apache/cassandra · error · ConfigurationException

Unable to create instance of generator of class

Error message

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

What it means

Cassandra guardrails can be configured with a custom generator class that generates values (e.g. passwords, role names). When instantiating the configured generator class fails (class missing, no public constructor, constructor throws, or class is not a ValueGenerator subtype), ValueGenerator.getGenerator wraps the reflective-instantiation failure in a ConfigurationException with this message.

Solutions

1. Verify the configured class name is a correct, fully-qualified name of a class that extends org.apache.cassandra.db.guardrails.ValueGenerator
2. Ensure the class (and any JAR containing it) is on Cassandra's classpath (lib/ or a JRE-specific lib dir)
3. Give the generator a public no-argument constructor
4. Fix the root cause reported in the nested message (the message field carries ex.getCause().getMessage() when the cause is a ConfigurationException)
5. Test instantiation in a unit test before deploying, e.g. ValueGenerator.getGenerator(name, config)

Example fix

// before (cassandra.yaml)
password_policy_generator: com.example.MyPasswordGenerator2  // typo, class not found
// after
password_policy_generator: com.example.MyPasswordGenerator  // class exists, public no-arg ctor, extends ValueGenerator

String cls = config.password_policy_generator;
if (cls != null && !cls.isBlank()) {
    try {
        Class<?> c = Class.forName(cls.trim());
        if (!org.apache.cassandra.db.guardrails.ValueGenerator.class.isAssignableFrom(c))
            throw new IllegalArgumentException(cls + " is not a ValueGenerator");
        c.getDeclaredConstructor().setAccessible(true); // throws if no no-arg ctor
    } catch (ReflectiveOperationException e) {
        throw new IllegalArgumentException("Cannot load generator " + cls + ": " + e, e);
    }
}

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

try {
    generator = ValueGenerator.getGenerator("password_policy", config);
} catch (ConfigurationException e) {
    logger.error("Bad generator config: {}", e.getMessage());
    generator = NoOpGenerator.INSTANCE;
}

When it happens

Trigger: Configuring a guardrail such as password_policy or role_name_policy with a generator class that cannot be instantiated: the class is not on the classpath, has no no-arg public constructor, its constructor throws, or it does not extend ValueGenerator.

Common situations: Typo in the fully-qualified class name in cassandra.yaml; custom generator JAR missing from the lib directory; generator class written for a different Cassandra version or with a constructor taking arguments; ConfigurationException raised inside the generator's own initialization.

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 validator 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/8b84c880dc20bdd7. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/db/guardrails/ValueGenerator.java:167

            Class<? extends ValueGenerator<VALUE>> generatorClass =
                (Class<? extends ValueGenerator<VALUE>>) rawGeneratorClass;

            @SuppressWarnings("unchecked")
            ValueGenerator<VALUE> generator = generatorClass.getConstructor(CustomGuardrailConfig.class)
                                                            .newInstance(config);
            logger.debug("Using {} generator for guardrail '{}' with parameters {}",
                         generator.getClass(), name, generator.getParameters());
            return generator;
        }
        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 generator of class %s: %s",
                                                    className, message), ex);
        }
    }
}

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