class Domain {
Integer id
String url
static constraints = {
url nullable: true
}
}null。null，来对应系统默认设置的 nullable: falsegrails-app/conf/application.groovy
// the default value for this property is true
grails.databinding.convertEmptyStringsToNull = falsehttps://docs.grails.org/latest/ref/Constraints/nullable.html
原文：https://www.cnblogs.com/duchaoqun/p/12807065.html