Identifiers model the primary key of an entity. They are used to uniquely identify each specific entity.
Hibernate and JPA both make the following assumptions about the corresponding database column(s):
UNIQUE-
The values must uniquely identify each row.
NOT NULL-
The values cannot be null. For composite ids, no part can be null.
IMMUTABLE-
The values, once inserted, can never be changed. This is more a general guide, than a hard-fast rule as opinions vary. JPA defines the behavior of changing the value of the identifier attribute to be undefined; Hibernate simply does not support that. In cases where the values for the PK you have chosen will be updated, Hibernate recommends mapping the mutable value as a natural id, and use a surrogate id for the PK. See Natural Ids.
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Note
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Technically the identifier does not have to map to the column(s) physically defined as the table primary key. They just need to map to column(s) that uniquely identify each row. However, this documentation will continue to use the terms identifier and primary key interchangeably. |
Every entity must define an identifier. For entity inheritance hierarchies, the identifier must be defined just on the entity that is the root of the hierarchy.
An identifier might be simple (single value) or composite (multiple values).
Simple identifiers map to a single basic attribute, and are denoted using the javax.persistence.Id annotation.
According to JPA only the following types should be used as identifier attribute types:
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any Java primitive type
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any primitive wrapper type
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java.lang.String -
java.util.Date(TemporalType#DATE) -
java.sql.Date -
java.math.BigDecimal -
java.math.BigInteger
Any types used for identifier attributes beyond this list will not be portable.
Values for simple identifiers can be assigned, as we have seen in the examples above. The expectation for assigned identifier values is that the application assigns (sets them on the entity attribute) prior to calling save/persist.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/AssignedIdentifierTest.java[role=include]Values for simple identifiers can be generated. To denote that an identifier attribute is generated, it is annotated with javax.persistence.GeneratedValue
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/GeneratedIdentifierTest.java[role=include]Additionally, to the type restriction list above, JPA says that if using generated identifier values (see below) only integer types (short, int, long) will be portably supported.
The expectation for generated identifier values is that Hibernate will generate the value when the save/persist occurs.
Identifier value generations strategies are discussed in detail in the Generated identifier values section.
Composite identifiers correspond to one or more persistent attributes. Here are the rules governing composite identifiers, as defined by the JPA specification:
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The composite identifier must be represented by a "primary key class". The primary key class may be defined using the
javax.persistence.EmbeddedIdannotation (see Composite identifiers with@EmbeddedId), or defined using thejavax.persistence.IdClassannotation (see Composite identifiers with@IdClass). -
The primary key class must be public and must have a public no-arg constructor.
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The primary key class must be serializable.
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The primary key class must define equals and hashCode methods, consistent with equality for the underlying database types to which the primary key is mapped.
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The restriction that a composite identifier has to be represented by a "primary key class" (e.g. Hibernate does allow composite identifiers to be defined without a "primary key class" via multiple |
The attributes making up the composition can be either basic, composite, @ManyToOne.
Note especially that collection and one-to-one are never appropriate.
Modeling a composite identifier using an EmbeddedId simply means defining an embeddable to be a composition for the one or more attributes making up the identifier, and then exposing an attribute of that embeddable type on the entity.
@EmbeddedIdlink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/EmbeddedIdTest.java[role=include]As mentioned before, EmbeddedIds can even contain @ManyToOne attributes:
@EmbeddedId with @ManyToOnelink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/EmbeddedIdManyToOneTest.java[role=include]|
Note
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Hibernate supports directly modeling However, that is not portably supported by the JPA specification. In JPA terms, one would use "derived identifiers". For more details, see Derived Identifiers. |
Modeling a composite identifier using an IdClass differs from using an EmbeddedId in that the entity defines each individual attribute making up the composition. The IdClass simply acts as a "shadow".
@IdClasslink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/IdClassTest.java[role=include]Non-aggregated composite identifiers can also contain ManyToOne attributes as we saw with aggregated ones (still non-portably).
@ManyToOnelink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/IdClassManyToOneTest.java[role=include]With non-aggregated composite identifiers, Hibernate also supports "partial" generation of the composite values.
@IdClass with partial identifier generation using @GeneratedValuelink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/IdClassGeneratedValueTest.java[role=include]|
Note
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This feature which allows auto-generated values in composite identifiers exists because of a highly questionable interpretation of the JPA specification made by the SpecJ committee. Hibernate does not feel that JPA defines support for this, but added the feature simply to be usable in SpecJ benchmarks. Use of this feature may or may not be portable from a JPA perspective. |
Hibernate allows defining a composite identifier out of entity associations.
