servlet中的过滤器格式：
<filter>
<filter-name> xxxx</filter-name>
<filter-class> xxxx.aaaa</filter-class>
</filter>
<init-param>(可选)
<parma-name>charset</param-name>
<param-value> UTF-8</param-value>
</init-param>
<filter-mapping>
<filter-name> xxxx</filter-name>
<url-pattern> /* </url-pattern>
</filter-mapping>
