Filter 简单应用实例求详解

web.xml


<filter>
<filter-name>AdminFilter</filter-name>
<filter-class>szy.shopping.filter.AdminFilter</filter-class>
<init-param>
<param-name>url</param-name>
<param-value>/admin/login.jsp</param-value>
</init-param>
</filter>

<filter-mapping>
<filter-name>AdminFilter</filter-name>
<url-pattern>/admin/index.jsp</url-pattern>
</filter-mapping>






AdminFilter.java

public class AdminFilter implements Filter{
String url = "/";
@Override
public void destroy() {

}

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
HttpServletRequest req = (HttpServletRequest) request;
HttpServletResponse res = (HttpServletResponse) response;
HttpSession session = req.getSession();
if (session.getAttribute("admin") == null) {
res.sendRedirect(req.getContextPath() + url);
return ;
}
chain.doFilter(request, response);
}

@Override
public void init(FilterConfig config) throws ServletException {
url = config.getInitParameter("url");
}


}



能不能详细说明以上两段代码的具体解释？大致的流程是怎么样的？

问题补充：
lianj_lee所说的这个权限判断是我写程序调用，还是它自动根据XML来调用验证是否登录？


还有

<filter>
<filter-name>AdminFilter</filter-name>
<filter-class>szy.shopping.filter.AdminFilter</filter-class>
<init-param>
<param-name>url</param-name>
<param-value>/admin/login.jsp</param-value>
</init-param>
</filter>

<filter-mapping>
<filter-name>AdminFilter</filter-name>
<url-pattern>/admin/index.jsp</url-pattern>
</filter-mapping>

可否在详细说明？谢谢～