593 words

Auto Generated Values

Marking a field with the @GeneratedValuejavax.persistence.GeneratedValueJPA annotationProvides for the specification of generation strategies for the values of primary keys.
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annotation specifies that a value will be automatically generated for that field. This is primarily intended for primary key fields but ObjectDB also supports this annotation for non-key numeric persistent fields as well. Several different value generation strategies can be used as explained below.

The Auto Strategy

ObjectDB maintains a special global number generator for every database. This number generator is used to generate automatic object IDs for entity objects with no primary key fields defined (as explained in the previous section).

The same number generator is also used to generate numeric values for primary key fields annotated by @GeneratedValuejavax.persistence.GeneratedValueJPA annotationProvides for the specification of generation strategies for the values of primary keys.
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with the AUTOGenerationType.AUTOenum constantIndicates that the persistence provider should pick an appropriate strategy for the particular database.
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strategy:

AUTOGenerationType.AUTOenum constantIndicates that the persistence provider should pick an appropriate strategy for the particular database.
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is the default strategy, so the following definition is equivalent:

During a commit the AUTOGenerationType.AUTOenum constantIndicates that the persistence provider should pick an appropriate strategy for the particular database.
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strategy uses the global number generator to generate a primary key for every new entity object. These generated values are unique at the database level and are never recycled, as explained in the previous section.

The Identity Strategy

The IDENTITYGenerationType.IDENTITYenum constantIndicates that the persistence provider must assign primary keys for the entity using a database identity column.
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strategy is very similar to the AUTOGenerationType.AUTOenum constantIndicates that the persistence provider should pick an appropriate strategy for the particular database.
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strategy:

The IDENTITYGenerationType.IDENTITYenum constantIndicates that the persistence provider must assign primary keys for the entity using a database identity column.
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strategy also generates an automatic value during commit for every new entity object. The difference is that a separate identity generator is managed per type hierarchy, so generated values are unique only per type hierarchy.

The Sequence Strategy

The sequence strategy consists of two parts - defining a named sequence and using the named sequence in one or more fields in one or more classes. The @SequenceGeneratorjavax.persistence.SequenceGeneratorJPA annotationDefines a primary key generator that may be referenced by name when a generator element is specified for the GeneratedValue annotation.
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annotation is used to define a sequence and accepts a name, an initial value (the default is 1) and an allocation size (the default is 50). A sequence is global to the application and can be used by one or more fields in one or more classes. The SEQUENCEGenerationType.SEQUENCEenum constantIndicates that the persistence provider must assign primary keys for the entity using a database sequence.
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strategy is used in the @GeneratedValuejavax.persistence.GeneratedValueJPA annotationProvides for the specification of generation strategies for the values of primary keys.
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annotation to attach the given field to the previously defined named sequence:

@Entityjavax.persistence.EntityJPA annotationSpecifies that the class is an entity.
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// Define a sequence - might also be in another class: @SequenceGeneratorjavax.persistence.SequenceGeneratorJPA annotationDefines a primary key generator that may be referenced by name when a generator element is specified for the GeneratedValue annotation.
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(nameSequenceGenerator.nameannotation element(Required) A unique generator name that can be referenced by one or more classes to be the generator for primary key values.
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="seq", initialValueSequenceGenerator.initialValueannotation element(Optional) The value from which the sequence object is to start generating.
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=1, allocationSizeSequenceGenerator.allocationSizeannotation element(Optional) The amount to increment by when allocating sequence numbers from the sequence.
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=100) public class EntityWithSequenceId { // Use the sequence that is defined above: @GeneratedValuejavax.persistence.GeneratedValueJPA annotationProvides for the specification of generation strategies for the values of primary keys.
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(strategyGeneratedValue.strategyannotation element(Optional) The primary key generation strategy that the persistence provider must use to generate the annotated entity primary key.
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=GenerationTypejavax.persistence.GenerationTypeJPA enumDefines the types of primary key generation strategies.
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.SEQUENCEGenerationType.SEQUENCEenum constantIndicates that the persistence provider must assign primary keys for the entity using a database sequence.
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, generatorGeneratedValue.generatorannotation element(Optional) The name of the primary key generator to use as specified in the SequenceGenerator or TableGenerator annotation.
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="seq") @Idjavax.persistence.IdJPA annotationSpecifies the primary key of an entity.
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long id; }

