Mapping Java Generic to String value

Question

I want to persist into DB 2 entities :

• Attribute

  @Entity
  public class Attribute<T> {
  
  @Id @GeneratedValue(strategy = AUTO)
  Long id;
  
  @ManyToOne @JoinColumn(name = "item_id")
  Item item;
  
  String name;
  
  T value;
  
  boolean isTemplate;
  // skip setter and getter
  }
  

• Item

  public class Item {
  
  @Id
  @GeneratedValue(strategy = AUTO)
  Long id;
  
  @OneToMany(cascade = ALL)
  @JoinColumn(name= "item_id")
  List<Attribute> attributes;
  
  private boolean isTemplate;
      // skip setter and getter
  }
  
  in short Item 1-->* Attribute
  

• Error message that i get because hibernate can't map T value;

  Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.hibernate.SessionFactory]: Factory method 'sessionFactory' threw exception; nested exception is org.hibernate.AnnotationException: Property domain.item.Attribute.value has an unbound type and no explicit target entity. Resolve this Generic usage issue or set an explicit target attribute (eg @OneToMany(target=) or use an explicit @Type

• I only need this simple table

  • Item
    | id:int | isTemplate:boolean |
  • Attribute
    | id:int | Name:String | type:String (i.e:String,Integer - > based value type) | value:String | fk_item_id |

Thanks in advance for any help or suggestion to solve this problem.

Answer 1

sbjavateam, thank you for your thoroughly explanation. I have arrived in almost the same conclusion after I try to implement AttributeConverter. I stuck to convert T value to to String. It ends up to use instance of to get Object type only but I cant map its value. It is well explained in your explained Also in relational databases you can't persist java object without appropriate type.

I ends up almost the same approach like you to create additional class wrapper but Attribute<T> really a good idea.... It brings me arsenal to prove design your entity first clean and nice and do mapping latter.

Answer 2

You can't persist generic T due to java type erasure Type Erasure. Type T exist only in source and at runtime it's type Object. hibernate doent's know how to store / present Object in database it might be any type - entity , collection ,embeded object , some simple object - string , integer .....

Also in relational databases you can't persist java object without appropriate type (you can try to serialize object and save it as blob and in java side de-serialize it :) :) )

General :

1. if T it's entity : need to provide an interface/superclass instead of T, be it only an empty marker one if the differences between subclasses are large.

   @ManyToOne(targetEntity=T_EntityClass.class) @JoinColumn(name = "") private T value;

2. if it's not entity - looks like your case : create abstract entity with all fields except value field , and extends from this entity child - implementation , like for String , Integer....

   @Entity
   @Inheritance(strategy = InheritanceType.JOINED)
   public abstract class Attribute<T> {
       @Id
       @GeneratedValue(strategy = GenerationType.AUTO)
       private Long id;
   
       @ManyToOne
       @JoinColumn(name = "item_id")
       private Item item;
   
       @Column
       private String name;
   
       @Column
       boolean isTemplate;
   
       public abstract T getValue();
   
       public abstract void setValue(T value);
       ........
      }  
   

String implementation :

@Entity
public class AttributeStringValue extends Attribute<String>{

    @Column
    String value;

    @Override
    public String getValue() {
        return value;
    }

    @Override
    public void setValue(String value) {
        this.value = value;
    }
}

Integer implementation :

@Entity
public class AttributeIntegerValue extends Attribute<Integer>{

    @Column
    Integer value;

    @Override
    public Integer getValue() {
        return value;
    }

    @Override
    public void setValue(Integer value) {
        this.value = value;
    }
}

as result you have 3 tables :

create table attribute (
    id bigint generated by default as identity,
    is_template boolean,
    name varchar(255),
    item_id bigint,
    primary key (id)
)

create table attribute_integer_value (
    value integer,
    id bigint not null,
    primary key (id)
)

create table attribute_string_value (
    value varchar(255),
    id bigint not null,
    primary key (id)
)