事务注释不能解决org.hibernate.LazyInitializationException

如何解决事务注释不能解决org.hibernate.LazyInitializationException

我正在编写一个简单的Spring Data JPA应用程序。我使用MySQL数据库。有两个简单的表：

1. 部门
2. 员工

每个员工都在某个部门工作（Employee.department_id）。

我有两个实体类：

@Entity
public class Department {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    private Long id;

    @Basic(fetch = FetchType.LAZY)
    @OneToMany(mappedBy = "department")
    List<Employee> employees;
}

@Entity
public class Person {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    private Long id;

    @ManyToOne
    @JoinColumn
    private Department department;
}

存储库：

@Repository
public interface DepartmentRepository extends JpaRepository<Department,Long> {}

@Repository
public interface EmployeeRepository extends JpaRepository<Employee,Long> {}

有一个DepartmentService类：

@Service
@Transactional
public class DepartmentService {
    @Autowired
    DepartmentRepository departmentRepository;

    @Transactional
    List<Department> getAll() {
        departmentRepository.findAll();
    }

    @Transactional
    void getEmployees() {
        for(Department department: getAll()) {
            List<Employee> employees = department.getEmployees();
            for(Employee employee: employees)
                System.out.println(employee);
        }
    }
}

还有主类（我不需要网络，因此我使用CommandLineRunner作为控制台应用程序运行该应用程序）

@SpringBootApplication
public class EmployeeDB implements CommandLineRunner {
    @Autowired
    DepartmentService departmentService;

    public static void main(String[] args) {
        SpringApplication.run(EmployeeDB.class,args);
    }

    @Override
    public void run(String... args) throws Exception {
        departmentService.getEmployees();
    }
}

我在类@Transactional之前及其所有方法之前添加了DepartmentService，但仍然出现以下错误：

 org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: employeeDB.Department.employees,could not initialize proxy - no Session

Stacktrace：

Caused by: org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: employeeDB.Department.employees,could not initialize proxy - no Session
    at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:602) ~[hibernate-core-5.4.2.Final.jar:5.4.2.Final]
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:217) ~[hibernate-core-5.4.2.Final.jar:5.4.2.Final]
    at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:581) ~[hibernate-core-5.4.2.Final.jar:5.4.2.Final]
    at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:148) ~[hibernate-core-5.4.2.Final.jar:5.4.2.Final]
    at org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:303) ~[hibernate-core-5.4.2.Final.jar:5.4.2.Final]
    at employeeDB.DepartmentService.getEmployees(DepartmentService.java:25) ~[main/:na]
    at employeeDB.EmployeeDB.run(EmployeeDB.java:41) [main/:na]
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:795) [spring-boot-2.3.3.RELEASE.jar:2.3.3.RELEASE]
    ... 5 common frames omitted

为什么会出现此错误？我该怎么解决？

解决方法

@Transactional不应用于获取延迟加载的实体，这将导致n+1 select problem。

您需要改用适当的映射或查询，例如在jpql中：

select d
from Department d
left join fetch d.employees

,

总结以上所有答案和评论：

发生这种情况是因为方法DepartmentService::getEmployees不是公开的。

我在getEmployees（）之前添加了public，这解决了这个问题。

@Transactional注释由AOP提供支持，因此代理只能处理公共方法。

这意味着@Transactional实际上并未应用到此方法，并且在读取惰性集合时会话已经关闭。