messageExpression property
MessageExpression declares a CEL expression that evaluates to the validation failure message that is returned when this rule fails. Since messageExpression is used as a failure message, it must evaluate to a string. If both message and messageExpression are present on a rule, then messageExpression will be used if validation fails. If messageExpression results in a runtime error, the runtime error is logged, and the validation failure message is produced as if the messageExpression field were unset. If messageExpression evaluates to an empty string, a string with only spaces, or a string that contains line breaks, then the validation failure message will also be produced as if the messageExpression field were unset, and the fact that messageExpression produced an empty string/string with only spaces/string with line breaks will be logged. messageExpression has access to all the same variables as the rule; the only difference is the return type. Example: "x must be less than max ("+string(self.max)+")".
Implementation
final String? messageExpression;