What causes "No implementation of interface ${interface} was found" in quarkusio/quarkus?

Create a class implementing the fragment interface, named with the 'Impl' suffix (e.g. UserRepositoryCustomImpl) and annotate it @Component (or @Repository)

quarkusio/quarkus · error · IllegalArgumentException

No implementation of interface ${interface} was found

Error message

No implementation of interface ${interface} was found

What it means

Every custom fragment interface used by a Spring Data repository must have exactly one implementing class. This error is thrown when the Jandex index contains no class implementing the fragment interface, so Quarkus has nothing to wire the generated forwarding methods to.

Source

Thrown at extensions/spring-data-jpa/deployment/src/main/java/io/quarkus/spring/data/deployment/generate/FragmentMethodsUtil.java:38

        if (knownImplementors.size() > 1) {
            DotName previouslyFound = null;
            for (ClassInfo knownImplementor : knownImplementors) {
                if (knownImplementor.name().toString().endsWith("Impl")) { // the default suffix that Spring Data JPA looks for is 'Impl'
                    if (previouslyFound != null) { // make sure we don't have multiple implementations suffixed with 'Impl'
                        throw new IllegalArgumentException(
                                "Interface " + customInterfaceToImplement
                                        + " must contain a single implementation whose name ends with 'Impl'. Multiple implementations were found: "
                                        + previouslyFound + "," + knownImplementor);
                    }
                    previouslyFound = knownImplementor.name();
                }
            }
            return previouslyFound;
        } else if (knownImplementors.size() == 1) {
            return knownImplementors.iterator().next().name();
        } else {
            throw new IllegalArgumentException(
                    "No implementation of interface " + customInterfaceToImplement + " was found");
        }
    }
}

View on GitHub (pinned to e1c734241f)

Solutions

Create a class implementing the fragment interface, named with the 'Impl' suffix (e.g. UserRepositoryCustomImpl) and annotate it @Component (or @Repository)
Check the implementor's 'implements' clause actually names the fragment interface (typos, wrong package import)
If the implementation lives in a dependency JAR, index that JAR (META-INF/jandex.idx or quarkus.index-dependency.*)
Clean rebuild to refresh the index

Example fix

// before
public interface UserRepositoryCustom { List<User> findByCustomQuery(String q); }
// (no implementation)
// after
public class UserRepositoryCustomImpl implements UserRepositoryCustom {
    @PersistenceContext EntityManager em;
    public List<User> findByCustomQuery(String q) { return em.createQuery(...).getResultList(); }
}

Defensive patterns

Strategy: validation

Validate before calling

// Verify each fragment interface has an implementation before building
for i in $(grep -rl "interface .*Custom" src/main/java --include=*.java); do
  name=$(basename $i .java)
  grep -rq "implements $name" src/main/java || echo "MISSING implementation for $name"
done

Prevention

Always create the matching <Interface>Impl class together with the fragment interface
Annotate implementations with @Component or @Repository
Keep interface and implementation in the same module

When it happens

Trigger: A repository extends a custom interface (e.g. UserRepositoryCustom) but no @Component/@Repository class implements it; the implementing class was deleted or renamed; the implementation exists in an un-indexed JAR.

Common situations: Declaring the fragment interface but forgetting the implementation class; typo in implementor's 'implements' clause; implementation removed during refactoring; implementor in an external un-indexed dependency.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/a216c118cde41128. Report an issue: GitHub.