Loading mechnism of plugin classes and singleton objects

[Enzo Michelangeli]

I have a question about the loading mechanism of plugin classes. I'm working 
with a custom URLFilter, and I need a singleton object loaded and 
initialized by the first instance of the URLFilter, and shared by other 
instances (e.g., instantiated by other threads). I was assuming that the 
URLFilter class was being loaded only once even when the filter is used by 
multiple threads, so I tried to use a static member variable of my URLFilter 
class to hold a reference to the object to be shared: but it appears that 
the supposed singleton, actually, isn't, because the method responsible for 
its instantiation finds the static field initialized to null. So: are 
URLFilter classes loaded multiple times by their classloader in Nutch? The 
wiki page at 
http://wiki.apache.org/nutch/WhichTechnicalConceptsAreBehindTheNutchPluginSystem 
seems to suggest otherwise:

   Until Nutch runtime, only one instance of such a plugin
   class is alive in the Java virtual machine.
(By the way, what does "Until Nutch runtime" mean here? Before Nutch 
runtime, no class whatsoever is supposed to be alive in the JVM, is it?)

Enzo