Hibernate映射子类被加载为父类的问题排查与解决

我定义了Project类，以及父类Resource和它的两个子类StorageResource、ComputeResource，使用Hibernate 5.3.7.Final版本配置如下映射：

@Entity
@Table(name = "PROJECTS", ...})
public class Project implements Serializable {
    ...
    @OneToMany(
        mappedBy = "project",
        cascade = CascadeType.ALL,
        orphanRemoval = true
    )
    private List<Resource> resources;
    ...
}

@Entity
@Table(name = "resources")
@Inheritance(strategy = InheritanceType.JOINED)
public class Resource implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private long id;

    @ManyToOne
    private Project project;
    ...
}

@Entity
@Table(name = "storage_resources")
@PrimaryKeyJoinColumn(name = "id")
public class StorageResource extends Resource implements Serializable {
    ...
}

@Entity
@Table(name = "compute_resources")
@PrimaryKeyJoinColumn(name = "id")
public class ComputeResource extends Resource implements Serializable {
    ...
}

保存两个子类实例到数据库时一切正常，但加载项目及其关联资源时出现异常：

• 存储资源记录能正确加载为StorageResource实例
• 计算资源记录被加载为Resource实例，而非预期的ComputeResource实例

对比两个子类的映射配置，确认代码层面配置完全一致，耗时两天排查后发现，问题出在运行环境的Hibernate配置文件中——缺失了ComputeResource的映射配置项：

<mapping class="....ComputeResource"/>

添加该配置项后，映射加载恢复正常。

内容的提问来源于stack exchange，提问作者Giorgio