Filter过滤器

定义：
FilterConfig对象提供对servlet环境及web.xml文件中指派的过滤器名的访问。
FilterConfig对象具有一个getInitParameter方法，它能够访问部署描述符文件（web.xml）中分配的过滤器初始化参数。

实例：
将下面的代码加入到web.xml中，试用FilterConfig就可以获得以 filter 作为描述标签内的参数。

<!-- The Cache Filter -->
<filter>
<!-- 设计过滤处理类，生成静态页面 -->
<filter-name>CacheFilter</filter-name>
<filter-class>com.jspbook.CacheFilter</filter-class>

<!-- 不需要缓存的URL -->
<init-param>
<param-name>/TimeMonger.jsp</param-name>
<param-value>nocache</param-value>
</init-param>

<init-param>
<param-name>/TestCache.jsp</param-name>
<param-value>nocache</param-value>
</init-param>

<!-- 缓存超时时间, 单位为秒 -->
<init-param>
<param-name>cacheTimeout</param-name>
<param-value>600</param-value>
</init-param>

<!-- 是否根据浏览器不同的地区设置进行缓存(生成的缓存文件为 test.jspid=1_zh_CN 的格式) -->
<init-param>
<param-name>locale-sensitive</param-name>
<param-value>true</param-value>
</init-param>

</filter>

<filter-mapping>
<filter-name>CacheFilter</filter-name>
<url-pattern>*.jsp</url-pattern>
</filter-mapping> 

用法：

filterConfig.getInitParameter("locale-sensitive"); 得到的就是 ture
filterConfig.getInitParameter("cacheTimeout"); 得到的就是 600
filterConfig.getInitParameter(request.getRequestURI()); 得到的就是param-name 对应的 param-value 值

过滤处理类：

public class CacheFilter implements Filter {
ServletContext sc;
FilterConfig fc;
long cacheTimeout = Long.MAX_VALUE;

public void doFilter(ServletRequest req, ServletResponse res,
FilterChain chain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;

// check if was a resource that shouldn't be cached.
String r = sc.getRealPath("");
String path = fc.getInitParameter(request.getRequestURI());
if (path != null && path.equals("nocache")) {
chain.doFilter(request, response);
return;
}
path = r + path;

}

public void init(FilterConfig filterConfig) {
this.fc = filterConfig;
String ct = fc.getInitParameter("cacheTimeout");
if (ct != null) {
cacheTimeout = 60 * 1000 * Long.parseLong(ct);
}
this.sc = filterConfig.getServletContext();
}

public void destroy() {
this.sc = null;
this.fc = null;
}
}