Dissecting The Nutch Crawler -Factory classes: '''URLFilterFactory'''

英文原文出处：DissectingTheNutchCrawler
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Factory classes: '''URLFilterFactory'''

> Class net.nutch.net.URLFilterFactory
> used by:
> - net.nutch.db.WebDBInjector
> - net.nutch.tools.UpdateDatabaseTool

URLFilterFactory is not strictly part of the crawler, but it is a good extension point within Nutch. Here's how it works:

1. When the class is loaded, URLFILTER_CLASS is set to the value returned by NutchConf for the key "urlfilter.class"

2. When getFilter() is called, it checks to see if the filter class has already been loaded. If not, we load it using Class.forName(URLFILTER_CLASS), and the class is returned.

It loads one class, which is configurable via "urlfilter.class". By default, nutch-default.xml specifies this as follows:

<!-- urlfilter properties -->

<property>
  <name>urlfilter.class</name>
  <value>net.nutch.net.RegexURLFilter</value>
  <description>Name of the class used to filterURLs.</description>
</property>

<property>
  <name>urlfilter.regex.file</name>
  <value>regex-urlfilter.txt</value>
  <description>Name of file onCLASSPATH containing default regular
  expressions used byRegexURLFilter.</description>
</property>

Now let's look at the crawler factories, which are a bit more complex.

---

工厂类：''URLFilterFactory'''

类 net.nutch.net.URLFilterFactory 被net.nutch.db.WebDBInjector 和net.nutch.tools.UpdateDatabaseTool 使用

URLFilterFactory is not strictly part of the crawler, but it is a good extension point within Nutch. Here's how it works:

URLFilterFactory 严格意义上并不属于crawler，但它是一个好的扩展点。让我们看看它的工作机制：

1. 当该类被加载时，属性URLFILTER_CLASS被赋值为NutchConf.get().get("urlfilter.class")
2. 当getFilter()方法被调用，它检查是否该类被加载，如果没有，通过Class.forName(URLFILTER_CLASS)来加载，否则直接返回该类

它通过可配置的urlfilter.class特性加载该类。默认情况下，nutch-default.xml定义如下

<!--urlfilterproperties-->

<property>
<name>urlfilter.class</name>
<value>net.nutch.net.RegexURLFilter</value>
<description>NameoftheclassusedtofilterURLs.</description>
</property>

<property>
<name>urlfilter.regex.file</name>
<value>regex-urlfilter.txt</value>
<description>NameoffileonCLASSPATHcontainingdefaultregular
expressionsusedbyRegexURLFilter.</description>
</property>

让我们再看看与crawler相关的工厂，那可是有点复杂。