Servlet 页面跳转 并弹出提示对话框

<pre name="code" class="html">简单的登录验证模块 
点击登录进入相应的Servlet，Servlet调用bean验证用户名是否存在 
如果存在则登录成功，跳转 
如果不存在则弹出错误对话框，并回到登录页面 
开始用的下面两句话 
out.print("<script language='javascript'>alert('the name doesnot exit')</script>"); 
response.sendRedirect("Login.jsp"); 
但不会显示对话框，而是直接回到Login.jsp 
后来改用 
JOptionPane.showMessageDialog(null, "name doesnot exits"); 
response.sendRedirect("Login.jsp"); 
则可以实现。

	@Override 
	public void doGet(HttpServletRequest request, HttpServletResponse response) 
			throws ServletException, IOException { 
			// TODO Auto-generated method stub 
			request.setCharacterEncoding("utf-8");  
			response.setCharacterEncoding("utf-8");  
		    String username=request.getParameter("username"); 
		    String password=request.getParameter("password"); 
		    String code=request.getParameter("code"); 
		    //状态码（0：默认值（用户未登入成功），1:用户登入成功,2:验证码通过） 
		    String message="0"; 
		    boolean target=false; 
		    if(username.equals("")){ 
		    	username="0"; 
			} 
		    if(password.equals("")){ 
		    	password="0"; 
		    } 
		     
		    HttpSession sessions = request.getSession();		     
		    if(sessions.getAttribute("safecode")!=null ){ 
		    	target=true; 
		    	message="2"; 
		    } 
		     
		    if(target){ 
		    	 String sql="select * from userinfo info where info.`username`='"+username.trim()+"' and info.`password`='"+password.trim()+"'"; 
				    try { 
						ResultSet rusultset=DataBase.select(sql); 
						while(rusultset.next()){ 
							message="1"; 
							HttpSession session = request.getSession(); 
							session.setAttribute("username",username); 
						} 
						DataBase.closeAll(rusultset);				 
					} catch (Exception e) { 
						// TODO Auto-generated catch block 
						e.printStackTrace(); 
					} 
		    	 
		    } 
		     
		    
		<span style="color:#ff0000;">	 
		    response.setContentType("text/;html"); 
	        PrintWriter out=response.getWriter();	 
		    //登入信息返回 
		    if(message.equals("0")){//默认 
		    		JOptionPane.showMessageDialog(null, "用户名或密码不正确");	 
				  response.sendRedirect("./login.html");    
			  }else if(message.equals("1")){//成功 
				  response.sendRedirect("./message.html");   
			  }else if(message.equals("2")){//用户名已经存在 
				  JOptionPane.showMessageDialog(null, "注册码不正确");	 
				  response.sendRedirect("./login.html");   
			  }else{//特殊情况 
				  JOptionPane.showMessageDialog(null, "用户名或密码不正确");	 
				  response.sendRedirect("./login.html");    
			  } 
		     
		    out.flush(); 
	        out.close();</span> 
		     
		     
		 
	} 
 
	@Override 
	public void doPost(HttpServletRequest request, HttpServletResponse response) 
			throws ServletException, IOException { 
		// TODO Auto-generated method stub 
		this.doGet(request, response); 
		 
	}