A method showing how to test to see if an EntityManager contains an arbitrary object, not just a known entity, without running the risk of rolling the transaction back.

gistfile1.java

/**
 * Returns {@code true} if the supplied {@link EntityManager} is
 * non-{@code null} and contains the supplied {@link Object} in its
 * persistence context.  Unlike the {@link EntityManager#contains(Object)}
 * method, this method will not throw any exceptions, and hence will
 * never roll the containing transaction back.
 *
 * @param em the {@link EntityManager} in question; may be {@code null}
 * in which case {@code false} will be returned
 *
 * @param o the {@link Object} to look for; may be {@code null} in which 
 * case {@code false} will be returned
 *
 * @return {@code true} if the supplied {@link Object} is a {@linkplain
 * Metamodel#getManagedTypes() managed type} with respect to the supplied
 * {@link EntityManager} and is also {@linkplain
 * EntityManager#contains(Object) contained} by the supplied {@link 
 * EntityManager}; {@code false} otherwise
 */
public static final boolean contains(final EntityManager em, final Object o) {
  boolean returnValue = false;
  if (em != null && o != null) {
    final Metamodel mm = em.getMetamodel();
    if (mm != null) {
      final Collection<?> types = mm.getManagedTypes();
      returnValue = types != null && types.contains(o) && em.contains(o);
    }
  }
  return returnValue;
}