引言
再回顾一下问题场景：
Iterable<Teacher> 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<open>)] 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$HibernateConnectionHandle@73b74615]
2019-06-06 11:36:40.449 INFO 11391 --- [ main] o.h.h.i.QueryTranslatorFactoryInitiator : HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select teacher0_.id as id1_2_, teacher0_.name as name2_2_ from teacher teacher0_
2019-06-06 11:36:40.598 DEBUG 11391 --- [ main] o.s.orm.jpa.JpaTransactionManager : Initiating transaction commit
2019-06-06 11:36:40.598 DEBUG 11391 --- [ main] o.s.orm.jpa.JpaTransactionManager : Committing JPA transaction on EntityManager [SessionImpl(1334204880<open>)]
2019-06-06 11:36:40.601 DEBUG 11391 --- [ main] o.s.orm.jpa.JpaTransactionManager : Closing JPA EntityManager [SessionImpl(1334204880<open>)] after transaction
2019-06-06 11:36:40.602 DEBUG 11391 --- [ main] com.yunzhiclub.jpa.JpaApplicationTests : 教师: 张三
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.yunzhiclub.jpa.entity.Teacher.klasses, could not initialize proxy - no Session分析
Creating new transaction with name [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findAll]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly
Opened new EntityManager [SessionImpl(1334204880<open>)] for JPA transaction
Exposing JPA transaction as JDBC [org.springframework.orm.jpa.vendor.HibernateJpaDialect$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<open>)]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<open>)]
Closing JPA EntityManager [SessionImpl(1334204880<open>)] 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<open>)] 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<open>)] 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<open>)]
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<open>)] 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<open>)]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<open>)] 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。
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