（收藏品）利用Servlet过滤器实现权限的控制

可以在字符集过滤器的基础上再增加一个过滤器实现对用户权限的控制，即只有用户登录以后才能进行页面的访问，这样就实现了对用户访问权限的控制。
首先完成权限过滤器的代码。
package myproj.filter;
import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LoginFilter extends HttpServlet implements Filter {
    //如果没有登录，就返回到指定页面
    String backUrl;
    public void destroy(){
    }
    public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException
    {
        HttpServletRequest httpRequest=(HttpServletRequest)request;
        HttpServletResponse httpResponse=(HttpServletResponse)response;
       
        //获取Session对象
        HttpSession session = request.getSession();
       
        //如果没有登录就中断过滤器链，返回到登录页面
        if(session.getAttribute("user") == null)
        {
            response.sendRedirect(backUrl);
        }
        else
            chain.doFilter(request,response);   
    }
   
    public void init(FilterConfig filterConfig)hrows ServletException
    {
        //从过滤器的配置中获得初始化参数，如果没有就使用默认值
        if(filterConfig.getInitParamter("backurl")!=null)
        {
            backUrl = filterConfig.getInitParamter("backurl");
        }
        else
            backUrl = "index.jsp";
    }

然后在web.xml中增加过滤器配置和过滤器的关联配置
<!--过滤器配置-->
<filter>
    <filter-name>Encoder</filter-name>
    <filter-class>myproj.filter.SetCharacterEncodingFilter</filter-class>
    <!--初始化参数-->
    <init-param>
         <param-name>newcharset</param-name>
         <param-value>gb2312</param-value>
    </init-param>
</filter>
<filter>
    <filter-name>LoginFilter</filter-name>
    <filter-class>myproj.filter.LoginFilter</filter-class>
    <!--初始化参数-->
    <init-param>
         <param-name>backurl</param-name>
         <param-value>index.jsp</param-value>
    </init-param>
</filter>

<!--过滤器与URL关联-->
<filter-mapping>
    <filter-name>LoginFilter</filter-name>
    <!--如果没有登录而直接通过url直接访问testfilter.jsp将会返回登录页面-->
    <url-pattern>/testfilter.jsp</url-pattern>
</filter-mapping>
<filter-mapping>
    <filter-name>Encoder</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>