In the following example, the PersonAddress entity identifier is formed of two @ManyToOne associations.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/IdManyToOneTest.java[role=include]Although the mapping is much simpler than using an @EmbeddedId or an @IdClass, there’s no separation between the entity instance and the actual identifier.
To query this entity, an instance of the entity itself must be supplied to the persistence context.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/IdManyToOneTest.java[role=include]When using composite identifiers, the underlying identifier properties must be manually assigned by the user.
Automatically generated properties are not supported to be used to generate the value of an underlying property that makes the composite identifier.
Therefore, you cannot use any of the automatic property generator described by the generated properties section like @Generated, @CreationTimestamp or @ValueGenerationType or database-generated values.
Nevertheless, you can still generate the identifier properties prior to constructing the composite identifier, as illustrated by the following examples.
Assuming we have the following EventId composite identifier and an Event entity which uses the aforementioned composite identifier.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/composite/Event.java[role=include]link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/composite/EventId.java[role=include]If you want to generate the composite identifier properties in-memory, you need to do that as follows:
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/composite/EmbeddedIdInMemoryGeneratedValueTest.java[role=include]Notice that the createdOn property of the EventId composite identifier was generated by the data access code and assigned to the
identifier prior to persisting the Event entity.
If you want to generate the composite identifier properties using a database function or stored procedure, you could to do it as illustrated by the following example.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/composite/EmbeddedIdDatabaseGeneratedValueTest.java[role=include]Notice that the createdOn property of the EventId composite identifier was generated by calling the CURRENT_TIMESTAMP database function,
and we assigned it to the composite identifier prior to persisting the Event entity.
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You can also auto-generate values for non-identifier attributes. For more details, see the Generated properties section. |
Hibernate supports identifier value generation across a number of different types. Remember that JPA portably defines identifier value generation just for integer types.
Identifier value generation is indicated using the javax.persistence.GeneratedValue annotation.
The most important piece of information here is the specified javax.persistence.GenerationType which indicates how values will be generated.
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Note
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The discussions below assume that the application is using Hibernate’s "new generator mappings" as indicated by the Starting with Hibernate 5, this is set to |
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Important
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In Hibernate 5.3, Hibernate attempts to delay the insert of entities if the flush-mode does not equal In Hibernate 5.4, Hibernate attempts to remedy the problem using an algorithm to decide if the insert should be delayed or if it requires immediate insertion. We wanted to restore the behavior prior to 5.3 only for very specific use cases where it made sense. Entity mappings can sometimes be complex and it is possible a corner case was overlooked. Hibernate offers a
way to completely disable the 5.3 behavior in the event problems occur with This configuration option is meant to act as a temporary fix and bridge the gap between the changes in this behavior across Hibernate 5.x releases. If this configuration setting is necessary for a mapping, please open a JIRA and report the mapping so that the algorithm can be reviewed. |
AUTO(the default)-
Indicates that the persistence provider (Hibernate) should choose an appropriate generation strategy. See Interpreting AUTO.
IDENTITY-
Indicates that database IDENTITY columns will be used for primary key value generation. See Using IDENTITY columns.
SEQUENCE-
Indicates that database sequence should be used for obtaining primary key values. See Using sequences.
TABLE-
Indicates that a database table should be used for obtaining primary key values. See Using the table identifier generator.
How a persistence provider interprets the AUTO generation type is left up to the provider.
The default behavior is to look at the Java type of the identifier attribute.
If the identifier type is UUID, Hibernate is going to use a UUID identifier.
If the identifier type is numerical (e.g. Long, Integer), then Hibernate is going to use the IdGeneratorStrategyInterpreter to resolve the identifier generator strategy.
The IdGeneratorStrategyInterpreter has two implementations:
FallbackInterpreter-
This is the default strategy since Hibernate 5.0. For older versions, this strategy is enabled through the
hibernate.id.new_generator_mappingsconfiguration property. When using this strategy,AUTOalways resolves toSequenceStyleGenerator. If the underlying database supports sequences, then a SEQUENCE generator is used. Otherwise, a TABLE generator is going to be used instead. LegacyFallbackInterpreter-
This is a legacy mechanism that was used by Hibernate prior to version 5.0 or when the
hibernate.id.new_generator_mappingsconfiguration property is false. The legacy strategy mapsAUTOto thenativegenerator strategy which uses the Dialect#getNativeIdentifierGeneratorStrategy to resolve the actual identifier generator (e.g.identityorsequence).
