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
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