validator

even( validator: {
   return (it % 2) == 0
})
password1( validator: {
   val, obj ->
      obj.properties['password2'] == val
})
magicNumber( validator:
   someClosureWithTwoParameters)
// This one assumes you have an error message defined like:
// classname.propertyName.custom.error=My error shows arguments {3} and {4} for value {0}
otherProperty( validator: { return ['custom.error', arg1, arg2] } )
// The following example does not use custom validation.
// A custom message may be defined in messages.properties:
// user.login.blank=Please enter a login
// which will be used instead of default.blank.message
class User {
    String login
    static constraints = {
        login(blank:false)
    }
}
// In the following example, custom validation is used:
// user.login.validator.invalid=Please enter a login
class User {
   String login
    static constraints = {
        login(validator: {
            return (it.length != 0)
    	})
    }
}
// The following might define the error message as:
// user.login.invalid.bountyhunter=Invalid bounty hunter ({2}) tried to log in.  (Class name = {1}.  Property name = {0})
class User {
    String login
    static constraints = {
        login(validator: {
            if (!it.startsWith('boba')) return ['invalid.bountyhunter']
        })
    }
}