Add available-locale discovery and a language selector to i18n

[codeconsole]

Summary

Grails already ships the switch-language half of i18n — I18nAutoConfiguration wires a SessionLocaleResolver and a ?lang=-driven LocaleChangeInterceptor, and FormTagLib has a g:localeSelect. What was missing was a way to discover which locales an application is actually translated into (g:localeSelect lists all ~150 Locale.getAvailableLocales(), which is not useful as a language switcher) and a ready-made selector in the generated app.

This PR adds that discovery to grails-i18n and surfaces a language dropdown in the default create-app layout. Because a generated web app already ships the messages_*.properties bundles (via the forge default plugins / the base profile skeleton), a freshly created app shows a fully-populated language switcher out of the box with no configuration.

Changes

grails-i18n

• New AvailableLocaleResolver — scans the classpath for messages_*.properties and returns the translated locales (plus the configurable default locale, grails.i18n.default.locale, default en), sorted by display name and cached.
• I18nAutoConfiguration registers it as a bean (@ConditionalOnMissingBean).
• I18nGrailsPlugin publishes the list to the servlet context (availableLocales) on startup and re-scans on dev-mode bundle changes. Reading a servlet-context attribute keeps consumers decoupled from this module (no new compile dependency in grails-gsp / grails-forge).

grails-gsp

• g:localeSelect gains an available="true" attribute that lists only the translated locales instead of every JVM locale. Default behaviour is unchanged.

Generated app (grails-forge template + classic web profile skeleton)

• The default main.gsp navbar gets a Bootstrap language dropdown that iterates the published locales and switches via the existing ?lang= interceptor. It renders only when more than one locale is available.

Docs

• localeSelect tag reference + the "Changing Locales" guide.

Tests

• AvailableLocaleResolverSpec (resolver discovery, sorting, default locale, caching, immutability)
• LocaleSelectAvailableSpec (available="true" behaviour + a render test of the generated navbar snippet producing correct ?lang= links)
• GrailsGspSpec gains a check that the generated layout contains the dropdown

All green.

Update: plugin-contributed locales (default on)

AvailableLocaleResolver now discovers locales contributed by plugins too — their bundles are namespaced (e.g. spring-security-core_fr.properties) and were previously missed. When enabled it scans every root-level *.properties bundle on the classpath and validates each candidate suffix against Locale.getISOLanguages(), so non-i18n files (application.properties, etc.) are ignored.

Controlled by grails.i18n.availableLocales.includePlugins, default true — a selector lists every locale the app or its plugins provide out of the box. Set it to false to restrict discovery to the application's own messages_*.properties.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.4143%. Comparing base (b5e583a) to head (8f9d7aa).
⚠️ Report is 4 commits behind head on 8.0.x.

Files with missing lines	Patch %	Lines
...g/grails/plugins/i18n/AvailableLocaleResolver.java	80.8511%	4 Missing and 5 partials ⚠️
...vy/org/grails/plugins/i18n/I18nGrailsPlugin.groovy	53.8462%	4 Missing and 2 partials ⚠️
...vy/org/grails/plugins/web/taglib/FormTagLib.groovy	66.6667%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #15940        +/-   ##
==================================================
+ Coverage     49.3909%   49.4143%   +0.0234%     
- Complexity      16808      16828        +20     
==================================================
  Files            1993       1994         +1     
  Lines           93495      93562        +67     
  Branches        16360      16375        +15     
==================================================
+ Hits            46178      46233        +55     
- Misses          40180      40184         +4     
- Partials         7137       7145         +8     

Files with missing lines	Coverage Δ
...org/grails/plugins/i18n/I18nAutoConfiguration.java	76.6667% <100.0000%> (+1.6667%)	⬆️
...vy/org/grails/plugins/web/taglib/FormTagLib.groovy	76.4808% <66.6667%> (-0.1448%)	⬇️
...vy/org/grails/plugins/i18n/I18nGrailsPlugin.groovy	14.2857% <53.8462%> (+14.2857%)	⬆️
...g/grails/plugins/i18n/AvailableLocaleResolver.java	80.8511% <80.8511%> (ø)

... and 4 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 8f9d7aa
▶️ Tests: 17712 executed
⚪️ Checks: 61/61 completed

---

Learn more about TestLens at testlens.app.

