2.6.8. Using sequences

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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.

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The preferred (and portable) way to configure this generator is using the JPA-defined javax.persistence.SequenceGenerator annotation.

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The simplest form is to simply request sequence generation; Hibernate will use a single, implicitly-named sequence (`hibernate_sequence`) for all such unnamed definitions.

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<div class="title">Example 116. Unnamed sequence</div>
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<pre class="prettyprint highlight">`@Entity
public class MyEntity {

    @Id
    @GeneratedValue( generation = SEQUENCE )
    public Integer id;

    ...
}`</pre>
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Or a specifically named sequence can be requested

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<div class="title">Example 117. Named sequence</div>
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<pre class="prettyprint highlight">`@Entity
public class MyEntity {

    @Id
    @GeneratedValue( generation = SEQUENCE, name = "my_sequence" )
    public Integer id;

    ...
}`</pre>
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Use javax.persistence.SequenceGenerator to specify additional
configuration.

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<div class="title">Example 118. Configured sequence</div>
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<pre class="prettyprint highlight">`@Entity
public class MyEntity {

    @Id
    @GeneratedValue( generation = SEQUENCE, name = "my_sequence" )
    @SequenceGenerator( name = "my_sequence", schema = "globals", allocationSize = 30 )
    public Integer id;

    ...
}`</pre>
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