apache/cassandra · error · ConfigurationException

Unable to create instance of validator of class

Error message

Unable to create instance of validator of class %s: %s

What it means

Cassandra guardrails support a custom validator class that validates generated/configured values. ValueValidator.getValidator reflectively instantiates the class configured for a guardrail (e.g. password_policy_validator); when that fails — missing class, missing public no-arg constructor, constructor throwing, wrong type — it wraps the failure in a ConfigurationException with this message.

Solutions

1. Check that the configured value is the correct fully-qualified class name of a ValueValidator subclass
2. Deploy the validator class/JAR to Cassandra's classpath
3. Add a public no-arg constructor to the validator class
4. Read the nested message in the exception — ConfigurationException causes are unwrapped into the message
5. Run ValueValidator.getValidator(name, config) in a unit test before rollout

Example fix

// before
role_name_policy_validator: com.example.RoleNameValidator  // ctor takes args
// after
public class RoleNameValidator extends ValueValidator<String> { public RoleNameValidator() {} ... }

String cls = config.password_policy_validator;
if (cls != null && !cls.isBlank()) {
    try {
        Class<?> c = Class.forName(cls.trim());
        if (!org.apache.cassandra.db.guardrails.ValueValidator.class.isAssignableFrom(c))
            throw new IllegalArgumentException(cls + " is not a ValueValidator");
        c.getDeclaredConstructor();
    } catch (ReflectiveOperationException e) {
        throw new IllegalArgumentException("Cannot load validator " + cls + ": " + e, e);
    }
}

boolean isValidValidator(String name) {
    try { return ValueValidator.class.isAssignableFrom(Class.forName(name)); }
    catch (Throwable t) { return false; }
}

try {
    validator = ValueValidator.getValidator("password_policy", config);
} catch (ConfigurationException e) {
    logger.error("Invalid validator class: {}", e.getMessage());
    validator = NoOpValidator.INSTANCE;
}

When it happens

Trigger: Setting a guardrail validator option to a class name that cannot be reflectively instantiated at startup or when the guardrail is first used.

Common situations: Misspelled fully-qualified class name in cassandra.yaml; validator JAR not deployed to lib/; validator class has a parameterized constructor; class was compiled against an incompatible Cassandra version; nested ConfigurationException from the validator's static/instance init.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

• Unable to create instance of generator of class
• Cannot find configured row cache provider class
• couldn't load cipher factory
• default_keyspace_rf to be set
• default_keyspace_rf to be set

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/5aa24dfba67f9dfa. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/db/guardrails/ValueValidator.java:151

            Class<? extends ValueValidator<VALUE>> validatorClass =
                (Class<? extends ValueValidator<VALUE>>) rawValidatorClass;

            @SuppressWarnings("unchecked")
            ValueValidator<VALUE> validator = validatorClass.getConstructor(CustomGuardrailConfig.class)
                                                            .newInstance(config);
            logger.debug("Using {} validator for guardrail '{}' with parameters {}",
                         validator.getClass(), name, validator.getParameters());
            return validator;
        }
        catch (Exception ex)
        {
            String message;
            if (ex.getCause() instanceof ConfigurationException)
                message = ex.getCause().getMessage();
            else
                message = ex.getMessage();

            throw new ConfigurationException(format("Unable to create instance of validator of class %s: %s",
                                                    className, message), ex);
        }
    }
}

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