CreateUserValidator.groovy

package examples.usermgr

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.opensymphony.workflow.InvalidInputException


/* CONFIGURABLE SECTION */

// custom field names
final def userNameField = "Username"
final def passwordField = "Password"
final def displayNameField = "Display Name"
final def emailField = "E-Mail"

/* END CONFIGURABLE SECTION */

def issue = issue as Issue


def customFieldManager = ComponentAccessor.getCustomFieldManager()
def customFields = customFieldManager.getCustomFieldObjects(issue)

[userNameField, passwordField, displayNameField, emailField].each {fieldName ->
    def field = customFields.find { it.name == fieldName }
    assert field // fields not found or not associated with issue

    if (! issue.getCustomFieldValue(field)) {
        throw new InvalidInputException("Please enter all of $userNameField, $passwordField, $displayNameField and $emailField")
    }
}

def username = issue.getCustomFieldValue(customFields.find {it.name == userNameField}) as String

def userUtil = ComponentAccessor.getUserUtil()

if (userUtil.getUserByKey(username)) {
    throw new InvalidInputException(userNameField, "User already exists with this key")
}

if (userUtil.getUserByName(username)) {
    throw new InvalidInputException(userNameField, "User already exists with this user name")
}