ad-hockery: /ad·hok'@r·ee/, n.
Gratuitous assumptions... which lead to the appearance of semi-intelligent behavior but are in fact entirely arbitrary. Jargon File

Stubbing access to the g:message tag in unit tests

Grails controllers and tag libs can access any tag as though it were a method. The most common use for this is probably accessing i18n messages via the [g:message][1] tag. However, because tag access is magic wired up by Grails it’s not available in unit tests without some effort.

This is a perennial nuisance. Not exactly difficult to solve and yet something I find myself solving over and over in different tests and different projects.

I’ve come up with what I think is a pretty good and re-usable solution. It allows you to specify messages if you want to, or just use the message code if it’s not something you care about in a particular test. As an aside, at a unit test level, I think testing that the correct message code is being used is probably the right thing to do; I’d leave testing actual message text to end-to-end tests.

Here’s an example. Imagine we’re testing the following controller that displays a simple greeting:

class GreetingController {
    def index = {
        [message: message(code: "greeting.message", args: [])]

Here’s a spec that shows the behaviour both when a message is specified and when it isn’t:

import grails.plugin.spock.*
import org.springframework.context.*
import spock.lang.*

class GreetingControllerSpec extends ControllerSpec {

    @Shared def messageSource = new StaticMessageSource()
    @Shared def pirateEnglish = new Locale("en", "BV")

    def setupSpec() {
        messageSource.useCodeAsDefaultMessage = true
        messageSource.addMessage "greeting.message", pirateEnglish, "Ahoy there {0}!"

    def setup() {
        mockMessageTag(controller, messageSource)

    def "greeting is rendered by index action"() {
        if (name) = name
        if (locale) controller.request.addPreferredLocale(locale)

        controller.index() == [message: message]

        name  | locale        | message
        null  | null          | "greeting.message"
        "Rob" | null          | "greeting.message"
        "Rob" | pirateEnglish | "Ahoy there Rob!"

    // in reality this would be static imported from a helper class
    static void mockMessageTag(artefact, MessageSource messageSource) {
        artefact.metaClass.message = { attrs ->
            messageSource.getMessage(attrs.code, attrs.args as Object[], delegate.request.locale)

A few things to note:

  1. The stubbed message tag returns the code if there is no message defined

  2. message arguments are ignored unless there is a message defined

Although in the example I’ve used Spock, this technique would work equally well with JUnit tests extending ControllerUnitTestCase. It will also work just as well for tag libs tests extending TagLibUnitTestCase or TagLibSpec.

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