Prior to 5.3.1.Final, only the javax.el API was necessary if you were using a ResourceBundleMessageInterpolator: the ExpressionFactory was initialized at runtime for each interpolation.

When we fixed https://hibernate.atlassian.net/browse/HV-1131, we decided to initialize the ExpressionFactory once and for all when initializing the ResourceBundleMessageInterpolator, making a javax.el implementation required if using the ResourceBundleMessageInterpolator.

The issue is that if the implementation is missing, ExpressionFactory.newInstance() throws a javax.el.ELException which is not caught and wrapped into a ValidationException with a nice message.