junit-team/junit5 · error · ExtensionConfigurationException

@DefaultLocale can only be used with language tag if…

Error message

@DefaultLocale can only be used with language tag if provider is not set

What it means

ExtensionConfigurationException from DefaultLocaleExtension.createFromParts, raised when @DefaultLocale is configured with language (and possibly country/variant) parts AND a custom localeProvider at the same time. Parts-based construction and a LocaleProvider are mutually exclusive.

Solutions

Drop the localeProvider attribute (it defaults to NullLocaleProvider) when using language/country/variant.
OR keep the provider and remove language/country/variant so getFromProvider is used instead.

Example fix

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

Defensive patterns

Strategy: validation

Validate before calling

if (!annotation.language().isEmpty() && annotation.localeProvider() != NullLocaleProvider.class) {
    // conflict: remove provider or remove parts
}

Prevention

Parts and provider are mutually exclusive in @DefaultLocale.
Reset localeProvider to default when using language/country/variant.
Validate annotation shape in a shared helper before tests run.

When it happens

Trigger: Annotating a test with @DefaultLocale(language = "en", localeProvider = MyProvider.class). createFromParts is chosen when value() is empty but language is non-empty; it rejects any non-default provider.

Common situations: Switching from a provider-based to a parts-based configuration but forgetting to reset localeProvider back to the default NullLocaleProvider; merging two annotation usages during refactor.

Related errors

AI-assisted analysis of junit-team/junit5@f070c699a0 (2026-08-11). Data as JSON: /api/errors/88cc3d2ccb8c5777. Report an issue: GitHub.

Appendix: source

Thrown at junit-jupiter-api/src/main/java/org/junit/jupiter/api/util/DefaultLocaleExtension.java:81

			return createFromParts(annotation);
		}
		else {
			return getFromProvider(annotation);
		}
	}

	private static Locale createFromLanguageTag(DefaultLocale annotation) {
		if (!annotation.language().isEmpty() || !annotation.country().isEmpty() || !annotation.variant().isEmpty()
				|| annotation.localeProvider() != NullLocaleProvider.class) {
			throw new ExtensionConfigurationException(
				"@DefaultLocale can only be used with language tag if language, country, variant and provider are not set");
		}
		return Locale.forLanguageTag(annotation.value());
	}

	private static Locale createFromParts(DefaultLocale annotation) {
		if (annotation.localeProvider() != NullLocaleProvider.class)
			throw new ExtensionConfigurationException(
				"@DefaultLocale can only be used with language tag if provider is not set");
		String language = annotation.language();
		String country = annotation.country();
		String variant = annotation.variant();
		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.");
		}

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