Class Loader confusion

Question

I am trying to use java.net.URLClassLoader to load classes dynamically from jar files. The strange thing is, I can only get this to work successfully with one particular Jar file. With any other Jar file I have tried, I get a java.lang.NoClassDefFoundError. I'm trying to figure out what is different about this particular jar file.
The first thing I should point out is that when I create the URLClassLoader, I can successfully load the appropriate classes using loadClass(). So I am not mistyping the filename of the Jar or anything like that.
It's only when I try to create an instance of a class that is not in the Jar but uses a class in the Jar file, that I get the NoClassDefFoundError.
The second thing I have noticed is that everything works fine when run using the Sun JDK1.3 VM. It is only in my production environment (Lotus Notes) that the problem occurs. It seems the Notes class loader is doing things differently to the sun class loader. Perhaps this is a security issue?
Can anyone shed any light on why this is occuring? Why does everything work fine with one particular Jar (Jakarta POI) but no others I have tried? And why does it stop working in the Notes environment?
Any help is appreciated.
And here is my code:
    public void test()
        URL[] urls=new URL[1];
        File file=new File("c:/software/libraries/tablelayout.jar");
        try
            urls[0]=file.toURL();
        catch (MalformedURLException e)
            e.printStackTrace();
            return;
        ClassLoader cl = this.getClass().getClassLoader();
        ClassLoader loader=URLClassLoader.newInstance(urls,cl);
        Class agentClass=null;
        try
            // Test if we can actually load the class
            Class testClass = loader.loadClass("com.lowagie.text.pdf.PdfReader");
            System.out.println("loaded test class");
            agentClass = loader.loadClass("test.RandomAgent");
        catch (ClassNotFoundException e)
            e.printStackTrace();
            return;
        AgentBase agent=null;
        try
            agent = (AgentBase) agentClass.newInstance(); // This is where the NoClassDefFoundError is thrown
        catch (Exception e)
            e.printStackTrace();
            return;
        agent.NotesMain();
    }And the stack trace I get:
loaded test class
java.lang.NoClassDefFoundError: com/lowagie/text/pdf/PdfReader
     at java.lang.Class.newInstance0(Native Method)
     at java.lang.Class.newInstance(Class.java:262)
     at TestAgentUsingClassLoader.NotesMain(TestAgentUsingClassLoader.java:64)
     at lotus.domino.AgentBase.runNotes(Unknown Source)
     at lotus.domino.NotesThread.run(NotesThread.java:218)

Answer 1

I figured it out. Thanks to this post: http://forum.java.sun.com/thread.jspa?threadID=484866&messageID=2267796
The class test.RandomAgent was actually being loaded by the system classloader, even though I had asked to create it using the URLClassLoader. The URLClassLoader delegates the creation to its parent (the system class loader) which found the test.RandomAgent class on the system class path. Thereon, all class created by the RandomAgent class would have been loaded by its ClassLoader, i.e. the system class loader.
To fix this, I had to ensure the test.RandomAgent class was not on the system classpath. To do that, I had to put it in its own Jar and add that jar to the URLClassLoader list.
It turned out that the reason it worked with one jar (Jakart POI) was that that Jar was on my system classpath! So that was a red herring.