Question

Here is some code I’ve been twiddling with to try and lazily fill in fields in object, mostly for object factories in JUnit but it could be quite a useful method to have.

    private void lazyObjectFill(Object profil) throws IllegalArgumentException, IllegalAccessException, InvocationTargetException {
    final Method[] list = profil.getClass().getDeclaredMethods();
    for (Method m : list) {
        if (Modifier.isPublic(m.getModifiers()) && m.getName().startsWith("set")) {

            final Class< ?>[] parameterTypes = m.getParameterTypes();
            if (parameterTypes.length == 1) {
                final Class< ?> clazz = parameterTypes[0];
                if (clazz == String.class) {
                    log.info("Invoking " + m.getName() + " with [\"\"]");
                    m.invoke("");
                } else if (clazz.isPrimitive() && Defaults.defaultValue(clazz) != null) {
                    log.info("Invoking " + m.getName() + " with [" + Defaults.defaultValue(clazz) + "]");
                    m.invoke(Defaults.defaultValue(clazz));
                }
            }

        }
    }
}

We get the following exception when running this code on an object.

java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:592)

The exception occurs when running m.invoke(“”); on a string setter.

---

Updated source code for the benefit of the googlers.

private void lazyObjectFill(Object obj) throws IllegalArgumentException, IllegalAccessException, InvocationTargetException {
    final Method[] list = obj.getClass().getDeclaredMethods();
    for (Method method : list) {
        method.setAccessible(true);
        if (Modifier.isPublic(method.getModifiers()) && method.getName().startsWith("set")) {

            final Class< ?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length == 1) {
                final Class< ?> clazz = parameterTypes[0];
                if (clazz == String.class) {
                    log.info("Invoking " + method.getName() + " with [\"\"]");
                    method.invoke(obj, "");
                } else if (clazz.isPrimitive() && Defaults.defaultValue(clazz) != null) {
                    log.info("Invoking " + method.getName() + " with [" + Defaults.defaultValue(clazz) + "]");
                    method.invoke(obj, Defaults.defaultValue(clazz));
                }
            }

        }
    }
}

Answer 1

You are almost there, but methods are static and they need an object on which to invoke them.

i.e.

m.invoke(profil, "");

and

m.invoke(profil, Defaults.defaultValue(clazz));

You were (unknowingly) trying to execute the method on a string object without parameters. And since the string class does not have that method, it had to fail. Details can be found in the Method javadoc.

BTW: Static methods are invoked like this:

method.invoke(null, params);