jwtk/jjwt · error · IllegalArgumentException

Class method parameter cannot be null.

Error message

Class method parameter cannot be null.

What it means

IllegalArgumentException thrown by io.jsonwebtoken.lang.Classes.newInstance(Class) when the Class argument is null. It is a fail-fast precondition check before attempting reflection-based instantiation.

Solutions

1. Check the Class argument for null before calling Classes.newInstance
2. Fix the upstream lookup (class name config key, Classes.forName call) that produced null
3. If a null is expected, guard with a default implementation class

Example fix

// before
Class<MyCodec> c = lookupCodecClass(); // may be null
MyCodec codec = Classes.newInstance(c);
// after
Class<MyCodec> c = lookupCodecClass();
if (c == null) {
    c = MyCodec.class; // or throw a clearer config error
}
MyCodec codec = Classes.newInstance(c);

if (clazz == null) {
    clazz = DefaultImplClass.class; // or fail with a clear config error
}

Class<?> resolveClass(Class<?> c) {
    return (c != null) ? c : DefaultImplClass.class;
}

try {
    return Classes.newInstance(clazz);
} catch (IllegalArgumentException e) {
    throw new IllegalStateException("Implementation class not configured; cannot instantiate null class", e);
}

When it happens

Trigger: Passing a null Class<T> to Classes.newInstance(clazz), commonly because an earlier lookup (e.g. Classes.forName, a config-driven class name, or a map lookup) returned or was assigned null.

Common situations: Configuration property naming a class is missing so a lookup yields null that is then handed to newInstance; calling getImplementationClass-style helpers without checking for null; test code constructing instances from a null constant.

Related errors

• Collection must not be null
• Map must not be null
• MAC key cannot be null.
• Reflection operation failed. This is likely due to an…
• SecretKey byte array cannot be null.

AI-assisted analysis of jwtk/jjwt@fb71496164 (2026-09-09). Data as JSON: /api/errors/65e25105280e9936. Report an issue: GitHub.

Appendix: source

Thrown at api/src/main/java/io/jsonwebtoken/lang/Classes.java:216

     * @param <T>  the type of the object created
     * @return the newly created object
     */
    @SuppressWarnings("unchecked")
    public static <T> T newInstance(String fqcn, Object... args) {
        return (T) newInstance(forName(fqcn), args);
    }

    /**
     * Creates a new instance of the specified {@code clazz} via {@code clazz.newInstance()}.
     *
     * @param clazz the class to invoke
     * @param <T>   the type of the object created
     * @return the newly created object
     */
    public static <T> T newInstance(Class<T> clazz) {
        if (clazz == null) {
            String msg = "Class method parameter cannot be null.";
            throw new IllegalArgumentException(msg);
        }
        try {
            return clazz.newInstance();
        } catch (Exception e) {
            throw new InstantiationException("Unable to instantiate class [" + clazz.getName() + "]", e);
        }
    }

    /**
     * Returns a new instance of the specified {@code clazz}, invoking the associated constructor with the specified
     * {@code args} arguments.
     *
     * @param clazz the class to invoke
     * @param args  the arguments matching an associated class constructor
     * @param <T>   the type of the created object
     * @return the newly created object
     */
    public static <T> T newInstance(Class<T> clazz, Object... args) {

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