code: public void setIdentifierMetadata(IdentifierMetadata identifierMetadata) {
      if (this.identifierMetadata != null) {
        String format = "Class %s has at least two fields, %s and %s, marked with %s annotation. "
            + "Only one field can be marked as an identifier. ";
        String message = String.format(format, entityClass.getName(),
            this.identifierMetadata.getName(), identifierMetadata.getName(),
            Identifier.class.getName());
        throw new EntityManagerException(message);
      }
      this.identifierMetadata = identifierMetadata;
    }
docs: Sets the metadata of the identifier. An exception will be thrown if there is an
  IdentifierMetadata already set.
  
  @param identifierMetadata
  the metadata of the identifier.