apache/kafka · error · KafkaException

klass is not an instance of t.getName()

Error message

klass is not an instance of t.getName()

What it means

KafkaException thrown by getConfiguredInstance after successfully instantiating a class when the resulting object is not an instance of the expected super-type T (i.e. t.isInstance(o) is false). The expected interface name (t.getName()) and the class are both in the message. It is a type-assignment guard: the configured class loaded fine but does not implement/extend the interface Kafka requires for that slot (e.g. Partitioner, Serializer, Converter, Transform, ConfigProvider, MetricsReporter).

Source

Thrown at clients/src/main/java/org/apache/kafka/common/config/AbstractConfig.java:416

    private <T> T getConfiguredInstance(Object klass, Class<T> t, Map<String, Object> configPairs) {
        if (klass == null)
            return null;
        Object o;

        if (klass instanceof String) {
            try {
                o = Utils.newInstance((String) klass, t);
            } catch (ClassNotFoundException e) {
                throw new KafkaException("Class " + klass + " cannot be found", e);
            }
        } else if (klass instanceof Class<?>) {
            o = Utils.newInstance((Class<?>) klass);
        } else
            throw new KafkaException("Unexpected element of type " + klass.getClass().getName() + ", expected String or Class");
        try {
            if (!t.isInstance(o))
                throw new KafkaException(klass + " is not an instance of " + t.getName());
            if (o instanceof Configurable)
                ((Configurable) o).configure(configPairs);
        } catch (Exception e) {
            maybeClose(o, "AutoCloseable object constructed and configured during failed call to getConfiguredInstance");
            throw e;
        }
        return t.cast(o);
    }

    /**
     * Get a configured instance of the give class specified by the given configuration key. If the object implements
     * Configurable configure it using the configuration.
     *
     * @param key The configuration key for the class
     * @param t   The interface the class should implement
     * @return A configured instance of the class
     */
    public <T> T getConfiguredInstance(String key, Class<T> t) {

View on GitHub (pinned to c31c9215e1)

Solutions

1. Confirm the configured class implements the required interface named in the message (Serializer, Deserializer, Partitioner, Converter, Transformation, ConfigProvider, MetricsReporter, etc.).
2. Check the fully-qualified name: a same-named class in the wrong package is a frequent cause.
3. If you wrote the class, add `implements <expected-interface>` and implement its methods.
4. Align versions: if the interface changed across Kafka versions, rebuild/upgrade the plugin against the matching kafka-clients version.

Example fix

// before: configured class does not implement Serializer
public class MyJsonEncoder { /* encodes but is not a Serializer */ }
props.put("value.serializer", "com.example.MyJsonEncoder");

// after
public class MyJsonEncoder implements org.apache.kafka.common.serialization.Serializer<MyType> {
    @Override public byte[] serialize(String topic, MyType data) { /* ... */ return null; }
}
props.put("value.serializer", "com.example.MyJsonEncoder");

// Verify the class is assignable to the expected interface before asking Kafka to build it.
String className = String.valueOf(config.get(key));
Class<?> candidate = Class.forName(className, false, Thread.currentThread().getContextClassLoader());
if (!expectedInterface.isAssignableFrom(candidate)) {
    throw new IllegalArgumentException(
        className + " does not implement " + expectedInterface.getName());
}
T plugin = config.getConfiguredInstance(key, expectedInterface);

boolean implementsT(Class<?> candidate, Class<?> t) {
    return candidate != null && t.isAssignableFrom(candidate);
}

try {
    T plugin = config.getConfiguredInstance(key, T.class);
} catch (org.apache.kafka.common.KafkaException ke) {
    if (ke.getMessage().contains("is not an instance of")) {
        // right class name, wrong interface — e.g. a Serializer used where Deserializer was required
        log.error("Plugin '{}' does not implement the required interface", config.get(key));
    } else {
        throw ke;
    }
}

When it happens

Trigger: A config key requiring a specific interface is pointed at a class that exists on the classpath but does not implement it: e.g. setting key.serializer to a class that does not implement org.apache.kafka.common.serialization.Serializer, or a Connect transform class that does not implement Transformation. The class instantiates via its no-arg/default constructor, then the isInstance check fails.

Common situations: See trigger scenarios.

Related errors

• Class klass cannot be found
• Unknown configuration '%s'
• Non-string value found in original settings for key entry.ge
• Unexpected element of type klass.getClass().getName(), expec
• {klass} ClassNotFoundException exception occurred

AI-assisted analysis of apache/kafka@c31c9215e1 (2026-08-03). Data as JSON: /data/errors/518b97522e02e1aa.json. Report an issue: GitHub.