alibaba/arthas · error · IllegalArgumentException

is not an interface

Error message

{iface} is not an interface

What it means

Thrown by ReflectUtils.findInterfaceMethod when the Class passed in is not an interface (iface.isInterface() returns false). The utility enforces a hard precondition: it only inspects interface types via getDeclaredMethods, so concrete classes, abstract classes, arrays, and primitives are rejected up front.

Solutions

1. Pass an interface Class (e.g. Factory.class, or your annotated interface), not a class.
2. Guard with iface.isInterface() before calling findInterfaceMethod, and pick a different code path for non-interfaces.
3. Inspect the value of iface logged in the message to confirm what was actually passed.

Example fix

// before
Method m = ReflectUtils.findInterfaceMethod(MyConcreteImpl.class);

// after
if (MyInterface.class.isInterface()) {
    Method m = ReflectUtils.findInterfaceMethod(MyInterface.class);
}

if (iface == null || !iface.isInterface()) {
    // reject before calling findInterfaceMethod
}

static boolean isSingleMethodInterface(Class<?> c) {
    return c != null && c.isInterface() && c.getDeclaredMethods().length == 1;
}

When it happens

Trigger: Call findInterfaceMethod(class) with a concrete class, abstract class, enum type, array type, or primitive Class object instead of a true Java interface.

Common situations: Accidentally passing a Class<? extends SomeImpl> where an interface type was expected; passing the implementation class of a CGLib factory rather than its interface; tooling that loads arbitrary Class objects and forwards them here.

Related errors

• expecting exactly 1 method in
• missing newInstance method
• Source is not an array
• Cannot convert to char
• Couldn't find rpcmethod

AI-assisted analysis of alibaba/arthas@21cf2e9ba5 (2026-08-14). Data as JSON: /api/errors/38407df2285df509. Report an issue: GitHub.

Appendix: source

Thrown at common/src/main/java/com/taobao/arthas/common/ReflectUtils.java:409

        for (int i = 0; i < interfaces.length; i++) {
            addAllMethods(interfaces[i], list);
        }

        return list;
    }

    public static List addAllInterfaces(Class type, List list) {
        Class superclass = type.getSuperclass();
        if (superclass != null) {
            list.addAll(Arrays.asList(type.getInterfaces()));
            addAllInterfaces(superclass, list);
        }
        return list;
    }

    public static Method findInterfaceMethod(Class iface) {
        if (!iface.isInterface()) {
            throw new IllegalArgumentException(iface + " is not an interface");
        }
        Method[] methods = iface.getDeclaredMethods();
        if (methods.length != 1) {
            throw new IllegalArgumentException("expecting exactly 1 method in " + iface);
        }
        return methods[0];
    }

    // SPRING PATCH BEGIN
    public static Class defineClass(String className, byte[] b, ClassLoader loader) throws Exception {
        return defineClass(className, b, loader, null, null);
    }

    public static Class defineClass(String className, byte[] b, ClassLoader loader, ProtectionDomain protectionDomain)
                    throws Exception {

        return defineClass(className, b, loader, protectionDomain, null);
    }

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