junit-team/junit5 · error · ExtensionConfigurationException

@DefaultLocale can only be used with a provider if value…

Error message

@DefaultLocale can only be used with a provider if value, language, country and variant are not set.

What it means

ExtensionConfigurationException from DefaultLocaleExtension.getFromProvider, raised when @DefaultLocale uses a localeProvider but also specifies country or variant. The provider form is exclusive: it must be the sole source of the Locale, so any country/variant (and value/language, enforced elsewhere) alongside it is rejected.

Solutions

Remove country and variant (and value/language) so only localeProvider remains.
If you need parts alongside the provider, fold that logic into the provider's get() instead of the annotation.

Example fix

// before
@DefaultLocale(localeProvider = MyProvider.class, country = "US")
void test() {}
// after
@DefaultLocale(localeProvider = MyProvider.class)
void test() {}

Defensive patterns

Strategy: validation

Validate before calling

if (annotation.localeProvider() != NullLocaleProvider.class
        && (!annotation.country().isEmpty() || !annotation.variant().isEmpty())) {
    // conflict: remove country/variant
}

Prevention

A LocaleProvider must be the sole source of the locale.
Clear country/variant/language/value when using a provider.
Encode the locale decision inside the provider, not the annotation.

When it happens

Trigger: Annotating a test with @DefaultLocale(localeProvider = MyProvider.class, country = "US") or with variant set. getFromProvider is selected when value and language are empty; it rejects non-empty country/variant.

Common situations: Adding a provider while leaving previously-set country/variant attributes in place; misunderstanding that the provider fully determines the locale.

Related errors

AI-assisted analysis of junit-team/junit5@f070c699a0 (2026-08-11). Data as JSON: /api/errors/eedcce7995afef20. Report an issue: GitHub.

Appendix: source

Thrown at junit-jupiter-api/src/main/java/org/junit/jupiter/api/util/DefaultLocaleExtension.java:104

		if (!language.isEmpty() && !country.isEmpty() && !variant.isEmpty()) {
			return JupiterLocaleUtils.createLocale(language, country, variant);
		}
		else if (!language.isEmpty() && !country.isEmpty()) {
			return JupiterLocaleUtils.createLocale(language, country);
		}
		else if (!language.isEmpty() && variant.isEmpty()) {
			return JupiterLocaleUtils.createLocale(language);
		}
		else {
			throw new ExtensionConfigurationException(
				"@DefaultLocale not configured correctly. When not using a language tag, specify either"
						+ " language, or language and country, or language and country and variant.");
		}
	}

	private static Locale getFromProvider(DefaultLocale annotation) {
		if (!annotation.country().isEmpty() || !annotation.variant().isEmpty())
			throw new ExtensionConfigurationException(
				"@DefaultLocale can only be used with a provider if value, language, country and variant are not set.");
		var providerClass = annotation.localeProvider();
		LocaleProvider provider;
		try {
			provider = ReflectionSupport.newInstance(providerClass);
		}
		catch (Exception exception) {
			throw new ExtensionConfigurationException(
				"LocaleProvider instance could not be constructed because of an exception", exception);
		}
		return invoke(provider);
	}

	@SuppressWarnings("ConstantValue")
	private static Locale invoke(LocaleProvider provider) {
		var locale = provider.get();
		if (locale == null) {
			throw new ExtensionConfigurationException("LocaleProvider instance returned with null");

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