java.lang.ClassCastException: org.hibernate.mapping.SingleTableSubclass cannot be cast to org.hibernate.mapping.RootClass

I have 2 classes one parent and one child, I used this workaround to define mixed inheritance strategy, however when compiling with maven, I am getting an error.

Parent class:

@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;
.
.
.}

Child class:

 @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}

error:

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)
);

I tried to use your mapping (with minor changes) with exactly your hibernate version 5.2.14.Final (spring-boot-starter-data-jpa 2.0.0.RELEASE):

Tables:

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');

Hibernate mapping:

@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
{

}

And simple code for select testing:

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

will generate the following 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

And insert testing:

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

will generate the following 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 (?, ?)

If it will not help, please provide the full stack trace of your exception and full code of all entities from your inheritance hierarchy.