apache/pulsar · error · RuntimeException

User class must have a no-arg constructor

Error message

User class must have a no-arg constructor

What it means

createInstance requires a no-arg constructor: it calls getDeclaredConstructor() with no arguments. If the class has no accessible zero-argument constructor, NoSuchMethodException is wrapped in this RuntimeException. All classes instantiated through this helper must expose a public no-arg constructor.

Source

Thrown at pulsar-common/src/main/java/org/apache/pulsar/common/util/Reflections.java:92

        if (!xface.isAssignableFrom(theCls)) {
            throw new RuntimeException(userClassName + " does not implement " + xface.getName());
        }
        @SuppressWarnings("unchecked") // safe: theCls is verified to be assignable to xface
        Class<T> tCls = (Class<T>) theCls.asSubclass(xface);
        T result;
        try {
            @SuppressWarnings("unchecked") // safe: constructor cache is keyed by theCls which extends T
            Constructor<T> meth = (Constructor<T>) constructorCache.get(theCls);
            if (null == meth) {
                meth = tCls.getDeclaredConstructor();
                meth.setAccessible(true);
                constructorCache.put(theCls, meth);
            }
            result = meth.newInstance();
        } catch (InstantiationException ie) {
            throw new RuntimeException("User class must be concrete", ie);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException("User class must have a no-arg constructor", e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException("User class must have a public constructor", e);
        } catch (InvocationTargetException e) {
            throw new RuntimeException("User class constructor throws exception", e);
        }
        return result;

    }

    /**
     * Create an instance of <code>userClassName</code> using provided <code>classLoader</code>.
     *
     * @param userClassName user class name
     * @param classLoader class loader to load the class.
     * @return the instance
     */
    public static Object createInstance(String userClassName,
                                        ClassLoader classLoader) {

View on GitHub (pinned to 820761864e)

Solutions

1. Add a public no-arg constructor to the class
2. Move required setup out of the constructor into an initialize/lifecycle method the framework calls
3. If parameters are unavoidable, instantiate the object yourself instead of via this reflective helper

Example fix

// before
public MyFilter(String topic) { this.topic = topic; }
// after
public MyFilter() { }
public MyFilter(String topic) { this.topic = topic; }

// ensure a declared no-arg constructor exists
Class<?> cls = Class.forName(className, true, classLoader);
try {
    cls.getDeclaredConstructor();
} catch (NoSuchMethodException e) {
    throw new IllegalArgumentException("Missing no-arg constructor: " + className);
}

boolean hasNoArgCtor(Class<?> cls) { try { cls.getDeclaredConstructor(); return true; } catch (NoSuchMethodException e) { return false; } }

try {
    T obj = Reflections.createInstance(className, XFace.class, classLoader);
} catch (RuntimeException e) {
    if (e.getMessage().equals("User class must have a no-arg constructor")) {
        throw new IllegalArgumentException("Add a public no-arg constructor to " + className, e);
    }
    throw e;
}

When it happens

Trigger: Configuring a class whose only constructors take arguments, so `theCls.getDeclaredConstructor()` fails with NoSuchMethodException.

Common situations: Plugin classes written with constructor injection; adding a constructor with parameters removed the implicit no-arg constructor; Kotlin/Scala classes without a default constructor.

Related errors

• Failed to instantiate ${className}
• Exception caused while converting configuration: ${message}
• Failed to compute configuration overrides
• Failed to create authentication: ${message}
• (wraps reflective instantiation failure for BrokerEntryMetad

AI-assisted analysis of apache/pulsar@820761864e (2026-09-06). Data as JSON: /api/errors/f3e6e29e0e4a318f. Report an issue: GitHub.