Ivocation II

Question

Hi,
Strange things happen with this invocation trickery.
I've class-loading related problem.
I have class file (Exec.class) and list of libraries (jars) it uses.
In my code (Invoke.java), I first, use URLClassLoader to create instance of Exec class and load all required jars. As second step I invoke exec() method of instance of Exec.
Exec uses xerces.jar, so when I load its instance, I specify xerces.jar as one of URLs passed to URLClassLoader. Getting instance works OK. When I invoke Exec's instance than, Exec throws error java.lang.ClassNotFoundException: org.apache.xerces.parsers.SAXParser which is saying, that it can't find class from xerces.jar file. If xerces wasn't loaded, first step would fail and would not let me proceed to invocation.
Interesting thing is, that I tried to run Invoke with -cp xerces.jar and invocation run ok.
As Exec.class can request any library it wants, it can't be solved by putting all needed jar files on command-line.
As a deed of desperation, I tried changing System property "java.class.path" on runtime, so that it contained xerces.jar but it didn't help either.
To me it seems that loaded libraries for some reason don't propagate to invoked class. Seems bizzare.
Any help ?
Michal

Answer 1

OK. To make things clear I describe my situation.
I have Invoke class which gets name of class to invoke and libraries it depends on. (Simplified) code of Invoke may look like this:Invoke.java:
public class Invoke{
    String classToInvoke = "Exec.class"
    String method = "exec"
    String library = "xerces.jar"
    public static void main(String []args){
        Object newInstance = loadExternalClass(classToInvoke,library);
        invokeClass(newInstance,method);
    public Object loadExternalClass(String classToInvoke,String library){
        URL[] urlsToLoad = new URL[]{
            new File(classToInvoke).toURL(),
            new File(library).toURL()
        URLClassLoader urlClassLoader = new URLClassLoader(urlsToLoad);
        Class newClass = urlClassLoader.loadClass()
        return newClass.newInstance();
    public void invokeClass(Object instanceToInvoke,String methodToCall){
        Method execMethod = ... // getting right method from instanceToInvoke
        execMethod.invoke(instanceToInvoke,invokeParameters);
}And Exec.class would look like:
import org.xml.sax.*;
public class Exec {
    public void exec(){
         // \/\/\/\/ here it fails
         XMLReader reader = XMLReaderFactory.createXMLReader ("org.apache.xerces.parsers.SAXParser");
        reader.setContentHandler(this);
        InputSource inputSource = new InputSource(new StringReader(xml));
        reader.parse(inputSource);
}Exec.class doesn't use reflection. (and isn't aware, there is any reflection business going on)
Concerning hierarchy of ClassLoaders. I know that you may pass reference to parentClassLoader when you create a new one, but never investigated why you would like to do it (I suppose, it's called when local ClassLoader doesn't succeed to find class it is ordered to load).
Thus I don't know what is implication of such a hierarchy and have no clue what to do to have my code working.
Any advice appreciated.
Michal