quarkusio/quarkus · error · IllegalArgumentException

The <clazz.getSimpleName()> class [<instanceClass>] could no

Error message

The <clazz.getSimpleName()> class [<instanceClass>] could not be instantiated!

What it means

After resolving the configured setting value to a Class, loadSettingInstance instantiates it via its no-arg constructor. If the class is abstract/an interface, lacks a public no-arg constructor, its constructor throws, or instantiation is otherwise illegal, the builder wraps the reflective exception in this IllegalArgumentException.

Source

Thrown at extensions/hibernate-orm/runtime/src/main/java/io/quarkus/hibernate/orm/runtime/boot/FastBootMetadataBuilder.java:705

                instanceClass = classLoaderService.classForName(settingStringValue);
            } else {
                try {
                    instanceClass = (Class<? extends T>) Class.forName(settingStringValue);
                } catch (ClassNotFoundException e) {
                    throw new IllegalArgumentException("Can't load class: " + settingStringValue, e);
                }
            }
        } else {
            throw new IllegalArgumentException(
                    "The provided " + settingName + " setting value [" + settingValue + "] is not supported!");
        }

        if (instanceClass != null) {
            try {
                instance = instanceClass.getConstructor().newInstance();
            } catch (InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
                throw new IllegalArgumentException(
                        "The " + clazz.getSimpleName() + " class [" + instanceClass + "] could not be instantiated!",
                        e);
            }
        }

        return instance;
    }

}

View on GitHub (pinned to e1c734241f)

Solutions

1. Make the class concrete with a public no-argument constructor.
2. Read the nested 'Caused by' (InstantiationException, NoSuchMethodException, InvocationTargetException) for the root cause.
3. Move logic depending on CDI/services out of the constructor into integrate()/contribute() callbacks.
4. Verify the class implements the expected interface (e.g. MetadataBuilderContributor).

Example fix

// before
public abstract class MyContributor implements MetadataBuilderContributor { ... }
// after
public class MyContributor implements MetadataBuilderContributor {
    public MyContributor() { }
    @Override public void contribute(MetadataBuilder mb) { ... }
}

// Pre-instantiate to catch problems before boot
Object c = Class.forName("com.example.MyContributor").getConstructor().newInstance();
if (!(c instanceof MetadataBuilderContributor)) throw new IllegalStateException("Wrong interface");

try {
    bootPersistenceUnit();
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("could not be instantiated")) {
        log.error("Contributor must be concrete with a public no-arg constructor", e.getCause());
    }
}

When it happens

Trigger: Configuring a contributor/naming-strategy class that is abstract, an interface, has only constructor arguments, or whose no-arg constructor throws at static-init time — reached through settings like metadata_builder_contributor or naming strategy settings.

Common situations: Pointing a strategy setting at an interface; contributor requiring DI; constructor failing because Quarkus beans/CDI aren't initialized during static init; shading/obfuscation removing default constructors.

Related errors

• Unable to instantiate integrator <integratorClass>
• Unable to create instance of Logging Filter ''
• RuntimeException(e)
• The Hibernate ORM configuration in Quarkus does not support
• Datasource must be defined for persistence unit '%s'. Settin

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/b099cd3439eaffa2. Report an issue: GitHub.