nutch-site.xml
在nutch2.2.1中，有两份配置文件：nutch-default.xml与nutch-site.xml。
其中前者是nutch自带的默认属性，一般情况下不要修改。
如果需要修改默认属性，可以在nutch-site.xml中增加一个同名的属性，并修改其值。nutch-site.xml中的属性值会覆盖nutch-default.xml中的值。
1、db.ignore.external.links
若为true，则只抓取本域名内的网页，忽略外部链接。
可以在 regex-urlfilter.txt中增加过滤器达到同样效果，但如果过滤器过多，如几千个，则会大大影响nutch的性能。
<property> <name>db.ignore.external.links</name> <value>true</value> <description>If true, outlinks leading from a page to external hosts will be ignored. This is an effective way to limit the crawl to include only initially injected hosts, without creating complex URLFilters. </description> </property>
2、fetcher.parse
能否在抓取的同时进行解释：可以，但不 建议这样做。
<property> <name>fetcher.parse</name> <value>false</value> <description>If true, fetcher will parse content. NOTE: previous releases would default to true. Since 2.0 this is set to false as a safer default.</description> </property>
官方解释
N.B. In a parsing fetcher, outlinks are processed in the reduce phase (at least when outlinks are followed). If a fetcher's reducer stalls you may run out of memory or disk space, usually after a very long reduce job. Behaviour typical to this is usually observed in this situation.
In summary, if it is possible, users are advised not to use a parsing fetcher as it is heavy on IO and often leads to the above outcome.
3、db.max.outlinks.per.page
默认情况下，Nutch只抓取某个网页的100个外部链接，导致部分链接无法抓取。若要改变此情况，可以修改此配置项。
<property> <name>db.max.outlinks.per.page</name> <value>100</value> <description>The maximum number of outlinks that we'll process for a page. If this value is nonnegative (>=0), at most db.max.outlinks.per.page outlinks will be processed for a page; otherwise, all outlinks will be processed. </description> </property>官方说明如下：http://wiki.apache.org/nutch/FAQ/
Nutch doesn't crawl relative URLs? Some pages are not indexed but my regex file and everything else is okay - what is going on?
The crawl tool has a default limitation of 100 outlinks of one page that are being fetched. To overcome this limitation change thedb.max.outlinks.per.page property to a higher value or simply -1 (unlimited).
file: conf/nutch-default.xml
<property> <name>db.max.outlinks.per.page</name> <value>-1</value> <description>The maximum number of outlinks that we'll process for a page. If this value is nonnegative (>=0), at most db.max.outlinks.per.page outlinks will be processed for a page; otherwise, all outlinks will be processed. </description> </property>
see also: http://www.mail-archive.com/nutch-user@lucene.apache.org/msg08665.html
4、file.content.limit http.content.limit ftp.content.limit
默认情况下，nutch只抓取网页的前65536个字节，之后的内容将被丢弃。
但对于某些大型网站，首页的内容远远不止65536个字节，甚至前面65536个字节里面均是一些布局信息，并没有任何的超链接。
因此修改默认值如下：
<property> <name>file.content.limit</name> <value>-1</value> <description>The length limit for downloaded content using the file protocol, in bytes. If this value is nonnegative (>=0), content longer than it will be truncated; otherwise, no truncation at all. Do not confuse this setting with the http.content.limit setting. </description> </property> <property> <name>http.content.limit</name> <value>-1</value> <description>The length limit for downloaded content using the http protocol, in bytes. If this value is nonnegative (>=0), content longer than it will be truncated; otherwise, no truncation at all. Do not confuse this setting with the file.content.limit setting. </description> </property> <property> <name>ftp.content.limit</name> <value>-1</value> <description>The length limit for downloaded content, in bytes. If this value is nonnegative (>=0), content longer than it will be truncated; otherwise, no truncation at all. Caution: classical ftp RFCs never defines partial transfer and, in fact, some ftp servers out there do not handle client side forced close-down very well. Our implementation tries its best to handle such situations smoothly. </description> </property>
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