[borinquenkid]

Test coverage audit for this PR (grounded in Codecov's patch report + a line-by-line read of the head commit 8f9d7aa)

Codecov: 75% patch coverage, 17 lines uncovered. Per-file: AvailableLocaleResolver.java 80.85% (4 missing/5 partial), I18nGrailsPlugin.groovy 53.85% (4 missing/2 partial), FormTagLib.groovy 66.67% (0 missing/2 partial branches). Below is what those numbers correspond to in the code, confirmed by grepping the actual test files (no guessing from the summary alone).

1. I18nGrailsPlugin.doWithApplicationContext() / publishAvailableLocales() — no plugin-lifecycle test at all

grails-core/grails-i18n/src/main/groovy/org/grails/plugins/i18n/I18nGrailsPlugin.groovy

Lines 53 to 68 in 8f9d7aa

	@Override
	void doWithApplicationContext() {
	publishAvailableLocales()
	}
	private void publishAvailableLocales() {
	def ctx = applicationContext
	if (!(ctx instanceof WebApplicationContext)) {
	return
	}
	def servletContext = ((WebApplicationContext) ctx).servletContext
	if (servletContext != null && ctx.containsBean('availableLocaleResolver')) {
	AvailableLocaleResolver resolver = ctx.getBean('availableLocaleResolver', AvailableLocaleResolver)
	servletContext.setAttribute(AVAILABLE_LOCALES_ATTRIBUTE, resolver.availableLocales)
	}
	}

Nothing in this PR (or pre-existing) boots the plugin against a WebApplicationContext + ServletContext and asserts servletContext.getAttribute('availableLocales') (L66) is actually populated. The two early-return branches are also unexercised:

• L60-62: ctx is not a WebApplicationContext
• L64: servletContext == null or the availableLocaleResolver bean is absent

The onChange() addition at L123-126 (clear the resolver's cache and republish when a bundle changes) has no test either — I grepped both new specs (AvailableLocaleResolverSpec, LocaleSelectAvailableSpec) and neither calls onChange or references plugin reload behavior.

2. I18nAutoConfiguration.availableLocaleResolver bean — wiring itself is unasserted

grails-core/grails-i18n/src/main/groovy/org/grails/plugins/i18n/I18nAutoConfiguration.java

Lines 151 to 158 in 8f9d7aa

	@Bean
	@ConditionalOnMissingBean(AvailableLocaleResolver.class)
	public AvailableLocaleResolver availableLocaleResolver(GrailsApplication grailsApplication,
	@Value("${grails.i18n.default.locale:en}") String defaultLocale,
	@Value("${grails.i18n.availableLocales.includePlugins:true}") boolean includePlugins) {
	return new AvailableLocaleResolver(grailsApplication.getClassLoader(),
	Locale.forLanguageTag(defaultLocale.replace('_', '-')), includePlugins);
	}

Every sibling bean in this class (localeResolver, localeChangeInterceptor, messageSource) has a WebApplicationContextRunner-based test in I18nAutoConfigurationSpec.groovy — property-driven variants and @ConditionalOnMissingBean back-off against a user-supplied bean. grep -n "availableLocaleResolver\|AvailableLocaleResolver" I18nAutoConfigurationSpec.groovy returns nothing: the new bean isn't checked for presence, isn't checked for property wiring (grails.i18n.default.locale, grails.i18n.availableLocales.includePlugins), and the @ConditionalOnMissingBean(AvailableLocaleResolver.class) back-off path is untested. AvailableLocaleResolverSpec only exercises new AvailableLocaleResolver(...) directly, not the Spring wiring.

