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From Andrzej Bialecki>
Subject Static initializers
Date Tue, 20 Dec 2005 13:19:14 GMT

This was mentioned before: there are many places in Nutch that rely on 
static initializers. This is so-so or sometimes plainly bad, depending 
on a situation.

I'm facing a problem now with URLFilters. I need to run several fetchers 
inside a single VM, with different parameters such as different url 
patterns (which is handled by URLFilters). But even if I specify 
different NutchConf-s to each fetcher, the list of implementations and 
the instances of URLFilter[] in URLFilters are initialized only once, 
and this happens from the default configuration obtained through a call 
to static NutchConf.get().

I would like to change it somehow, but I'm not sure how... One way to 
solve this would be to instantiate the plugins based on a concrete 
NutchConf instance, like this:


    private URLFilters(NutchConf) {
       // initialize plugins based on this instance of NutchConf

    public static URLFilters get(NutchConf conf) {
       URLFilters res = (URLFilters)conf.get("urlfilters.key");
       if (res == null) {
          res = new URLFilters(conf);
          conf.put("urlfilters.key", res);
       return res;

In case you are running with a single NutchConf per JVM it doesn't change anything. In case
you want to run several different configs in a single JVM this approach provides the solution.
We could follow this strategy for other plugin registry facades. Comments?

Best regards,
Andrzej Bialecki     <><
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