For implementing database sequence-based identifier value generation Hibernate makes use of its org.hibernate.id.enhanced.SequenceStyleGenerator id generator.
It is important to note that SequenceStyleGenerator is capable of working against databases that do not support sequences by switching to a table as the underlying backing.
This gives Hibernate a huge degree of portability across databases while still maintaining consistent id generation behavior (versus say choosing between SEQUENCE and IDENTITY).
This backing storage is completely transparent to the user.
The preferred (and portable) way to configure this generator is using the JPA-defined javax.persistence.SequenceGenerator annotation.
The simplest form is to simply request sequence generation; Hibernate will use a single, implicitly-named sequence (hibernate_sequence) for all such unnamed definitions.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/SequenceGeneratorUnnamedTest.java[role=include]Using javax.persistence.SequenceGenerator, you can specify a specific database sequence name.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/SequenceGeneratorNamedTest.java[role=include]The javax.persistence.SequenceGenerator annotation allows you to specify additional configurations as well.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/SequenceGeneratorConfiguredTest.java[role=include]For implementing identifier value generation based on IDENTITY columns,
Hibernate makes use of its org.hibernate.id.IdentityGenerator id generator which expects the identifier to be generated by INSERT into the table.
IdentityGenerator understands 3 different ways that the INSERT-generated value might be retrieved:
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If Hibernate believes the JDBC environment supports
java.sql.Statement#getGeneratedKeys, then that approach will be used for extracting the IDENTITY generated keys. -
Otherwise, if
Dialect#supportsInsertSelectIdentityreports true, Hibernate will use the Dialect specific INSERT+SELECT statement syntax. -
Otherwise, Hibernate will expect that the database supports some form of asking for the most recently inserted IDENTITY value via a separate SQL command as indicated by
Dialect#getIdentitySelectString.
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Important
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It is important to realize that using IDENTITY columns imposes a runtime behavior where the entity row must be physically inserted prior to the identifier value being known. This can mess up extended persistence contexts (long conversations). Because of the runtime imposition/inconsistency, Hibernate suggests other forms of identifier value generation be used (e.g. SEQUENCE). |
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Note
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There is yet another important runtime impact of choosing IDENTITY generation: Hibernate will not be able to batch INSERT statements for the entities using the IDENTITY generation. The importance of this depends on the application-specific use cases. If the application is not usually creating many new instances of a given entity type using the IDENTITY generator, then this limitation will be less important since batching would not have been very helpful anyway. |
Hibernate achieves table-based identifier generation based on its org.hibernate.id.enhanced.TableGenerator which defines a table capable of holding multiple named value segments for any number of entities.
The basic idea is that a given table-generator table (hibernate_sequences for example) can hold multiple segments of identifier generation values.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/TableGeneratorUnnamedTest.java[role=include]link:extras/id/identifiers-generators-table-unnamed-mapping-example.sql[role=include]If no table name is given Hibernate assumes an implicit name of hibernate_sequences.
Additionally, because no javax.persistence.TableGenerator#pkColumnValue is specified,
Hibernate will use the default segment (sequence_name='default') from the hibernate_sequences table.
However, you can configure the table identifier generator using the {jpaJavadocUrlPrefix}TableGenerator.html[@TableGenerator] annotation.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/TableGeneratorConfiguredTest.java[role=include]link:extras/id/identifiers-generators-table-configured-mapping-example.sql[role=include]Now, when inserting 3 Product entities, Hibernate generates the following statements:
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/TableGeneratorConfiguredTest.java[role=include]link:extras/id/identifiers-generators-table-configured-persist-example.sql[role=include]As mentioned above, Hibernate supports UUID identifier value generation.
This is supported through its org.hibernate.id.UUIDGenerator id generator.
UUIDGenerator supports pluggable strategies for exactly how the UUID is generated.
These strategies are defined by the org.hibernate.id.UUIDGenerationStrategy contract.
The default strategy is a version 4 (random) strategy according to IETF RFC 4122.
Hibernate does ship with an alternative strategy which is a RFC 4122 version 1 (time-based) strategy (using IP address rather than mac address).
