How to load .class from working directory at runtime from jar?

Attempting to implement a plugin system for my application, the code of which extends my Plugin class to define custom functionality.

This code to load a class from within a jar, in the same directory as the .jar of the application, works fine:

if(pluginName.endsWith(".jar"))
{
    JarFile jarFile = new JarFile(pluginName);
    Enumeration jarComponents = jarFile.entries();

    while (jarComponents.hasMoreElements())
    {
        JarEntry entry = (JarEntry) jarComponents.nextElement();
        if(entry.getName().endsWith(".class"))
        {
            className = entry.getName();
        }
    }

    File file = new File(System.getProperty("user.dir")+"/"+pluginName);
    URLClassLoader loader = new URLClassLoader(new URL[]{file.toURI().toURL()});

    Class newClass = Class.forName(className.replace(".class", ""), true, loader);
    newPlugin = (Plugin) newClass.newInstance();
}

Which results in the correct response of:

benpalmer$ java -jar assignment.jar test_subproject.jar
- INFO: Add new plugin: source.com

The issue is I also want to provide the user the option to specify a .class file instead, as that is realistically all that is packed into a .jar for the purposes of my plugin system.

else if(pluginName.endsWith(".class"))
{
    File file = new File(System.getProperty("user.dir")+"/"+pluginName);
    URLClassLoader loader = new URLClassLoader(new URL[]{file.toURI().toURL()});

    Class newClass = Class.forName(className.replace(".class", ""), true, loader);
    newPlugin = (Plugin) newClass.newInstance();
}   
... exception handling

Which results in the following:

benpalmer$ java -jar assignment.jar TestPlugin.class
SEVERE: Error: Plugin class not found. Class name: 
java.lang.ClassNotFoundException: 
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at newsfeed.model.Plugin.loadClassFromJarFile(Plugin.java:144)
    at newsfeed.controller.NFWindowController.initPlugins(NFWindowController.java:81)
    at newsfeed.controller.NewsFeed$1.run(NewsFeed.java:20)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

I've tried the code above and many variations of path names/other options, but get ClassNotFoundException every time. Originally had trouble loading the jars but using the fully qualified path fixed that, now I can't load the Classes.

Any ideas? :)