3. grails-profiles/web/skeleton/.../main.gsp — zero coverage (the forge twin got a test, this didn't)

grails-core/grails-profiles/web/skeleton/grails-app/views/layouts/main.gsp

Lines 21 to 36 in 8f9d7aa

	<g:set var="availableLocales" value="${application.getAttribute('availableLocales')}"/>
	<g:if test="${availableLocales && availableLocales.size() > 1}">
	<g:set var="currentLocale" value="${org.springframework.web.servlet.support.RequestContextUtils.getLocale(request)}"/>
	<li class="nav-item dropdown">
	<a class="nav-link dropdown-toggle" href="#" id="localeDropdown" role="button" data-bs-toggle="dropdown" aria-expanded="false">
	<i class="bi bi-globe me-1"></i>${currentLocale.getDisplayName(currentLocale)}
	</a>
	<ul class="dropdown-menu dropdown-menu-end" aria-labelledby="localeDropdown">
	<g:each in="${availableLocales}" var="availableLocale">
	<li>
	<a class="dropdown-item${availableLocale == currentLocale ? ' active' : ''}" href="?lang=${availableLocale.toLanguageTag()}">${availableLocale.getDisplayName(availableLocale)}</a>
	</li>
	</g:each>
	</ul>
	</li>
	</g:if>

Identical dropdown markup was added to grails-forge/grails-forge-core/src/main/resources/gsp/main.gsp, and that copy got a new GrailsGspSpec test ("test default layout includes the language selector dropdown"). This classic-profile copy has no equivalent — grails-profiles has no src/test source set in the repo at all, so nothing verifies this template even renders, let alone that the g:if gate on size() > 1 behaves.

4. AvailableLocaleResolver — well-tested class, but specific branches are still open (matches Codecov's 4 missing/5 partial)

https://github.com/apache/grails-core/blob/8f9d7aa2bc65769b371bc392fa185df5f3edb261/grails-i18n/src/main/groovy/org/grails/plugins/i18n/AvailableLocaleResolver.java

• L128: defaultLocale == null or empty-language (Locale.ROOT) — the "don't add a default" branch is never hit by any spec.
• L143-144: the IOException catch in computeAvailableLocales() — unreachable without mocking ResourcePatternResolver.
• L157: filename == null / non-.properties filename branch.
• L162-163: a *.properties file with no underscore, reached only when includePluginBundles=true — currently only covered incidentally by whatever happens to be on the test classpath (e.g. application.properties), not by a deliberate fixture.
• L166-167: an empty suffix after the underscore (messages_.properties).
• L170: a non-ISO-language suffix on a deliberate fixture — same incidental-vs-intentional gap as above.

5. FormTagLib.localeSelect — one untested branch

grails-core/grails-gsp/plugin/src/main/groovy/org/grails/plugins/web/taglib/FormTagLib.groovy

Lines 979 to 989 in 8f9d7aa

	def localeSelect(Map attrs) {
	def availableAttr = attrs.remove('available')
	boolean availableOnly = availableAttr != null && Boolean.valueOf(availableAttr.toString())
	if (availableOnly) {
	def availableLocales = request.servletContext?.getAttribute('availableLocales')
	attrs.from = availableLocales ?: [RCU.getLocale(request)]
	}
	else {
	attrs.from = Locale.getAvailableLocales()
	}
	attrs.value = (attrs.value ?: RCU.getLocale(request))?.toString()

LocaleSelectAvailableSpec covers available="true" and the attribute being absent, but never an explicit available="false". L981 (Boolean.valueOf(availableAttr.toString())) has an untested false-but-present path. Confirmed: grep -n "available=\"false\"" in the spec returns nothing.

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Findings generated by automated analysis of the Codecov patch report and the PR head commit; reviewed by a human before posting.