How to enable the Hibernate HiLo entity identifier

问题:

I'm initializing Hibernate without any XML by something like

org.hibernate.SessionFactory sessionFactory = 
    new org.hibernate.cfg.Configuration().
    .setProperty(...)
    .setProperty(...)
    ...
    .buildSessionFactory();

My classes use an ID like

@Id @Generated(GenerationTime.INSERT) @GeneratedValue private Integer id;

The generator used is SequenceStyleGenerator, which seems to be the replacement for the deprecated SequenceGenerator and SequenceHiLoGenerator and whatever. It uses

public static final int DEFAULT_INCREMENT_SIZE = 1;

and seems to allow configuration via

public static final String INCREMENT_PARAM = "increment_size";

but that's all I could find out. I guess I have to set some property "xxx.yyy.increment_size" or pass it in another way to Hibernate, but I can't see how.

---

I'm aware of @SequenceGenerator, but it seems to be completely ignored

回答1:

I guess you are looking for how to set increment_size property for your SequenceSytleGenerator.

Sample snippet below setting increment_size using @GenericGenerator annotation with hilo optimizer and SEQUENCE strategy.

@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "hilo_generator")
@GenericGenerator(
        name = "hilo_generator",
        strategy = "org.hibernate.id.enhanced.SequenceStyleGenerator",
        parameters = {
                // Or leave it out to get "hibernate_sequence".
                @Parameter(name = "sequence_name", value = "hilo_sequence"),
                // Or leave it out as this is the default.
                @Parameter(name = "initial_value", value = "1"),
                @Parameter(name = "increment_size", value = "5"),
                @Parameter(name = "optimizer", value = "hilo")
        })

There's no way you can globally set the DEFAULT_INCREMENT_SIZE with a Hibernate configuration property. You need to use the @Id configuration properties instead.

回答2:

@Generated vs @GeneratedValue

You don't need to use @Generated along with @GeneratedValue. The @Generated annotation is for non-id entity attributes that are generated by the database during INSERT or UPDATE. For more details about the @Generated annotation, check out this article.

On the other hand, the @GeneratedValue is only for the entity identifier attributes, and it's what you need to use when the entity identifier is generated automatically upon persisting an entity.

Sequence generator

The sequence generator requires an extra database roundtrip to call the sequence object when you persist the entity. For this reason, Hibernate offers the sequence-based optimizers to reduce the number of roundtrips needed to fetch entity identifier values.

Now, if you want to use hilo, the identifier mapping will look as follows:

@Id
@GeneratedValue(
    strategy = GenerationType.SEQUENCE, 
    generator = "post_sequence"
)
@GenericGenerator(
    name = "post_sequence",
    strategy = "sequence",
    parameters = {
        @Parameter(name = "sequence_name",  value = "post_sequence"),
        @Parameter(name = "initial_value",  value = "1"),
        @Parameter(name = "increment_size",  value = "3"),
        @Parameter(name = "optimizer", value = "hilo")
    }
)
private Long id;

Apart from having to use the Hibernate-specific @GenericGenerator, the problem with hilo is that the generated identifiers don't include the database sequence value, so an 3rd-party client using the database will not know how to generate the next identifier value unless they know the hilo algorithm and the allocationSize.

For this reason, it's better to use pooled or pooled-lo.

Pooled optimizer

The pooled optimizer is very easy to set up. All you need to do is set the allocationSize of the JPA @SequenceGenerator annotation, and Hibernate is going to switch to using the pooled optimizer:

@Id
@GeneratedValue(
    strategy = GenerationType.SEQUENCE,
    generator = "post_sequence"
)
@SequenceGenerator(
    name = "post_sequence",
    sequenceName = "post_sequence",
    allocationSize = 3
)
private Long id;

Pooled-lo optimizer

To use the pooled-lo optimizer, just add the following configuration property:

<property name="hibernate.id.optimizer.pooled.preferred" value="pooled-lo" />

Now, the entity identifier mapping is identical to the one I showed you before for the pooled optimizer.

To understand how the pooled-lo works, check out this diagram:

If you have been using the legacy hilo optimizer, you might want to switch to using pooled or pooled-lo, as hilo is not interoperable with other clients that might not be aware of the hilo identifier allocation strategy.

For more details about how you can migrate from hilo to pooled or pooled-lo, check out this article.