Java for Web Learning Notes (109): Again on the inheritance __java of entity Mapping (2) Entity

Let's start with a simple case where multiple known table has the same schema and can be mapped to the same entity. Then, in the next study, we'll discuss a complex situation where we might dynamically build tables that have the same schema, if you use JPA.

Assuming that the Test_a,test_b and Test_c tables have the same table structure, that is, the schema is the same, we cannot map it to the same entity because each entity class is mapped to a table. If we don't want to write duplicate code, we can use the inheritance method.

The following approach applies equally to the schemas of these tables, and the differences are reflected by extends. @Inheritance: Table per class

@Entity
@Inheritance (strategy=inheritancetype.table_per_class) public
abstract CLASS abstestentity {
    @ ID
    private Long ID;
    private String name;
    .....
}

If there is a test table, we don't need abstract. @Inheritance that this entity schema can be inherited.

@Entity
@Table (name= "test_a") public
Lass Testa extends abstestentity {
}

@Entity
@Table ( Name= "Test_b") public
Lass Testb extends abstestentity {
    private String data;
    ......
}

But there is a problem with this approach if:

@Entity
@Inheritance (strategy=inheritancetype.table_per_class) public
abstract CLASS abstestentity {
    @ ID
    @GeneratedValue (strategy=generationtype.identity)  //will cause an exception 
    private Long ID;
    private String name;
    .....
}

This can cause an exception:

caused By:org.hibernate.MappingException:Cannot use identity column key generation with <union-subclass> mapping F Or:cn.wei.flowingflying.chapter22.site.entity.AbsTestEntity at
	Org.hibernate.persister.entity.unionsubclassentitypersister.<init> (unionsubclassentitypersister.java:82) At Sun.reflect.NativeConstructorAccessorImpl.newInstance0 (Native method) at Sun.reflect.NativeConstructorAccessorImpl.newInstance (Unknown Source) at Sun.reflect.DelegatingConstructorAccessorImpl.newInstance (Unknown Source) at Java.lang.reflect.Constructor.newInstance (Unknown Source) at Org.hibernate.persister.internal.PersisterFactoryImpl.createEntityPersister (persisterfactoryimpl.java:96) at Org.hibernate.persister.internal.PersisterFactoryImpl.createEntityPersister (persisterfactoryimpl.java:77) at Org.hibernate.metamodel.internal.MetamodelImpl.initialize (metamodelimpl.java:128) at Org.hibernate.internal.sessionfactoryimpl.<init> (sessionfactoryimpl.java:301) at Org.hibernate.boOt.internal.SessionFactoryBuilderImpl.build (sessionfactorybuilderimpl.java:452) at Org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build (entitymanagerfactorybuilderimpl.java:889 )
	... More

The identity builder is not supported in this way. Need special attention. @MappedSuperclass

Our table is passed with Auto_increment, which leads to the inability to use the above way, which we can use @mappedsuperclass.

@MappedSuperclass public
abstract class Abstestentity {
    private Long ID;
    private String name;
  
    @Id
    @GeneratedValue (strategy=generationtype.identity) public
    Long GetId () {return
        Id;
    }    
    .....
}

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