apache/dubbo · error · RuntimeException

Class {} is not a interface.

Error message

Class {} is not a interface.

What it means

Thrown by Mixin.assertInterfaceArray at the start of mixin(). It verifies every element of the ics array is actually an interface; passing a concrete or abstract class is a programming error because Dubbo generates an implementor, not a subclass. The message names the offending class.

Source

Thrown at dubbo-common/src/main/java/org/apache/dubbo/common/bytecode/Mixin.java:226

    private static int findMethod(Class<?>[] dcs, String desc) {
        Class<?> cl;
        Method[] methods;
        for (int i = 0; i < dcs.length; i++) {
            cl = dcs[i];
            methods = cl.getMethods();
            for (Method method : methods) {
                if (desc.equals(ReflectUtils.getDesc(method))) {
                    return i;
                }
            }
        }
        return -1;
    }

    private static void assertInterfaceArray(Class<?>[] ics) {
        for (int i = 0; i < ics.length; i++) {
            if (!ics[i].isInterface()) {
                throw new RuntimeException("Class " + ics[i].getName() + " is not a interface.");
            }
        }
    }

    /**
     * new Mixin instance.
     *
     * @param ds delegates instance.
     * @return instance.
     */
    public abstract Object newInstance(Object[] ds);

    public static interface MixinAware {
        void setMixinInstance(Object instance);
    }
}

View on GitHub (pinned to 3a3043227f)

Solutions

1. Pass the interface type, not a concrete/abstract class, in the ics array.
2. If you intended to proxy a class rather than implement an interface, use a different mechanism (Proxy only handles interfaces too).
3. Add a compile-time or pre-call check ic.isInterface() for each element.

Example fix

// before
Mixin.mixin(new Class[]{ ServiceImpl.class }, dc); // ServiceImpl is a class

// after
Mixin.mixin(new Class[]{ Service.class }, dc); // Service is the interface

for (Class<?> ic : ics) {
    if (!ic.isInterface()) {
        throw new IllegalArgumentException(ic.getName() + " is not an interface");
    }
}

When it happens

Trigger: Mixin.mixin(new Class[]{ SomeClass.class }, dc) where SomeClass is not declared with the interface keyword.

Common situations: Passing an implementation class instead of its interface by mistake; passing a class that was refactored from interface to abstract class; copy-paste of the wrong Class reference.

Related errors

• Missing method [{}] implement.
• non-public interfaces class from different packages
• non-public delegate class from different packages
• {} is not a interface.
• interface limit exceeded

AI-assisted analysis of apache/dubbo@3a3043227f (2026-08-14). Data as JSON: /api/errors/99f6e2b54bfb96e3. Report an issue: GitHub.