quarkusio/quarkus · error · IllegalArgumentException

The use of interfaces as the generic type of Lists fields /…

Error message

The use of interfaces as the generic type of Lists fields / methods is not allowed. Offending field is ' + member.name() + ' of class ' + member.declaringClass().name().toString() + '

What it means

YamlListObjectHandler resolves the concrete element class of a List<...> field/method so it can instantiate elements when binding YAML config. Interfaces cannot be instantiated, so validation rejects them outright with IllegalArgumentException.

Solutions

Change the list element type from the interface to a concrete class with a public no-args constructor
Introduce a concrete base/default implementation class and use List<Impl> in the config
Move polymorphism out of the config binding (bind concrete class, expose interface separately)

Example fix

// before
List<Handler> handlers();
// after
List<DefaultHandler> handlers();

Defensive patterns

Strategy: type-guard

Validate before calling

static <T> void checkListElementType(List<T> listField, Class<?> cfg) {
    // element type must be a concrete class
}

Type guard

boolean isConcreteListElement(Type t) {
    return t instanceof Class<?> c && !c.isInterface() && !c.isAnnotation();
}

Try / catch

catch (IllegalArgumentException e) { if (e.getMessage().contains("not allowed")) { /* use concrete element class */ } throw e; }

Prevention

Use concrete classes as config list element types
Keep config binding types simple and concrete; expose interfaces only in service layer

When it happens

Trigger: A @ConfigProperties class/interface (processed via quarkus-config-yaml list-of-object support) declares a field or method of type List<SomeInterface> where SomeInterface is an interface, not a class.

Common situations: Modeling config lists against an abstraction (List<Handler> where Handler is an interface) carried over from Spring Boot style configuration.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/c7d35435b46e70a7. Report an issue: GitHub.

Appendix: source

Thrown at extensions/spring-boot-properties/deployment/src/main/java/io/quarkus/spring/boot/properties/deployment/YamlListObjectHandler.java:201

                    mc.public_();
                    mc.body(b -> {
                        LocalVar resultHandle = b.localVar("resultHandle", b.new_(ConstructorDesc.of(HashMap.class)));
                        for (Map.Entry<String, String> entry : fieldNameMap.entrySet().stream()
                                .sorted(Map.Entry.comparingByKey()).toList()) {
                            b.invokeVirtual(
                                    MethodDesc.of(HashMap.class, "put", Object.class, Object.class, Object.class),
                                    resultHandle, Const.of(entry.getKey()), Const.of(entry.getValue()));
                        }
                        b.return_(resultHandle);
                    });
                });
            }
        });
    }

    private void validateClass(ClassInfo classInfo, Member member) {
        if (Modifier.isInterface(classInfo.flags())) {
            throw new IllegalArgumentException(
                    "The use of interfaces as the generic type of Lists fields / methods is not allowed. Offending field is '"
                            + member.name() + "' of class '" + member.declaringClass().name().toString() + "'");
        }
        if (!classInfo.hasNoArgsConstructor()) {
            throw new IllegalArgumentException(
                    String.format("Class '%s' which is used as %s in class '%s' must have a no-args constructor", classInfo,
                            member.phraseUsage(), member.declaringClass().name().toString()));
        }
        if (!Modifier.isPublic(classInfo.flags())) {
            throw new IllegalArgumentException(
                    String.format("Class '%s' which is used as %s in class '%s' must be a public class", classInfo,
                            member.phraseUsage(), member.declaringClass().name().toString()));
        }
    }

    private ClassInfo validateType(Type type) {
        if (type.kind() != Type.Kind.PARAMETERIZED_TYPE) {
            throw new IllegalArgumentException(ILLEGAL_ARGUMENT_MESSAGE);

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