junit-team/junit5 · error · JUnitException

There should not be more than one ClasspathScanner…

Error message

There should not be more than one ClasspathScanner implementation present on the classpath but there were %d: %s

What it means

JUnitException thrown by ClasspathScannerLoader when java.util.ServiceLoader finds more than one ClasspathScanner implementation on the classpath. The SPI contract requires at most one provider; ambiguity is treated as a configuration error and the message lists both the count and the fully-qualified class names of all detected providers.

Solutions

Inspect the error message - it lists the conflicting implementation class names
Run mvn dependency:tree or gradle dependencies to find duplicate providers
Exclude the duplicate junit-platform-commons artifact or the third-party scanner
Keep a single ClasspathScanner provider on the test runtime classpath

Example fix

// build.gradle.kts - force a single version
configurations.all {
  resolutionStrategy.eachDependency {
    if (requested.group == "org.junit.platform") useVersion("1.13.3")
  }
}

Defensive patterns

Strategy: validation

Validate before calling

long count = ServiceLoader.load(ClasspathScanner.class).stream().count();
if (count > 1) {
    // fail fast with a clear build error before tests run
}

Prevention

Run mvn dependency:tree or gradle dependencies before test runs to spot duplicate junit-platform-commons
Avoid shading test framework artifacts into uber-jars
Pin a single junit-bom version across the build

When it happens

Trigger: Two or more JARs each register a META-INF/services/org.junit.platform.commons.support.scanning.ClasspathScanner entry and both are visible to the context ClassLoader used by ServiceLoader.load.

Common situations: Duplicate or conflicting junit-platform-commons JARs pulled in transitively plus shading; a third-party test library that ships its own ClasspathScanner alongside the default; fat-jar/uber-jar assembly merging multiple service entries.

Related errors

AI-assisted analysis of junit-team/junit5@f070c699a0 (2026-08-11). Data as JSON: /api/errors/2bde776ef72d0d0b. Report an issue: GitHub.

Appendix: source

Thrown at junit-platform-commons/src/main/java/org/junit/platform/commons/util/ClasspathScannerLoader.java:36

import org.junit.platform.commons.support.scanning.ClasspathScanner;

/**
 * @since 1.12
 */
class ClasspathScannerLoader {

	static ClasspathScanner getInstance() {
		ServiceLoader<ClasspathScanner> serviceLoader = ServiceLoader.load(ClasspathScanner.class,
			ClassLoaderUtils.getDefaultClassLoader());

		List<Provider<ClasspathScanner>> classpathScanners = serviceLoader.stream().toList();

		if (classpathScanners.size() == 1) {
			return classpathScanners.get(0).get();
		}

		if (classpathScanners.size() > 1) {
			throw new JUnitException(
				"There should not be more than one ClasspathScanner implementation present on the classpath but there were %d: %s".formatted(
					classpathScanners.size(),
					classpathScanners.stream().map(Provider::type).map(Class::getName).toList()));
		}

		return new DefaultClasspathScanner(ClassLoaderUtils::getDefaultClassLoader, ReflectionUtils::tryToLoadClass);
	}

	private ClasspathScannerLoader() {
	}

}

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