过滤器以及Severlet 的实例

<body>

   <!--  登录页面 -->
     <form action="loginServlet" method = "post">
        <input name = "username"  placeholder = "请输入用户名.."  />
        <input name = "password"  placeholder = "请输入密码.." />
         <input type = "submit" value = "提交"  />
     </form>
          <%= request.getAttribute("msg") == null ? "": request.getAttribute("msg") %> 
              
        
</body>

index.jsp 登陆成功后进入的页面

<body>
     <h1>欢迎登陆</h1>

</body>

loginSeverlet

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	      String user =  request.getParameter("username");
	      String password =  request.getParameter("password");
	      if( "admin".equals(user)&&"321".equals(password)) {  //如果用户名和密码
	    	    request.getSession().setAttribute("currentuser", user);
	    	    response.sendRedirect("index.jsp");
	      } else {
	    	  request.setAttribute("msg", "error");
	    	  request.getRequestDispatcher("login.jsp").forward(request, response); //转发
	      }
	}

Sessionfilter

public class Sessionfilter implements Filter {
      private  String[] pages;


            @Override
	public void doFilter(ServletRequest req, ServletResponse resp , FilterChain chain)
			throws IOException, ServletException {  
		//ServletRequest 是serclet 里面的HttpServletRequest的父类
		
		   
	}

	@Override
	public void init(FilterConfig config) throws ServletException {
          String mis =  config.getInitParameter("misspages");   //从web.xml里面获取的 <param-name>	
             String [] ms = mis.split(";");    //把其转换成数组
	}

	public String[] getPages() {
		return pages;
	}

	public void setPages(String[] pages) {
		this.pages = pages;
	}

}