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RequiredFieldValidator checks if the specified field is not null.
fieldName
- field name if plain-validator syntax is used, not needed if field-validator syntax is used.<validators> <!-- Plain Validator Syntax --> <validator type="required"> <param name="fieldName">username</param> <message>username must not be null</message> </validator> <!-- Field Validator Syntax --> <field name="username"> <field-validator type="required"> <message>username must not be null</message> </field-validator> </field> </validators>