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 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: [params.name])]
	}
	}

view raw GreetingController.groovy hosted with ❤ by GitHub

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 org.springframework.context.support.*
	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)
	}
	@Unroll
	def "greeting is rendered by index action"() {
	given:
	if (name) controller.params.name = name
	if (locale) controller.request.addPreferredLocale(locale)
	expect:
	controller.index() == [message: message]
	where:
	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)
	}
	}
	}

view raw GreetingControllerSpec.groovy hosted with ❤ by GitHub

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.