Introduction

One of my readers has recently asked me to answer this StackOverflow question, and because the question is very interesting, I decided to turn the answer into a blog post.

In this article, I’m going to show you how you can get access to database table metadata using Hibernate 5 API.

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;

    public Database getDatabase() {
        return database;
    }

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

        database = metadata.getDatabase();
    }

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

The org.hibernate.boot.model.relational.Database is what we are interested in since it contains all the database-related metadata.

To register MetadataExtractorIntegrator with Hibernate we have two possibilities based on the 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();

Domain Model

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

When running the following test case:

for(Namespace namespace : MetadataExtractorIntegrator.INSTANCE
    .getDatabase()
    .getNamespaces()) {

    for( Table table : namespace.getTables()) {
        LOGGER.info( "Table {} has the following columns: {}",
             table,
             StreamSupport.stream(
                Spliterators.spliteratorUnknownSize( 
                    table.getColumnIterator(), 
                    Spliterator.ORDERED
                ), 
                false
            )
            .collect( Collectors.toList()) 
        );
    }
}

Hibernate generates the following output:

Table org.hibernate.mapping.Table(post) has the following columns: [
    org.hibernate.mapping.Column(id), 
    org.hibernate.mapping.Column(title), 
    org.hibernate.mapping.Column(version)
]
Table org.hibernate.mapping.Table(post_comment) has the following columns: [
    org.hibernate.mapping.Column(id), 
    org.hibernate.mapping.Column(review), 
    org.hibernate.mapping.Column(version), 
    org.hibernate.mapping.Column(post_id)
]
Table org.hibernate.mapping.Table(post_details) has the following columns: [
    org.hibernate.mapping.Column(id), 
    org.hibernate.mapping.Column(created_by), 
    org.hibernate.mapping.Column(created_on), 
    org.hibernate.mapping.Column(version)
]
Table org.hibernate.mapping.Table(post_tag) has the following columns: [
    org.hibernate.mapping.Column(post_id), 
    org.hibernate.mapping.Column(tag_id)
]
Table org.hibernate.mapping.Table(tag) has the following columns: [
    org.hibernate.mapping.Column(id), 
    org.hibernate.mapping.Column(name), 
    org.hibernate.mapping.Column(version)
]

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Conclusion

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

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