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Hibernate's entity version (audit/history) support

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/*
 * Hibernate, Relational Persistence for Idiomatic Java
 *
 * License: GNU Lesser General Public License (LGPL), version 2.1 or later.
 * See the lgpl.txt file in the root directory or .
 */
package org.hibernate.envers;

import javax.persistence.EntityManager;

import org.hibernate.Session;
import org.hibernate.engine.spi.SessionImplementor;
import org.hibernate.envers.boot.internal.EnversService;
import org.hibernate.envers.exception.AuditException;
import org.hibernate.envers.internal.reader.AuditReaderImpl;
import org.hibernate.service.ServiceRegistry;

/**
 * @author Adam Warski (adam at warski dot org)
 */
public class AuditReaderFactory {
	private AuditReaderFactory() {
	}

	/**
	 * Create an audit reader associated with an open session.
	 *
	 * @param session An open session.
	 *
	 * @return An audit reader associated with the given sesison. It shouldn't be used
	 *         after the session is closed.
	 *
	 * @throws AuditException When the given required listeners aren't installed.
	 */
	public static AuditReader get(Session session) throws AuditException {
		SessionImplementor sessionImpl;
		if ( !(session instanceof SessionImplementor) ) {
			sessionImpl = (SessionImplementor) session.getSessionFactory().getCurrentSession();
		}
		else {
			sessionImpl = (SessionImplementor) session;
		}

		final ServiceRegistry serviceRegistry = sessionImpl.getFactory().getServiceRegistry();
		final EnversService enversService = serviceRegistry.getService( EnversService.class );

		return new AuditReaderImpl( enversService, session, sessionImpl );
	}

	/**
	 * Create an audit reader associated with an open entity manager.
	 *
	 * @param entityManager An open entity manager.
	 *
	 * @return An audit reader associated with the given entity manager. It shouldn't be used
	 *         after the entity manager is closed.
	 *
	 * @throws AuditException When the given entity manager is not based on Hibernate, or if the required
	 * listeners aren't installed.
	 */
	public static AuditReader get(EntityManager entityManager) throws AuditException {
		if ( entityManager.getDelegate() instanceof Session ) {
			return get( (Session) entityManager.getDelegate() );
		}

		if ( entityManager.getDelegate() instanceof EntityManager ) {
			return get( (EntityManager) entityManager.getDelegate() );
		}

		throw new AuditException( "Hibernate EntityManager not present!" );
	}
}