quarkusio/quarkus · error · RuntimeException

The provided hostname verifier " + verifier + " is not an in

Error message

The provided hostname verifier " + verifier + " is not an instance of HostnameVerifier

What it means

The configured class loaded and was constructed successfully, but the resulting object is not a HostnameVerifier, so the cast to HostnameVerifier threw ClassCastException, wrapped in this RuntimeException. The property is meant to point exclusively at HostnameVerifier implementations.

Source

Thrown at extensions/resteasy-reactive/rest-client/runtime/src/main/java/io/quarkus/rest/client/reactive/runtime/RestClientCDIDelegateBuilder.java:301

        oneOf(restClientConfig.verifyHost(), configRoot.verifyHost()).ifPresent(builder::verifyHost);
    }

    private void registerHostnameVerifier(String verifier, QuarkusRestClientBuilder 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, QuarkusRestClientBuilder builder) {
        Optional<String> keyStorePassword = oneOf(restClientConfig.keyStorePassword(), configRoot.keyStorePassword());
        Optional<String> keyStoreType = oneOf(restClientConfig.keyStoreType(), configRoot.keyStoreType());

        try {
            KeyStore keyStore = KeyStore.getInstance(keyStoreType.orElse("JKS"));
            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

1. Implement jakarta.net.ssl.HostnameVerifier (javax.net.ssl.HostnameVerifier on older stacks) in the configured class.
2. Verify the property value references the intended verifier class, not another SSL type.
3. If wrapping a third-party verifier, create a small adapter class implementing HostnameVerifier.

Example fix

// before
public class MyChecker { public boolean check(String h, SSLSession s) {...} } // not a HostnameVerifier

// after
public class MyChecker implements HostnameVerifier {
    public boolean verify(String hostname, SSLSession session) { ... }
}

Object v = Class.forName("com.example.MyVerifier").getDeclaredConstructor().newInstance();
if (!(v instanceof javax.net.ssl.HostnameVerifier)) {
    throw new IllegalStateException("Configured class is not a HostnameVerifier");
}

boolean isHostnameVerifier(String className) {
    try {
        Object o = Thread.currentThread().getContextClassLoader().loadClass(className).getDeclaredConstructor().newInstance();
        return o instanceof javax.net.ssl.HostnameVerifier;
    } catch (Exception e) { return false; }
}

try {
    // build client with configured verifier
} catch (RuntimeException e) {
    if (e.getMessage() != null && e.getMessage().contains("is not an instance of HostnameVerifier")) {
        builder.hostnameVerifier(new DefaultVerifier());
    } else throw e;
}

When it happens

Trigger: Setting the hostname-verifier property to a class that implements a similar interface (e.g. a custom checker) but not jakarta/javax.net.ssl.HostnameVerifier.

Common situations: Pointing the property at an X509TrustManager, a Spring-style HostnameVerifier, or a similarly-named class from another library; mixing javax/jakarta or different SSL utility types; copy-paste errors in configuration.

Related errors

• Could not find a public, no-argument constructor for the hos
• Could not find hostname verifier class " + verifier
• Failed to instantiate hostname verifier class " + verifier +
• No password provided for keystore
• Failed to initialize trust store from classpath resource " +

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/e5900ee0d9b473a1. Report an issue: GitHub.