java reflection call methods of an already loaded object

Question

Hi,

How can I call the method of an object that has already been loaded in the JVM using reflection? I tried

Class myClass = Class.forName("myClass");
Method m = com.test.class.getDeclaredMethod("getValue",new Class[] {});
Object result = m.invoke(myClass,null);

but i get java.lang.IllegalArgumentException: object is not an instance of declaring class. The method I want to call is void i.e. does not take parameters

UPDATE I have an application that has already loaded a class "A". Another class "B" will be instantiated by a framework. When class "B" is initialized, class "A" has already been loaded in the JVM. I want to call a method from loaded instance of class "A" BUT without having a reference to "A" in class "B". In the answers, it seems I must create a new instance of "A" in class "B" but I want access to an already loaded object. If I create a new instance of "A" in "B" why would I want to use reflection? Am I missunderstanding something?

Thanks

Answer 1

You're passing the instance of Class as the first parameter to Method.invoke(..), but that's wrong; you want to pass the instance you're interested in.

result = m.invoke(myInstance, null);

Answer 2

Instead of:

Object result = m.invoke(myClass, null);

You should be passing in an instance of myClass. The illegal argument exception is due to invoke getting an argument of type Class instead of type myClass:

Object result = m.invoke(myInstance, null);

Answer 3

I think you need

Class myClass = myObject.GetClass();
Method m = com.test.class.getDeclaredMethod("getValue",new Class[] {});
Object result = m.invoke(myObject,null);

Answer 4

The thing about invoke, is that you dont have to have the exact instance of the class in question, if the method being called dosnt require any instance variables of the parent class. In fact you can add the static modifier to the Method and just call invoke with null, null, Object[] which is the same as:

public void methLab(Method m){ try{ m.invoke(m.getDeclaringClass().newInstance(), new Object[0]); }catch(IllegalAccessException iae){ // The method or parent class is declared private or protected }catch(IllegalArgumentException iae){ // unsatisfied input parameters ( in this case, no params were passed) }catch(InstantiationException ie){ // could be several things
}catch(InvocationTargetException it){ // Method specific, exception chain. call: it.getTargetException(). } }