Utils class for doing transactions with Google App Engine

TXUtils.java

public final class TXUtils {

    public static final Logger log = Logger.getLogger(TXUtils.class.getName());

    public interface Transaction<T> {
    	T doit(EntityManager em);
    }

    private static final EntityManagerFactory emfInstance = Persistence
	    .createEntityManagerFactory("my-persistence-unit");

    public static <T> T doTransaction(Transaction<T> transaction) {
     	EntityManager em = emfInstance.createEntityManager();
  	  EntityTransaction tx = null;
  	  try {
  	    tx = em.getTransaction();
  	    tx.begin();
  	    T retVal = transaction.doit(em);
  	    tx.commit();
  	    return retVal;
  	  } catch (RuntimeException e) {
  	    if (tx != null && tx.isActive())
  		    tx.rollback();
  	    log.log(Level.SEVERE, "error during transaction: "
  		    + transaction.getClass().getName(), e);
  	    throw e;
  	  } finally {
  	    em.close();
  	  }
    }
}