LazyInitializationException：未能懒惰地初始化一个关系收集角色，无法初始化代理 - 无会话
我知道使用Eager获取类型可以正常工作，但这将是一个头痛和性能滞后于应用程序，所以，我必须继续考虑Lazy fetch类型。
我读到使用Spring Data JPA时， @Transactional 会初始化惰性关系，但它似乎对我没有影响。
这是我的应用程序的外观：
实体
课程实体 $ b
@Entity @Table（name =course） @Transactional public class Course { @Id @GeneratedValue（strategy = GenerationType.AUTO） private Intege r id; @Column（name =course_name） private String courseName; @Column（name =course_description） private String courseDescription; @ManyToMany（cascade = {CascadeType.MERGE，CascadeType.DETACH，CascadeType.REFRESH，CascadeType.PERSIST}） @JoinTable（ name =course_student， joinColumns = @JoinColumn（name =course_id）， inverseJoinColumns = @JoinColumn（name =student_id）） private List< Student>学生们;学生实体
@Entity @Table（name =students） public class Student { @I $ @GeneratedValue（strategy = GenerationType.AUTO）私人整数ID; 私人字符串名称; 私人字符串等级; @ManyToMany（cascade = {CascadeType.MERGE，CascadeType.DETACH，CascadeType.REFRESH，CascadeType.PERSIST}，fetch = FetchType.EAGER） @JoinTable（ name = course_student， joinColumns = @JoinColumn（name =student_id）， inverseJoinColumns = @JoinColumn（name =course_id）） List< Course>培训班;控制器 @Controller @RequestMapping（/ courses） @Transactional public class CourseController { ... 。 @GetMapping（get-joined-students） ModelAndView getJoined（）{ 课程course = courseRepo.findById（6）.get（） ; （学生学生： course.getStudents（））{ System.out.println（Student：+ student）; } ModelAndView modelAndView = new ModelAndView（redirect：list）; 返回modelAndView; }
}
存储库
课程库
@Repository public interface CourseRepo扩展JpaRepository< Course，Integer> {}学生资源库
@Repository public interface StudentRepo扩展JpaRepository< Student，Integer> {}Servlet配置文件
<？xml version =1.0encoding =UTF-8？> < beans xmlns =www.springframework/schema/beans....> < context：annotation-config /> < context：component-scan base-package =company.springdemo/> < mvc：annotation-driven /> ...我可能会做什么错误？
解决方案我认为你在这里混淆了一些东西。
您有多个选择： $ b
另请阅读我的文章以了解更多详情： blog.arnoldgalovics/2017/02/27/lazyinitializationexception-demystified/ blog.arnoldgalovics/2017/03/14/using-projections-in-your-data-access-层/
I have been receiving this error all the time when using Lazy evaluation in a ManyToMany relationship:
LazyInitializationException: failed to lazily initialize a collection of roles, could not initialize proxy - no Session
I know that using Eager fetch type will work fine, but this will be a headache and a performance lag to the application, so, I have to continue considering the Lazy fetch type.
I read that when using Spring Data JPA, @Transactional will initialize the lazy relations, but it doesn't seem to take effect with me.
Here is how my app look like:
Entities
Course Entity
@Entity @Table(name = "course") @Transactional public class Course { @Id @GeneratedValue(strategy = GenerationType.AUTO) private Integer id; @Column(name = "course_name") private String courseName; @Column(name = "course_description") private String courseDescription; @ManyToMany(cascade = {CascadeType.MERGE,CascadeType.DETACH,CascadeType.REFRESH,CascadeType.PERSIST}) @JoinTable( name = "course_student", joinColumns = @JoinColumn(name = "course_id"), inverseJoinColumns = @JoinColumn(name = "student_id") ) private List<Student> students;Student Entity
@Entity @Table(name = "students") public class Student { @Id @GeneratedValue(strategy = GenerationType.AUTO) private Integer id; private String name; private String grade; @ManyToMany(cascade = {CascadeType.MERGE,CascadeType.DETACH,CascadeType.REFRESH,CascadeType.PERSIST},fetch = FetchType.EAGER) @JoinTable( name = "course_student", joinColumns = @JoinColumn(name = "student_id"), inverseJoinColumns = @JoinColumn(name = "course_id") ) List<Course> courses;Controller
@Controller @RequestMapping("/courses") @Transactional public class CourseController { .... @GetMapping("get-joined-students") ModelAndView getJoined() { Course course = courseRepo.findById(6).get(); for (Student student : course.getStudents()) { System.out.println("Student: " + student); } ModelAndView modelAndView = new ModelAndView("redirect:list"); return modelAndView; }}
Repositories
Course Repository
@Repository public interface CourseRepo extends JpaRepository<Course,Integer> {}Student Repository
@Repository public interface StudentRepo extends JpaRepository<Student,Integer> { }Servlet Configuration File
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="www.springframework/schema/beans" .... > <context:annotation-config/> <context:component-scan base-package="company.springdemo"/> <mvc:annotation-driven/> .... <jpa:repositories transaction-manager-ref="transactionManagerJPA" base-package="company.springdemo" entity-manager-factory-ref="EMF" /> <bean id="transactionManagerJPA" class="org.springframework.orm.jpa.JpaTransactionManager" > <property name="dataSource" ref="myDataSource" /> </bean> <tx:annotation-driven transaction-manager="transactionManagerJPA" /> <bean id="EMF" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="packagesToScan" value="company.springdemo.entity" /> <property name="dataSource" ref="myDataSource" /> <property name="jpaProperties"> <props> <prop key="hibernate.show_sql">true</prop> <prop key="hibernate.hbm2ddl.auto">update</prop> <prop key="hibernate.dialect">org.hibernate.dialect.MySQL57Dialect</prop> <prop key="hibernate.show_sql">false</prop> <prop key="hibernate.format_sql">true</prop> </props> </property> <property name="persistenceProvider"> <bean class="org.hibernate.jpa.HibernatePersistenceProvider" /> </property> </bean>What potentially could I be doing wrong?
解决方案I think you are mixing up things here. LazyInitializationException is popping up as you are trying to fetch the relation when the transaction is already over.
You have multiple choices:
Also read my article for more details: blog.arnoldgalovics/2017/02/27/lazyinitializationexception-demystified/ blog.arnoldgalovics/2017/03/14/using-projections-in-your-data-access-layer/
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