java.lang.ClassCastException：org.hibernate.mapping.SingleTableSubclass 不能转换为 org.hibernate.mapping.RootClass

Question

我有 2 个类，一父一子，我使用 this 解决方法来定义混合继承策略，但是在使用 maven 编译时，出现错误。

父类：

@Entity
@Table(name="ITEMS")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="ITEM_CATEGORY",discriminatorType=DiscriminatorType.INTEGER)
  public class Item {
  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
  @Column(name = "ITEM_ID")
  protected Long itemId;
.
.
.}

儿童班：

 @Entity
    @SecondaryTable(name="TASKS", pkJoinColumns = 
    {@PrimaryKeyJoinColumn(name = "ITEM_ID", referencedColumnName = "ITEM_ID")})
    @DiscriminatorValue(value=ItemCategory.Values.TASK)
    public class Task extends Item {
        //no @Id field}

错误：

java.lang.IllegalStateException: Failed to load ApplicationContext
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Invocation of init method failed; nested exception is java.lang.ClassCastException: org.hibernate.mapping.SingleTableSubclass cannot be cast to org.hibernate.mapping.RootClass
Caused by: java.lang.ClassCastException: org.hibernate.mapping.SingleTableSubclass cannot be cast to org.hibernate.mapping.RootClass

pom.xml：

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.0.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-rest</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>

DDL：

CREATE TABLE ITEMS(
ITEM_ID bigint auto_increment primary key,
.
.
.
FOREIGN KEY(RELATED_TO_ITEM) references ITEMS(ITEM_ID)
);


CREATE TABLE TASKS( #INCLUDES REPEATED TASKS, ONETIME TASKS, LEARNING DUTY AND RESPONSES
ITEM_ID bigint primary key,
.
.
.
FOREIGN KEY(ITEM_ID) references ITEMS(ITEM_ID)
);

Answer 1

我尝试将您的映射（稍作改动）与您的休眠版本 5.2.14.Final (spring-boot-starter-data-jpa 2.0.0.RELEASE) 完全相同：

表格：

CREATE TABLE ITEMS (
   ITEM_ID bigint auto_increment primary key,
   ITEM_CATEGORY bigint,
   RELATED_TO_ITEM bigint,
   FOREIGN KEY(RELATED_TO_ITEM) references ITEMS(ITEM_ID)
);

insert into ITEMS
values (1, 1, null), (2, 1, 1), (3, 0, 1), (4, 1, 2);

CREATE TABLE TASKS(
   ITEM_ID bigint primary key,
   code varchar(100),
   FOREIGN KEY(ITEM_ID) references ITEMS(ITEM_ID)
);

insert into TASKS
values (1, 'TASK 1'), (2, 'Task 2'), (4, 'Task 4');

休眠映射：

@Entity
@Table(name = "ITEMS")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="ITEM_CATEGORY", discriminatorType = DiscriminatorType.INTEGER)
public class Item
{
   private Long itemId;

   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   @Column(name = "ITEM_ID")
   public Long getItemId()
   {
      return itemId;
   }
   public void setItemId(Long itemId)
   {
      this.itemId = itemId;
   }
}

@Entity
@SecondaryTable(name="TASKS", pkJoinColumns = {
   @PrimaryKeyJoinColumn(name = "ITEM_ID", referencedColumnName = "ITEM_ID")
})
@DiscriminatorValue("1")
public class Task extends Item
{
   private String code;

   @Column(table = "TASKS", name = "code")
   public String getCode()
   {
      return code;
   }
   public void setCode(String code)
   {
      this.code = code;
   }
}

@Entity
@DiscriminatorValue("0")
public class Job extends Item
{

}

以及用于选择测试的简单代码：

List<Item> items = session.createQuery("select i from Item i", Item.class).getResultList();

会生成如下sql：

/* select i from Item i */
select
  item0_.ITEM_ID as ITEM_ID2_0_,
  item0_1_.code as code1_1_,
  item0_.ITEM_CATEGORY as ITEM_CAT1_0_ 
from DB_A.ITEMS item0_ 
left outer join DB_A.TASKS item0_1_ on item0_.ITEM_ID=item0_1_.ITEM_ID

并插入测试：

Task task = new Task();
task.setCode("my new task 1");
session.persist(task);

会生成如下sql：

 /* insert com.sternkn.hibernate.model.Task */
insert into DB_A.ITEMS(ITEM_CATEGORY)  
values (1)

/* insert com.sternkn.hibernate.model.Task */
insert into DB_A.TASKS(code, ITEM_ID) 
values (?, ?)

如果没有帮助，请提供您的异常的完整堆栈跟踪和继承层次结构中所有实体的完整代码。