事務注解不能解決 org.hibernate.LazyInitializationException

Question

我正在編寫一個簡單的 Spring 數據 JPA 應用程序。 我使用 MySQL 數據庫。 有兩個簡單的表：

1. 部門
2. 員工

每個員工在某個部門工作 (Employee.department_id)。

我有兩個實體類：

@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;
}

存儲庫：

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

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

有一個DepartmentService class：

@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);
        }
    }
}

和主要的 class（我不需要 web，所以我使用CommandLineRunner將應用程序作為控制台應用程序運行）：

@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 class DepartmentService之前以及它的所有方法之前添加了@Transactional ，但我仍然不斷收到以下錯誤：

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

堆棧跟蹤：

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

為什么會出現此錯誤？ 我該如何解決？

Answer 1

@Transactional不應該用於獲取延遲加載的實體，這將導致n+1 select 問題。

您需要改用適當的映射或查詢，例如在 jpql 中：

select d
from Department d
left join fetch d.employees

Answer 2

總結以上所有答案和評論：

發生這種情況是因為方法DepartmentService::getEmployees不是公開的。

我在 getEmployees() 之前添加了 public，這解決了這個問題。

@Transactional注釋由 AOP 提供支持，那么代理只能處理公共方法。

這意味着@Transactional實際上並未應用於此方法，並且 session 在讀取惰性集合時已經關閉。