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/UuidGeneratedValueTest.java[role=include]To specify an alternative generation strategy, we’d have to define some configuration via @GenericGenerator.
Here we choose the RFC 4122 version 1 compliant strategy named org.hibernate.id.uuid.CustomVersionOneStrategy.
link:../../../../../test/java/org/hibernate/userguide/mapping/identifier/UuidCustomGeneratedValueTest.java[role=include]Most of the Hibernate generators that separately obtain identifier values from database structures support the use of pluggable optimizers. Optimizers help manage the number of times Hibernate has to talk to the database in order to generate identifier values. For example, with no optimizer applied to a sequence-generator, every time the application asked Hibernate to generate an identifier it would need to grab the next sequence value from the database. But if we can minimize the number of times we need to communicate with the database here, the application will be able to perform better, which is, in fact, the role of these optimizers.
- none
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No optimization is performed. We communicate with the database each and every time an identifier value is needed from the generator.
- pooled-lo
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The pooled-lo optimizer works on the principle that the increment-value is encoded into the database table/sequence structure. In sequence-terms, this means that the sequence is defined with a greater-than-1 increment size.
For example, consider a brand new sequence defined as
create sequence m_sequence start with 1 increment by 20. This sequence essentially defines a "pool" of 20 usable id values each and every time we ask it for its next-value. The pooled-lo optimizer interprets the next-value as the low end of that pool.So when we first ask it for next-value, we’d get 1. We then assume that the valid pool would be the values from 1-20 inclusive.
The next call to the sequence would result in 21, which would define 21-40 as the valid range. And so on. The "lo" part of the name indicates that the value from the database table/sequence is interpreted as the pool lo(w) end.
- pooled
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Just like pooled-lo, except that here the value from the table/sequence is interpreted as the high end of the value pool.
- hilo; legacy-hilo
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Define a custom algorithm for generating pools of values based on a single value from a table or sequence.
These optimizers are not recommended for use. They are maintained (and mentioned) here simply for use by legacy applications that used these strategies previously.
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Applications can also implement and use their own optimizer strategies, as defined by the |
@GenericGenerator allows integration of any Hibernate org.hibernate.id.IdentifierGenerator implementation, including any of the specific ones discussed here and any custom ones.
To make use of the pooled or pooled-lo optimizers,
the entity mapping must use the @GenericGenerator annotation:
@GenericGenerator mappinglink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/PooledOptimizerTest.java[role=include]Now, when saving 5 Person entities and flushing the Persistence Context after every 3 entities:
@GenericGenerator mappinglink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/PooledOptimizerTest.java[role=include]link:extras/id/identifiers-generators-pooled-lo-optimizer-persist-example.sql[role=include]As you can see from the list of generated SQL statements, you can insert 3 entities with just one database sequence call. This way, the pooled and the pooled-lo optimizers allow you to reduce the number of database roundtrips, therefore reducing the overall transaction response time.
JPA 2.0 added support for derived identifiers which allow an entity to borrow the identifier from a many-to-one or one-to-one association.
@MapsIdlink:../../../../../test/java/org/hibernate/userguide/associations/OneToOneMapsIdTest.java[role=include]In the example above, the PersonDetails entity uses the id column for both the entity identifier and for the one-to-one association to the Person entity.
The value of the PersonDetails entity identifier is "derived" from the identifier of its parent Person entity.
@MapsId persist examplelink:../../../../../test/java/org/hibernate/userguide/associations/OneToOneMapsIdTest.java[role=include]The @MapsId annotation can also reference columns from an @EmbeddedId identifier as well.
The previous example can also be mapped using @PrimaryKeyJoinColumn.
@PrimaryKeyJoinColumnlink:../../../../../test/java/org/hibernate/userguide/associations/OneToOnePrimaryKeyJoinColumnTest.java[role=include]|
Note
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Unlike |
If you annotate a given entity with the @RowId annotation and the underlying database supports fetching a record by ROWID (e.g. Oracle),
then Hibernate can use the ROWID pseudo-column for CRUD operations.
@RowId entity mappinglink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/RowIdTest.java[role=include]Now, when fetching an entity and modifying it, Hibernate uses the ROWID pseudo-column for the UPDATE SQL statement.
@RowId examplelink:../../../../../test/java/org/hibernate/userguide/mapping/identifier/RowIdTest.java[role=include]link:extras/id/identifiers-rowid-example.sql[role=include]