:58:28.944 DEBUG 11443 --- [nio-8080-exec-1] o.j.s.OpenEntityManagerInViewInterceptor : Closing JPA EntityManager in OpenEntityManagerInViewInterceptor
分析 
Opening JPA EntityManager in OpenEntityManagerInViewInterceptor
Found thread-bound EntityManager [SessionImpl(193939447)] for JPA transaction
Creating new transaction with name [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly
Exposing JPA transaction as JDBC [org.springframework.orm.jpa.vendor.HibernateJpaDialect$HibernateConnectionHandle@56eb5438]
HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select teacher0_.id as id1_2_, teacher0_.name as name2_2_ from teacher teacher0_
Initiating transaction commit
Committing JPA transaction on EntityManager [SessionImpl(193939447)]

這段沒什么說的，和上面一樣，創(chuàng)建事務，執(zhí)行完提交事務。

Not closing pre-bound JPA EntityManager after transaction
教師: 張三
Hibernate: select klasses0_.teacher_id as teacher_3_0_0_, klasses0_.id as id1_0_0_, klasses0_.id as id1_0_1_, klasses0_.name as name2_0_1_, klasses0_.teacher_id as teacher_3_0_1_ from klass klasses0_ where klasses0_.teacher_id=?
班級: 軟件工程
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
學生: Hello Kitty
學生: 史努比
班級: 網絡工程
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
學生: 米老鼠
學生: 唐老鴨
教師: 李四
Hibernate: select klasses0_.teacher_id as teacher_3_0_0_, klasses0_.id as id1_0_0_, klasses0_.id as id1_0_1_, klasses0_.name as name2_0_1_, klasses0_.teacher_id as teacher_3_0_1_ from klass klasses0_ where klasses0_.teacher_id=?
班級: 計算機科學與技術
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
學生: 哪吒
學生: 小竹熊
班級: 物聯網
Hibernate: select students0_.klass_id as klass_id4_1_0_, students0_.id as id1_1_0_, students0_.id as id1_1_1_, students0_.email as email2_1_1_, students0_.klass_id as klass_id4_1_1_, students0_.name as name3_1_1_ from student students0_ where students0_.klass_id=?
學生: 喜羊羊
學生: 灰太狼
Closing JPA EntityManager in OpenEntityManagerInViewInterceptor

這塊就有意思了。

第一行：Not closing pre-bound JPA EntityManager after transaction

最后一行：Closing JPA EntityManager in OpenEntityManagerInViewInterceptor

在事務之后沒有關閉Session，一直到最后，才將Session關閉，所以沒出錯。

而在單元測試中呢？Closing JPA EntityManager [SessionImpl(1334204880)] after transaction，事務執(zhí)行之后就關閉了Session，所以出錯了。

原因

找著關鍵了，Service里好使是因為Session在findAll的事務執(zhí)行完之后沒有關閉。

之前怎么沒發(fā)現呢？StackOverflow上這老哥和我是一樣的問題：Hibernate jpa entity manager not being closed in spring service layer - StackOverflow

User和Address一對多。

我寫了一個getUser方法，惰性加載的一對多，但是為什么序列化的時候去找addresses的時候，它不報lazy initialization的錯誤呢？

這是回答，大家注意一下我圈起來的幾個關鍵詞：

OpenEntityManagerInViewInterceptor：在Spring Boot項目中，Session是歸OpenEntityManagerInViewInterceptor管理的，這個是干什么的呢？

它是確保EntityManager(Session)一直保持開啟的狀態(tài)，直到請求結束之后(complete request)。所以session在本次請求中，一直open著，惰性加載的數據隨便查。

如果你不想這么干，你可以配置spring.jpa.open-in-view=false來禁用此行為。

作

將spring.jpa.open-in-view配置為false作一把。

果然，session關閉了，報錯位置在TeacherServiceImpl第28行，就是查詢惰性加載的klasses出錯了。

總結

從上周想到這個問題開始，到今天解決，也是花了許久的時間。

所以以后再遇到no session的問題，如果是在項目里的，就先去想想是不是complete request了，請求結束前，session一直有效。

如果是在單元測試中，就去想想是不是事務配錯了，導致session掛掉了。

茲可謂一勞而久逸。暫費而永無寧者也。 ——班固《封燕然山銘》

一勞永逸，這是程序員最快樂的時候。

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