@generatedvalue (estrategia = generationtype.identity) ejemplo de código

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Ejemplo 1: significado de generationtype.auto

Simply,@Id:This annotation specifies the primary key of the entity.@GeneratedValue:This annotation is used tospecify the primary key generation strategy touse. i.e Instructs database togenerate a value forthis field automatically. If the strategy is not specified by default AUTO will be used. 

GenerationTypeenum defines four strategies:1.GenerationType. TABLE,2.GenerationType. SEQUENCE,3.GenerationType. IDENTITY   
4.GenerationType. AUTO

GenerationType.SEQUENCE

Withthis strategy, underlying persistence provider must use a database sequence toget the next unique primary key for the entities. 

GenerationType.TABLE

Withthis strategy, underlying persistence provider must use a database table togenerate/keep the next unique primary key for the entities. 

GenerationType.IDENTITY
ThisGenerationType indicates that the persistence provider must assign primary keys for the entity using a database identity column. IDENTITY column is typically used in SQL Server. This special type column is populated internally by the table itself without using a separate sequence. If underlying database doesn't support IDENTITY column or some similar variant then the persistence provider can choose an alternative appropriate strategy. In this examples we are using H2 database which doesn't support IDENTITY column.

GenerationType.AUTO
ThisGenerationType indicates that the persistence provider should automatically pick an appropriate strategy for the particular database. This is the defaultGenerationType, i.e.if we just use @GeneratedValue annotation then this value of GenerationType will be used.

Ejemplo 2: id jpa generado automáticamente

@Entityjavax.persistence.EntityJPA annotationSpecifies that the class is an entity.SeeJavaDocReferencePage...publicclassEntityWithAutoId1@Idjavax.persistence.IdJPA annotationSpecifies the primary key of an entity.SeeJavaDocReferencePage...@GeneratedValuejavax.persistence.GeneratedValueJPA annotationProvides for the specification of generation strategies for the
 values of primary keys.SeeJavaDocReferencePage...(strategyGeneratedValue.strategyannotation element(Optional)The primary key generation strategy
 that the persistence provider must use togenerate the annotated entity primary key.SeeJavaDocReferencePage...=GenerationTypejavax.persistence.GenerationTypeJPA enumDefines the types of primary key generation strategies.SeeJavaDocReferencePage....AUTOGenerationType.AUTOenum constantIndicates that the persistence provider should pick an 
 appropriate strategy for the particular database.SeeJavaDocReferencePage...)long id;:

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