JPA Session 一劳永逸

引言

再回顾一下问题场景：

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

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

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

在单元测试中跑这段代码，是报错的，no Session，说明执行完teacherRepository.findAll()之后，session就已经关闭了。继续执行，session已经关闭，再去数据库查教师关联的班级信息，就错了。

然而呢？把这段代码再放到Service里，写一个接口，交给浏览器去调用，却正常执行，说明session还在。

然后就一直研究为什么不好使？如果能把这个原因分析明白，以后再遇到no session错误的时候就可以一劳永逸了。

探究

调试

调试最简单的方法就是中断，但是咱水平还不行，也不知道JPA内部去找Hibernate怎么调用的，中断哪个方法呢？

后台发现了另一种调试的方法，JPA的源码中也是像我们开发时经常写日志的，logger.debug()什么的。

slf4j中常用的日志级别就ERROR、WARN、INFO、DEBUG四种，我们可以将JPA的日志级别设置为DEBUG级别，这样我们就可以根据日志推测到JPA内部到底是怎么执行的了。

修改日志级别

logging.level.org.springframework.orm.jpa=debug

修改配置文件，将JPA的日志级别设置为DEBUG。

在单元测试中执行

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   : 教师: 张三

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)]

上来是先执行了一个事务，为什么会有事务呢？

JPA创建的仓库实现是SimpleJpaRepository，我们看看源码：

实现类上添加了事务注解，并采用了默认的REQUIRED传播级别。

如果当前存在事务，则使用当前事务。如果不存在任何事务，则创建一个新的事务。

当前不存在事务，所以是teacherRepository.findAll()方法自己创建的事务。

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
教师: 张三

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

再接着看下面的日志，执行了数据库的查询操作，提交了一个事务，然后Closing JPA EntityManager [SessionImpl(1334204880)] after transaction。

事务执行之后，就关闭了EntityManager，也就是Hibernate中的Session。

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

EntityManager和Session还是有一些差别的，但是我们目前还未接触到底层的实现，只需要把他们当成一个东西，只不过在不同领域叫法不同罢了。

在SpringMVC中执行

执行得很顺利，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       : 教师: 张三
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       : 班级: 软件工程
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       : 学生: Hello Kitty
2019-06-06 11:58:28.917 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 学生: 史努比
2019-06-06 11:58:28.917 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 班级: 网络工程
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       : 学生: 米老鼠
2019-06-06 11:58:28.919 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 学生: 唐老鸭
2019-06-06 11:58:28.919 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 教师: 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       : 班级: 计算机科学与技术
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       : 学生: 哪吒
2019-06-06 11:58:28.923 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 学生: 小竹熊
2019-06-06 11:58:28.923 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 班级: 物联网
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       : 学生: 喜羊羊
2019-06-06 11:58:28.925 DEBUG 11443 --- [nio-8080-exec-1] c.y.jpa.service.TeacherServiceImpl       : 学生: 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)]

这段没什么说的，和上面一样，创建事务，执行完提交事务。

Not closing pre-bound JPA EntityManager after transaction
教师: 张三
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=?
班级: 软件工程
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=?
学生: Hello Kitty
学生: 史努比
班级: 网络工程
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=?
学生: 米老鼠
学生: 唐老鸭
教师: 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=?
班级: 计算机科学与技术
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=?
学生: 哪吒
学生: 小竹熊
班级: 物联网
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=?
学生: 喜羊羊
学生: 灰太狼
Closing JPA EntityManager in OpenEntityManagerInViewInterceptor

这块就有意思了。

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

最后一行：Closing JPA EntityManager in OpenEntityManagerInViewInterceptor

在事务之后没有关闭Session，一直到最后，才将Session关闭，所以没出错。

而在单元测试中呢？Closing JPA EntityManager [SessionImpl(1334204880)] after transaction，事务执行之后就关闭了Session，所以出错了。

原因

找着关键了，Service里好使是因为Session在findAll的事务执行完之后没有关闭。

之前怎么没发现呢？StackOverflow上这老哥和我是一样的问题：Hibernate jpa entity manager not being closed in spring service layer - StackOverflow

User和Address一对多。

我写了一个getUser方法，惰性加载的一对多，但是为什么序列化的时候去找addresses的时候，它不报lazy initialization的错误呢？

这是回答，大家注意一下我圈起来的几个关键词：

OpenEntityManagerInViewInterceptor：在Spring Boot项目中，Session是归OpenEntityManagerInViewInterceptor管理的，这个是干什么的呢？

它是确保EntityManager(Session)一直保持开启的状态，直到请求结束之后(complete request)。所以session在本次请求中，一直open着，惰性加载的数据随便查。

如果你不想这么干，你可以配置spring.jpa.open-in-view=false来禁用此行为。

作

将spring.jpa.open-in-view配置为false作一把。

果然，session关闭了，报错位置在TeacherServiceImpl第28行，就是查询惰性加载的klasses出错了。

总结

从上周想到这个问题开始，到今天解决，也是花了许久的时间。

所以以后再遇到no session的问题，如果是在项目里的，就先去想想是不是complete request了，请求结束前，session一直有效。

如果是在单元测试中，就去想想是不是事务配错了，导致session挂掉了。

兹可谓一劳而久逸。暂费而永无宁者也。 ——班固《封燕然山铭》

一劳永逸，这是程序员最快乐的时候。