Reflection IllegalArgumentException java

You are trying to retrieve the value of the field on a Class<B> instance instead of doing it on an instance of the class representend by what you (quite clumsily) called instance in your code. To be clear, you are doing like if f3 was a static field of the class B (a field of Class<B> is indeed a static field of B) whereas it is actually a non-static field. Thus, you need to get an instance of B before calling the get method :

Class<?> clazz  = loadClass("b");
Object instance = clazz.newInstance();
Fied fieldf3    = B.class.getDeclaredField("f3");
fieldf3.setAccessible(true);
fieldf3.get(instance);

Let's see with an example (this time let's assume that B is a class defined in the main file) :

static class B {
    private int f3;
    public B(int f) {
        f3 = f;
    }
}

public static void main(String[] args) {
    B b = new b(17);
    Field field = B.class.getDeclaredField("f3");
    field.setAccessible(true);
    System.out.println(field.get(b));
}

This code will output 17 as expected. Now, imagine that f3 is a static field, then it becomes a field on any Class<B> instance (actually, I think it is a singleton so there is only a single such instance), then you could add System.out.println(field.get(B.class)); and it would still work. The value of the field will also be accessible from any instance of B so System.out.println(field.get(b)); will also work.