Filter 过滤器 详解

<filter>  
   <filter-name>SessionInvalidate</filter-name>    
   <filter-class>com.nonglin.action.SessionCheckFilter</filter-class> <!-- 过滤器核心类的class地址  -->
   <init-param>
            <param-name>charset</param-name> <!-- 定义编码格式 -->
            <param-value>UTF-8</param-value>
    </init-param>
    <init-param>
            <param-name>logonStrings</param-name><!-- 对登录页面不进行过滤 -->
            <param-value>login.jsp</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>SessionInvalidate</filter-name> 
    <url-pattern>*.jsp</url-pattern>   <!-- 过滤文件类型 -->
</filter-mapping>

SessionCheckFilter.java

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;  
/** 
 * 用于检测用户是否登陆的过滤器，如果未登录，则重定向到指的登录页面 配置参数 checkSessionKey 需检查的在 Session 中保存的关键字 
 * redirectURL 如果用户未登录，则重定向到指定的页面，URL不包括 ContextPath notCheckURLList 
 * 不做检查的URL列表，以分号分开，并且 URL 中不包括 ContextPath 
 */  
public class SessionCheckFilter implements Filter {  
	public FilterConfig config;

    /** *客户请求服务器时经过（相当于一个门卡）
     *  *@param request 客户请求 
     *  *@param response 服务器响应 
     *  *@param chain 滤镜链，通过 chain。doFilter(request,response)方法将请求传给下一个Filter或者Servlet 
     *  
     */
   
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    	HttpServletRequest req= (HttpServletRequest) request;
        HttpServletResponse resp= (HttpServletResponse) response;
        HttpSession session=req.getSession();  //获取serssion；
        String path= req.getRequestURI();	   //获取url链接；
        String username =(String) session.getAttribute("username");

        if(path.endsWith("login.jsp")){//登录页面不过滤  
            chain.doFilter(req, resp);  
        }else if(username!=null){//已经登录
        	chain.doFilter(request, response);
        	System.out.println("登录过");
        	return ;
        }
    }

    public void init(FilterConfig filterConfig) throws ServletException {
       
    }

	public void destroy() {
		// TODO Auto-generated method stub
		System.out.println("销毁...");  
	}
}