Hibernate Tips: How to override the primary key generation strategy

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Hibernate Tips is a series of posts in which I describe a quick and easy solution for common Hibernate questions. If you have a question for a future Hibernate Tip, please leave a comment below.

Question:

You explained that the GenerationType.SEQUENCE is the most efficient primary key generation strategy and that MySQL doesn’t support it.

What shall I do if I need to support PostgreSQL and MySQL with the same application?
Do I need to use the slower GenerationType.IDENTITY with PostgreSQL as well?

Solution:

No, you can use an external mapping file to override the mappings defined via annotations. So, you can use annotations to define your default mapping and override them if necessary.

Define The Default Mapping

In this case, you can use the @GeneratedValue annotation to define the GenerationType.SEQUENCE to generate the primary key values for the Author entity. This is the default strategy for your application. You can use it with all databases that support sequences.

@Entity
public class Author {

	@Id
	@GeneratedValue(strategy = GenerationType.SEQUENCE)
	@Column(name = "id", updatable = false, nullable = false)
	private Long id;

	...
}

When you now persist a new Author entity, Hibernate retrieves a new primary key value from the database sequence.

20:45:18,203 DEBUG [org.hibernate.SQL] - 
    select
        nextval ('hibernate_sequence')
20:45:18,245 DEBUG [org.hibernate.SQL] - 
    insert 
    into
        Author
        (firstName, lastName, version, id) 
    values
        (?, ?, ?, ?)

Override Parts Of Your Mapping

And when you install your application with a MySQL database, you need to override the generation strategy with an additional mapping file.

By default, JPA and Hibernate check if an orm.xml file exists in the META-INF directory and use it to override the mappings defined by the annotations. So, you just need to provide the mappings you want to change.

In this case, it’s only the generation strategy for the id attribute of the Author entity.

<entity-mappings>
    <entity class="org.thoughts.on.java.model.Author" name="Author">
        <attributes>
            <id name="id">
            	<generated-value strategy="identity"/>
            </id>
        </attributes>
   </entity>
</entity-mappings>

As you can see in the following log message, Hibernate now uses the GenerationType.IDENTITY which uses an auto-incremented database column to generate the primary key value.

20:42:47,414 DEBUG [org.hibernate.SQL] - 
    insert 
    into
        Author
        (firstName, lastName, version) 
    values
        (?, ?, ?)

Learn More

You can read more about the different strategies to generate unique primary key values in:

• How To Generate Primary Keys With JPA And Hibernate
• How To Generate UUIDs As Primary Keys

And if you want to learn more database-specific mapping and query features, you should take a look at:

• 5 Things You Need To Know When Using Hibernate With MySQL
• Hibernate With PostgreSQL – 6 Things You Need To Know