Best How To :

I suppose you have your class MessageMapping.java which has in turn a list (or one? dunno) of message of type Message.java. Message.java in turn will be structured with a list of Field of type Field.java. The classes will be as follow:

    @XmlAccessorType(XmlAccessType.FIELD)
    public class Field {

        @XmlAttribute
        private String tag;

        @XmlAtrribute
        private String source;

        @XmlAtrribute
        private String tranData;

        @XmlAtrribute
        private String dataType;

        @XmlAtrribute
        private String defaultValue;

        /*
        GETTER AND SETTER HERE
        */

    }

And Message.java like this:

        @XmlAccessorType(XmlAccessType.FIELD)
        public class Message {
            private List<Field> fields;
            private String Rtype;
            private String direction;
            private String name;
            private String mode;

            /*
            GETTER AND SETTER HERE
            */
        }

And ultimately message_mapping class will need to be built as you see fit to accomodate a list of message or a single one, dunno what are your specification. Hope it helps.