Introduction

Lukas Eder has posted a very interesting question on Stack Overflow, asking about a way of getting access to the binding metadata between entity mappings and database tables.

While I have previously answered a similar question using this article, I realized that case was much simpler since it only extracted the database metadata. In this article, you are going to see how easily you can get the bridge information between your JPA entities and the underlying database schema.

Integrator

Hibernate is very flexible, so it defines many SPI (Service Provider Interfaces) that you can register to customize Hibernate internals. One of these interfaces is org.hibernate.integrator.spi.Integrator which is used by many technologies that integrate with Hibernate ORM, like Bean Validation, Envers or JACC Security Provider.

Using the Hibernate Integrator API, we can write our own component that captures the SessionFactory build-time metadata which, otherwise, is only available during bootstrap.

public class MetadataExtractorIntegrator 
    implements org.hibernate.integrator.spi.Integrator {

    public static final MetadataExtractorIntegrator INSTANCE = 
        new MetadataExtractorIntegrator();

    private Database database;

    private Metadata metadata;

    public Database getDatabase() {
        return database;
    }

    public Metadata getMetadata() {
        return metadata;
    }

    @Override
    public void integrate(
            Metadata metadata,
            SessionFactoryImplementor sessionFactory,
            SessionFactoryServiceRegistry serviceRegistry) {

        this.database = metadata.getDatabase();
        this.metadata = metadata;

    }

    @Override
    public void disintegrate(
        SessionFactoryImplementor sessionFactory,
        SessionFactoryServiceRegistry serviceRegistry) {

    }
}

The org.hibernate.boot.Metadata is what we are interested in since it contains the PersistentClass entity bindings.

To register MetadataExtractorIntegrator with Hibernate we have two possibilities based on the underlying bootstrap method.

Hibernate-native boostrap

If you’re using the Hibernate-native bootstrap, then you can register the Integrator with the BootstrapServiceRegistryBuilder as follows:

final BootstrapServiceRegistry bootstrapServiceRegistry = 
    new BootstrapServiceRegistryBuilder()
    .enableAutoClose()
    .applyIntegrator( MetadataExtractorIntegrator.INSTANCE )
    .build();

final StandardServiceRegistry serviceRegistry = 
    new StandardServiceRegistryBuilder(bootstrapServiceRegistry)
    .applySettings(properties())
    .build();

JPA boostrap

If you’re using the JPA bootstrap, then you can register the Integrator with the BootstrapServiceRegistryBuilder as follows:

Map<String, Object> configuration = new HashMap<>();

Integrator integrator = integrator();
if (integrator != null) {
    configuration.put("hibernate.integrator_provider", 
        (IntegratorProvider) () -> Collections.singletonList(
            MetadataExtractorIntegrator.INSTANCE
        )
    );
}

EntityManagerFactory entityManagerFactory = new EntityManagerFactoryBuilderImpl(
    new PersistenceUnitInfoDescriptor(persistenceUnitInfo), 
    configuration
)
.build();

That’s for when you want to bootstrap JPA without any persistence.xml configuration file.

If you want to use the persistence.xml configuration file, then you just need to provide a new property to declare the custom Integrator:

<property
    name="hibernate.integrator_provider"
    value="com.vladmihalcea.book.hpjp.hibernate.metadata.MetadataExtractorIntegrator"
/>

Domain Model

Assuming we have the following database tables mapped by our JPA application:

When running the following test case:

Metadata metadata = MetadataExtractorIntegrator.INSTANCE.getMetadata();

for ( PersistentClass persistentClass : metadata.getEntityBindings()) {

    Table table = persistentClass.getTable();

    LOGGER.info( "Entity: {} is mapped to table: {}",
                 persistentClass.getClassName(),
                 table.getName()
    );

    for(Iterator propertyIterator = persistentClass.getPropertyIterator(); 
            propertyIterator.hasNext(); ) {
        Property property = (Property) propertyIterator.next();

        for(Iterator columnIterator = property.getColumnIterator(); 
                columnIterator.hasNext(); ) {
            Column column = (Column) columnIterator.next();

            LOGGER.info( "Property: {} is mapped on table column: {} of type: {}",
                         property.getName(),
                         column.getName(),
                         column.getSqlType()
            );
        }
    }
}

Hibernate generates the following output:

Entity: com.vladmihalcea.book.hpjp.util.providers.entity.BlogEntityProvider$Tag is mapped to table: tag
Property: name is mapped on table column: name of type: varchar(255)
Property: version is mapped on table column: version of type: integer

Entity: com.vladmihalcea.book.hpjp.util.providers.entity.BlogEntityProvider$PostComment is mapped to table: post_comment
Property: post is mapped on table column: post_id of type: bigint
Property: review is mapped on table column: review of type: varchar(255)
Property: version is mapped on table column: version of type: integer

Entity: com.vladmihalcea.book.hpjp.util.providers.entity.BlogEntityProvider$Post is mapped to table: post
Property: title is mapped on table column: title of type: varchar(255)
Property: version is mapped on table column: version of type: integer

Entity: com.vladmihalcea.book.hpjp.util.providers.entity.BlogEntityProvider$PostDetails is mapped to table: post_details
Property: createdBy is mapped on table column: created_by of type: varchar(255)
Property: createdOn is mapped on table column: created_on of type: datetime(6)
Property: version is mapped on table column: version of type: integer

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Conclusion

Hibernate is highly customizable, and the Integrator SPI allows you to get access to the Hibernate Metadata which you can later inspect from your enterprise application.