java的Hibernate框架报错“LazyInitializationException”的原因和解决方法

当使用Java的Hibernate框架时，可能会遇到“LazyInitializationException”错误。这个错误通常是由于以下原因之一引起的：

• 延迟加载问题：如果您尝试访问延迟加载的属性或关联实体时，则可能会出现此错误。在这种情况下，需要使用Hibernate的“FetchType.EAGER”选项或手动加载关联实体。

• 会话关闭问题：如果会话已关闭，则可能会出现此错误。在这种情况下，需要确保会话处于打开状态。

以下是两个实例说明：

实例 1

如果您尝试访问延迟加载的属性或关联实体，则可以尝试使用Hibernate的“FetchType.EAGER”选项或手动加载关联实体。例如，如果您尝试使用以下代码时出现“LazyInitializationException”错误：

@Service
public class MyService {
    @Autowired
    private MyRepository myRepository;

    public List<MyEntity> getMyEntities() {
        List<MyEntity> myEntities = myRepository.findAll();
        for (MyEntity myEntity : myEntities) {
            System.out.println(myEntity.getRelatedEntity().getName());
        }
        return myEntities;
    }
}

@Repository
public interface MyRepository extends JpaRepository<MyEntity, Long> {
}

@Entity
@Table(name = "my_entity")
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "related_entity_id")
    private RelatedEntity relatedEntity;

    // getters and setters
}

@Entity
@Table(name = "related_entity")
public class RelatedEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    // getters and setters
}

则可以尝试使用Hibernate的“FetchType.EAGER”选项或手动加载关联实体。

@Service
public class MyService {
    @Autowired
    private MyRepository myRepository;

    public List<MyEntity> getMyEntities() {
        List<MyEntity> myEntities = myRepository.findAll();
        for (MyEntity myEntity : myEntities) {
            Hibernate.initialize(myEntity.getRelatedEntity());
            System.out.println(myEntity.getRelatedEntity().getName());
        }
        return myEntities;
    }
}

@Repository
public interface MyRepository extends JpaRepository<MyEntity, Long> {
}

@Entity
@Table(name = "my_entity")
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "related_entity_id")
    private RelatedEntity relatedEntity;

    // getters and setters
}

@Entity
@Table(name = "related_entity")
public class RelatedEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    // getters and setters
}

实例 2

如果会话已关闭，则可以尝试确保会话处于打开状态。例如，如果您尝试使用以下代码时出现“LazyInitializationException”错误：

@Service
public class MyService {
    @Autowired
    private MyRepository myRepository;

    public List<MyEntity> getMyEntities() {
        List<MyEntity> myEntities = myRepository.findAll();
        for (MyEntity myEntity : myEntities) {
            System.out.println(myEntity.getRelatedEntity().getName());
        }
        return myEntities;
    }
}

@Repository
public interface MyRepository extends JpaRepository<MyEntity, Long> {
}

@Entity
@Table(name = "my_entity")
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "related_entity_id")
    private RelatedEntity relatedEntity;

    // getters and setters
}

@Entity
@Table(name = "related_entity")
public class RelatedEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    // getters and setters
}

则可以尝试确保会话处于打开状态。

@Service
public class MyService {
    @Autowired
    private MyRepository myRepository;

    @Transactional
    public List<MyEntity> getMyEntities() {
        List<MyEntity> myEntities = myRepository.findAll();
        for (MyEntity myEntity : myEntities) {
            System.out.println(myEntity.getRelatedEntity().getName());
        }
        return myEntities;
    }
}

@Repository
public interface MyRepository extends JpaRepository<MyEntity, Long> {
}

@Entity
@Table(name = "my_entity")
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "related_entity_id")
    private RelatedEntity relatedEntity;

    // getters and setters
}

@Entity
@Table(name = "related_entity")
public class RelatedEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    // getters and setters
}

总之，要解决“LazyInitializationException”错误，您需要使用Hibernate的“FetchType.EAGER”选项或手动加载关联实体，或确保会话处于打开状态。如果问题仍然存在，请查看Hibernate文档或寻求其他的帮助。