quarkusio/quarkus · error · RuntimeException

The provided hostname verifier ${verifier} is not an instanc

Error message

The provided hostname verifier ${verifier} is not an instance of HostnameVerifier

What it means

The class configured as hostname verifier loaded and instantiated fine, but the resulting object does not implement javax.net.ssl.HostnameVerifier, so the cast in registerHostnameVerifier failed with ClassCastException. The configured class must implement HostnameVerifier.

Source

Thrown at extensions/resteasy-classic/resteasy-client/runtime/src/main/java/io/quarkus/restclient/runtime/RestClientBase.java:166

        }
    }

    private void registerHostnameVerifier(String verifier, RestClientBuilder builder) {
        try {
            Class<?> verifierClass = Thread.currentThread().getContextClassLoader().loadClass(verifier);
            builder.hostnameVerifier((HostnameVerifier) verifierClass.getDeclaredConstructor().newInstance());
        } catch (NoSuchMethodException e) {
            throw new RuntimeException(
                    "Could not find a public, no-argument constructor for the hostname verifier class " + verifier, e);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Could not find hostname verifier class " + verifier, e);
        } catch (InstantiationException | IllegalAccessException | InvocationTargetException e) {
            throw new RuntimeException(
                    "Failed to instantiate hostname verifier class " + verifier
                            + ". Make sure it has a public, no-argument constructor",
                    e);
        } catch (ClassCastException e) {
            throw new RuntimeException("The provided hostname verifier " + verifier + " is not an instance of HostnameVerifier",
                    e);
        }
    }

    private void registerKeyStore(String keyStorePath, RestClientBuilder builder) {
        try {
            Optional<String> keyStoreType = oneOf(restClientConfig.keyStoreType(), configRoot.keyStoreType());
            KeyStore keyStore = KeyStore.getInstance(keyStoreType.orElse("JKS"));

            Optional<String> keyStorePassword = oneOf(restClientConfig.keyStorePassword(), configRoot.keyStorePassword());
            if (keyStorePassword.isEmpty()) {
                throw new IllegalArgumentException("No password provided for keystore");
            }
            String password = keyStorePassword.get();

            try (InputStream input = locateStream(keyStorePath)) {
                keyStore.load(input, password.toCharArray());
            } catch (IOException | CertificateException | NoSuchAlgorithmException e) {

View on GitHub (pinned to e1c734241f)

Solutions

Make the configured class implement javax.net.ssl.HostnameVerifier and its verify(String, SSLSession) method
Check for import mistakes — implement javax.net.ssl.HostnameVerifier, not another HostnameVerifier type
Verify the configured class name points at the intended verifier

Example fix

// before
import org.apache.http.conn.ssl.NoopHostnameVerifier;
public class MyVerifier extends NoopHostnameVerifier {}
// after
import javax.net.ssl.HostnameVerifier;
public class MyVerifier implements HostnameVerifier {
    public boolean verify(String host, SSLSession session) { return true; }
}

Defensive patterns

Strategy: type-guard

Validate before calling

Class<?> c = Class.forName(verifierName);
if (!javax.net.ssl.HostnameVerifier.class.isAssignableFrom(c)) {
    throw new IllegalStateException(verifierName + " does not implement javax.net.ssl.HostnameVerifier");
}

Type guard

static boolean isHostnameVerifier(Class<?> c) {
    return javax.net.ssl.HostnameVerifier.class.isAssignableFrom(c);
}

Try / catch

try {
    Object o = Class.forName(verifierName).getDeclaredConstructor().newInstance();
    if (!(o instanceof HostnameVerifier)) throw new IllegalStateException(verifierName + " is not a HostnameVerifier");
    builder.hostnameVerifier((HostnameVerifier) o);
} catch (ClassCastException e) {
    log.error("Wrong HostnameVerifier interface implemented", e);
}

Prevention

Import javax.net.ssl.HostnameVerifier exactly — beware same-named interfaces from other libraries
Add an instanceof assert/unit test for every configured verifier
Check implements clauses after refactors

When it happens

Trigger: Setting quarkus.rest-client.<name>.hostname-verifier to a class that implements a similarly-named but different interface, or an unrelated class entirely.

Common situations: Implementing the wrong HostnameVerifier (e.g. an application-specific interface of the same name); refactoring that changed the interface the class implements; copy-paste of a verifier from another library.

Related errors

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/576b600686ad42c4. Report an issue: GitHub.