Validators check that any input available to the transition (such as user-supplied input) is valid before the transition is performed. If a transition's validator 'fails', the transition's post functions will not be executed and the issue will not progress to the destination step of the transition.
You can use JJupin and SIL to define a large set of validators beside the default validators offered by Jira, by just adding a (k) SIL Validator to a transition and publishing the workflow.
Below are just a few common examples:
Required plugins
Comment Required
A validator that forces users to enter a comment during a transition, otherwise displaying a customized error message:
SIL Code
string errorMsg = "You must enter a comment!";
if(!hasInput("comment")) {
return false, "comment", errorMsg;
}