Unlike AUTOGenerationType.AUTOenum constantIndicates that the persistence provider should pick an appropriate strategy for the particular database.
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and IDENTITYGenerationType.IDENTITYenum constantIndicates that the persistence provider must assign primary keys for the entity using a database identity column.
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, the SEQUENCEGenerationType.SEQUENCEenum constantIndicates that the persistence provider must assign primary keys for the entity using a database sequence.
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strategy generates an automatic value as soon as a new entity object is persisted (i.e. before commit). This may be useful when the primary key value is needed earlier. To minimize round trips to the database server, IDs are allocated in groups. The number of IDs in each allocation is specified by the allocationSizeSequenceGenerator.allocationSizeannotation element(Optional) The amount to increment by when allocating sequence numbers from the sequence.
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attribute. It is possible that some of the IDs in a given allocation will not be used. Therefore, this strategy does not guarantee there will be no gaps in sequence values.

The Table Strategy

The TABLEGenerationType.TABLEenum constantIndicates that the persistence provider must assign primary keys for the entity using an underlying database table to ensure uniqueness.
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strategy is very similar to the SEQUENCEGenerationType.SEQUENCEenum constantIndicates that the persistence provider must assign primary keys for the entity using a database sequence.
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strategy:

@Entityjavax.persistence.EntityJPA annotationSpecifies that the class is an entity.
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@TableGeneratorjavax.persistence.TableGeneratorJPA annotationDefines a primary key generator that may be referenced by name when a generator element is specified for the GeneratedValue annotation.
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(nameTableGenerator.nameannotation element(Required) A unique generator name that can be referenced by one or more classes to be the generator for id values.
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="tab", initialValueTableGenerator.initialValueannotation element(Optional) The initial value to be used to initialize the column that stores the last value generated.
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=0, allocationSizeTableGenerator.allocationSizeannotation element(Optional) The amount to increment by when allocating id numbers from the generator.
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=50) public class EntityWithTableId { @GeneratedValuejavax.persistence.GeneratedValueJPA annotationProvides for the specification of generation strategies for the values of primary keys.
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(strategyGeneratedValue.strategyannotation element(Optional) The primary key generation strategy that the persistence provider must use to generate the annotated entity primary key.
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=GenerationTypejavax.persistence.GenerationTypeJPA enumDefines the types of primary key generation strategies.
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.TABLEGenerationType.TABLEenum constantIndicates that the persistence provider must assign primary keys for the entity using an underlying database table to ensure uniqueness.
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, generatorGeneratedValue.generatorannotation element(Optional) The name of the primary key generator to use as specified in the SequenceGenerator or TableGenerator annotation.
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="tab") @Idjavax.persistence.IdJPA annotationSpecifies the primary key of an entity.
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long id; }

ORM-based JPA providers (such as Hibernate, TopLink, EclipseLink, OpenJPA, JPOX, etc.) simulate a sequence using a table to support this strategy. ObjectDB does not have tables, so the TABLEGenerationType.TABLEenum constantIndicates that the persistence provider must assign primary keys for the entity using an underlying database table to ensure uniqueness.
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and SEQUENCEGenerationType.SEQUENCEenum constantIndicates that the persistence provider must assign primary keys for the entity using a database sequence.
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strategies are almost identical.

A tiny difference is related to the initial value attribute. Whereas the SEQUENCEGenerationType.SEQUENCEenum constantIndicates that the persistence provider must assign primary keys for the entity using a database sequence.
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strategy maintains the next sequence number to be used the TABLEGenerationType.TABLEenum constantIndicates that the persistence provider must assign primary keys for the entity using an underlying database table to ensure uniqueness.
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strategy maintains the last value that was used. The implication for the initialValue attribute is that if you want sequence numbers to start with 1 in the TABLEGenerationType.TABLEenum constantIndicates that the persistence provider must assign primary keys for the entity using an underlying database table to ensure uniqueness.
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strategy initialValue=0 has to be specified in the @SequenceGeneratorjavax.persistence.SequenceGeneratorJPA annotationDefines a primary key generator that may be referenced by name when a generator element is specified for the GeneratedValue annotation.
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annotation.


This documentation explains how to use JPA in the context of the ObjectDB Object Database but mostly relevant
also for ORM JPA implementations, such as Hibernate (and HQL), EclipseLink, TopLink, OpenJPA and DataNucleus.
ObjectDB is not an ORM JPA implementation but an Object Database (ODBMS) for Java with built in JPA 2 support.