URLClassLoader with Eclipse

Question

I am currently writing an Eclipse plugin using SWT and JFace. My program also runs as a standalone outside of the Eclipse IDE. I'm having a problem using a URLClassLoader when running as a plugin. It works fine when working as a standalone.
My program dynamically loads classes from a directory or Jar file using a URLClassLoader. I have code like the following
Class c = loader.loadClass(someString);
// I want to check if it is derived from a Widget so I do the following
if (Widget.class.isAssignableFrom(c)) {
This works fine when I run as a standalone program. But when it runs as an Eclipse plugin the behaviour is different. The method isAssignableFrom returns false. However, if I use my new classloader to load the Widget class and use that in the if statement above, it works grand i.e.
Class widgetClass = loader.loadClass("org.eclipse.swt.widgets");
if (widgetClass.isAssignableFrom(c) {
It works, but there are other problems further down the road that I think are related to this. Here's my theory as to what's happening. When a stand alone program is kicked off from Eclipse, a new instance of javaw.exe is run, and I guess Eclipse tells it what jars to load by using the -cp option. These classes are loaded by the system class loader and it's the system class loader that is used to load the class Widget.class. Similarly Class.forNew will also use the system class loader.
Eclipse itself is also an instance of javaw.exe but I think it uses its own class loader to load the jars in the plugin directory. Therefore the SWT classes are not part of the system class loader when running as an eclipse plugin. Thats why the above code no longer works.
The problem is that further down the line, I try to use the class I loaded to create an instance of itself. But it throws an exception straight away. It's difficullt to debug why exactly its throwing the exception. But I think if I could solve the above problem, the second one would go away.
Thanks for any help with this
Sean

Answer 1

After some additional debugging this problem I'm having, I discovered that a strange thing is happening to SWT's static members. I call a function Display.getCurrent() from my own code - works fine and and then call Constructor.newInstance on an object - the newInstance code also ends up calling Display.getCurrent() and I'm seeing in the debugger that the Display object's static members are now null. This is why it crashes later.
As far as I can see from the debugger, any time I'm at a breakpoint in Display.getCurrent(), the call stack stems back from the main() call, so I have no idea why these statics are being treated as different.
Anyone got any ideas?
Thanks for any help