Head First Servlets and JSP, 2nd Edition

Declaring and ordering filters

When you configure filters in the DD, you’ll usually do three things:

• Declare your filter

• Map your filter to the web resources you want to filter

• Arrange these mappings to create filter invocation sequences

Declaring a filter

<filter>
  <filter-name>BeerRequest</filter-name>
  <filter-class>com.example.web.BeerRequestFilter
        </filter-class>
  <init-param>
    <param-name>LogFileName</param-name>
    <param-value>UserLog.txt</param-value>
  </init-param>
</filter>

Declaring a filter mapping to a URL pattern

<filter-mapping>
  <filter-name>BeerRequest</filter-name>
  <url-pattern>*.do</url-pattern>
</filter-mapping>

Declaring a filter mapping to a servlet name

<filter-mapping>
  <filter-name>BeerRequest</filter-name>
  <servlet-name>AdviceServlet</servlet-name>
</filter-mapping>

Rules for <filter>

• The <filter-name> is mandatory.

• The <filter-class> is mandatory.

• The <init-param> is optional, and you can have many.

Rules for <filter-mapping>

• The <filter-name> is mandatory and it is used to link to the correct <filter> element.

• Either the <url-pattern> or the the <servlet-name> element is mandatory.

• The <url-pattern> element defines which web app resources will use this filter.

• The <servlet-name> element defines which single web app resource will use this filter.

Note

IMPORTANT: The Container’s rules for ordering filters:

When more than one filter is mapped to a given resource, the Container uses the following rules:

1) ALL filters with matching URL patterns are located first. This is NOT the same as ...