Bootstrap Classloader Utility

Question

I am trying to write an application launching utility that will behave somewhat how the Apache Tomcat Server does only for a single application.
The bootstrap class will be passed a string representing the path to the desired application. The bootstrap will then configure a URLClassLoader that includes any resources (jars and/or classes) found in a common/lib and common/classes directory. A second URLClassLoader is constructed with the application's specific jars and/or classes (based upon the string that was passed in). The first (or "common") ClassLoader is to be the parent of the app specific ClassLoader.
A common interface is used for each application that the bootstrap class then instantiates like this:
Class appClass = appLoader.loadClass(<fully-qualified classname here>);
appClass.newInstance();
Where appLoder is the URLClassLoader that has the "common" ClassLoader as its parent.
This much works great! But, my concrete application class that gets instantiated tries to use a class from a third-party vendor. This third-party class is in a jar file that resided in the common/lib directory (and was successfully loaded by the "common" ClassLoader). But, I get a NoClassDefFoundError thrown from within my application class.
How can I get my application's class to use the ClassLoader that loaded it? I have tried to pick through the Tomcat code to see how they are doing things, but quickly get lost in the complexity of a multi-threaded, multi-app container.
Any thoughts/suggestions would be greatly appreciated.

Answer 1

First (and maybe this is part of my problem), I'm not subclassing URLClassLoader. I am using it as is. Let me give you more detail. This is what the body of my Bootstrap class's main method currently looks (I've simplified it some, but still has same problem) like:
public static void main(String[] args) {
  ClassLoader commonLoader = null;
  ClassLoader frameworkLoader = null;
  try {
    // prepare the common class loader
    File[] classDir = { new File(".", "common" + File.separator + "bin")};
    File[] jarDir = { new File(".", "common" + File.separator + "lib") };
    commonLoader = ClassLoaderFactory.createClassLoader(classDir, jarDir, null);
    // prepare the framework class loader
    classDir[0] = new File(".", "bin");
    jarDir[0] = new File(".", "lib");
    frameworkLoader =     ClassLoaderFactory.createClassLoader(classDir, jarDir, commonLoader);
  } catch (Throwable t) {
    t.printStackTrace();
    System.exit(1);
  try {
    Class startupClass = frameworkLoader.loadClass("com.jaf.JafAppImpl");
    startupClass.newInstance();
  } catch (ClassNotFoundException e) {
    e.printStackTrace();
  } catch (InstantiationException e) {
    e.printStackTrace();
  } catch (IllegalAccessException e) {
    e.printStackTrace();
}The ClassLoaderFactory simly iterates across the File arrays buiding up a collection of URLs which ultimately get passed into the constructor of a URLClassLoader (not a subclass of one) and returns it. Again, as I mentioned before, this is loosely based on what I saw happening in Apache's Tomcat Bootstrap class. The third-party class/jar is located in the common/lib directory that gets picked up in the common ClassLoader. I have verified that it successfully found the jar by attempting to instantiate one of the third-party classes directly in the same manner as my framework class (loadClass/new Instance) and it works fine. But, my problem is that when I try and use that third-party class in the constructor of my framework class, it can't find the class' definition (NoClassDefFoundError).
Two assumptions that I've been making:
1) That my framework class will continue to use the ClassLoader that loaded it for any subsequent classes that it needs.
2) That I didn't have to subclass URLClassLoader because I can pass in all of the URLs that I need (and another parent ClassLoader) and it will first look in its (the framework ClassLoader) available URLs and if it can't find the class there, it will look in its parent (the common ClassLoader) URLs.
I see that you have suggested passing in ClassLoader.getSystemClassLoader() as a parent. I wasn't sure exactly how this would help, but I did try passing the system ClassLoader into my common ClassLoader as its parent and that did not help matters. However, your subclass of URLClassLoader is how I thought URLClassLoader worked internally anyway.
Hope I've helped clear up what I'm trying to do. Thanks again for your input.