How do you programmatically generate a Hibernate JPA schema?

Question

I'd like to use the Hibernate/HBM2DDL schema generation as a starting point for managing my application's SQL schema using a tool like Liquibase or Flyway. To assist with that, I need a small utility in my project that I can run that will print out the auto-generated schema.

With older versions or Hibernate, this was relatively simple. Something like the following would work:

EntityManagerFactory emf = null; // TODO: create your EMF the usual way.
Class<? extends Dialect> hibernateDialectType = null; // TODO: e.g. HSQLDialect.class.

Configuration hibernateConfig = new Configuration();
hibernateConfig.setProperty(Environment.DIALECT, hibernateDialectType.getName());
for (EntityType<?> entityType : emf.getMetamodel().getEntities()) {
        hibernateConfig.addAnnotatedClass(entityType.getJavaType());
}

SchemaExport schemaExporter = new SchemaExport(hibernateConfig);
schemaExporter.setFormat(true);
schemaExporter.setDelimiter(";");
schemaExporter.create(Target.SCRIPT);

But as of at least Hibernate 5.2, the SchemaExport utility can no be built from a Hibernate Configuration instance.

So how can this be done nowadays?

Answer 1

After digging through the Hibernate Ant task's source on GitHub, I came up with the following solution:

/**
 * Uses Hibernate's HBM2DDL {@link SchemaExport} utility to generate SQL
 * database schemas.
 */
public final class HibernateSchemaPrinter {
    /**
     * A small application driver that calls
     * {@link #printHibernateSchemaToStdout(String, Class)}.
     * 
     * @param args
     *            (unused)
     */
    public static void main(String[] args) {
        printHibernateSchemaToStdout("gov.hhs.cms.bluebutton.data", PostgreSQL95Dialect.class);
    }

    /**
     * Prints the Hibernate-/HDM2DDL- auto-generated SQL schema to
     * {@link System#out}.
     * 
     * @param persistenceUnitName
     *            the name of the JPA persistence unit to generate the schema
     *            for
     * @param dialectType
     *            the Hibernate {@link Dialect} type to generate the schema for,
     *            e.g. {@link PostgreSQL95Dialect}
     */
    public static void printHibernateSchemaToStdout(String persistenceUnitName, Class<? extends Dialect> dialectType) {
        Map<Object, Object> properties = new HashMap<>();
        properties.put(AvailableSettings.DIALECT, dialectType.getName());

        /*
         * Use a Hibernate EntityManagerFactoryBuilderImpl to create a JPA
         * EntityManagerFactory, then grab the (now populated) Hibernate
         * Metadata instance out of it.
         */
        EntityManagerFactoryBuilderImpl entityManagerFactoryBuilder = new CustomHibernatePersistenceProvider()
                .getEntityManagerFactoryBuilder(persistenceUnitName, properties);
        entityManagerFactoryBuilder.build();
        Metadata metadata = entityManagerFactoryBuilder.getMetadata();

        SchemaExport schemaExport = new SchemaExport();
        schemaExport.setHaltOnError(true);
        schemaExport.setFormat(true);
        schemaExport.setDelimiter(";");
        schemaExport.execute(EnumSet.of(TargetType.STDOUT), Action.CREATE, metadata);
    }

    /**
     * A small hack, needed to extract the
     * {@link EntityManagerFactoryBuilderImpl} from
     * {@link HibernatePersistenceProvider}. Taken from the Hibernate Ant task
     * here: <a href=
     * "https://github.com/hibernate/hibernate-tools/blob/321dba082f0cd11a2295063e0cbcf4f34a5b8bdd/main/src/java/org/hibernate/tool/ant/JPAConfigurationTask.java">
     * JPAConfigurationTask.java</a>.
     */
    private static final class CustomHibernatePersistenceProvider extends HibernatePersistenceProvider {
        /**
         * (See overridden method; we're just making it <code>public</code>.)
         * 
         * @param persistenceUnit
         *            (see overridden method)
         * @param properties
         *            (see overridden method)
         * @return (see overridden method)
         */
        public EntityManagerFactoryBuilderImpl getEntityManagerFactoryBuilder(String persistenceUnit,
                Map<Object, Object> properties) {
            return (EntityManagerFactoryBuilderImpl) getEntityManagerFactoryBuilderOrNull(persistenceUnit, properties);
        }
    }
}

Takes a bit more code these days, but still works, so good enough.

Answer 2

If, instead, you want to generate a Liquibase changelog right from the Hibernate metadata, you can use the following code:

// Create a "connection" to the offline JPA data.
String url = "jpa:persistence:META-INF/persistence.xml";
Database jpaDatabase = CommandLineUtils.createDatabaseObject(RESOURCE_ACCESSOR, url, null, null, null, null,
        null, false, false, null, null, null, null, null, null, null);

DiffResult schemaDiff = DiffGeneratorFactory.getInstance().compare(jpaDatabase, null,
        CompareControl.STANDARD);
DiffToChangeLog diffChangeLogProducer = new DiffToChangeLog(schemaDiff, new DiffOutputControl());
diffChangeLogProducer.print(System.out);

Answer 3

I see no good reason to not use standard JPA, via

Persistence.generateSchema(String persistenceUnitName, Map properties);

and that way you don't tie yourself to any particular implementation, and still can get a DDL script by use of the javax.persistence.schema-generation.* properties.