為何我每次第一次執(zhí)行程序的話都會(huì)出現(xiàn)session中的保存驗(yàn)證碼取出來(lái)為空？
public?class?LoginServlet?extends?HttpServlet?{
public?void?doPost(HttpServletRequest?req,?HttpServletResponse?resp)
throws?ServletException,?IOException?{
doGet(req,?resp);
}
public?void?doGet(HttpServletRequest?req,?HttpServletResponse?resp)
throws?ServletException,?IOException?{
//后臺(tái)記錄的驗(yàn)證碼（均為大寫字母）
String?verifyPic?=?(String)?req.getSession().getAttribute("verifyPicture");
//前臺(tái)提交過(guò)來(lái)的驗(yàn)證碼（用戶不一定輸入大寫字母）
String?verifyCode?=?req.getParameter("verifyCode");
System.out.println("verifyPicture為："+verifyPic);
//前臺(tái)輸入的全部轉(zhuǎn)換為大寫字母
verifyCode?=?verifyCode.toUpperCase();
System.out.println("verifyCode為："+verifyCode);
resp.setContentType("text/html;charset=UTF-8");
PrintWriter?out?=?resp.getWriter();
if(verifyCode.equals(verifyPic)){
out.print("驗(yàn)證碼輸入正確！");
}else{
out.print("驗(yàn)證碼輸入錯(cuò)誤！");
}
out.flush();
out.close();
}
}public?class?ImageServlet?extends?HttpServlet?{
public?void?doGet(HttpServletRequest?req,?HttpServletResponse?resp)
throws?ServletException,?IOException?{
//1、定義一個(gè)BufferedImage對(duì)象
BufferedImage?bfi?=?new?BufferedImage(65,?23,?BufferedImage.TYPE_INT_RGB);
//2、獲取Graphics對(duì)象
Graphics?g?=?bfi.getGraphics();
Color?c?=?new?Color(180,200,255);//Color(int?r,?int?g,?int?b)?指定紅色、綠色和藍(lán)色值的不透明的?sRGB顏色,最后一個(gè)值默認(rèn)為255
g.setColor(c);
g.fillRect(0,?0,?65,?23);
//3、通過(guò)Random隨機(jī)產(chǎn)生驗(yàn)證碼信息
char[]?ch?=?"ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".toCharArray();
Random?random?=?new?Random();
int?len?=?ch.length;
int?index;//索引
StringBuffer?sb?=?new?StringBuffer();
for(int?i=0;i<4;i++){//生成4位驗(yàn)證碼
index?=?random.nextInt(len);//在ch數(shù)組中隨機(jī)字符
//4、使用Graphics繪制圖片
g.setColor(new?Color(random.nextInt(88),?random.nextInt(188),?random.nextInt(255)));
g.drawString(ch[index]+"",?(i*15)+3,?18);
sb.append(ch[index]);
}
//5、記錄驗(yàn)證碼信息到session中
req.getSession().setAttribute("verifyPicture",?sb.toString());
//6、使用ImageIO輸出圖片
ImageIO.write(bfi,?"JPG",?resp.getOutputStream());
}
}index.jsp頁(yè)面：
<%@?page?language="java"?import="java.util.*"?pageEncoding="UTF-8"%>
<%
String?path?=?request.getContextPath();
String?basePath?=?request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE?HTML?PUBLIC?"-//W3C//DTD?HTML?4.01?Transitional//EN">
<html>
??<head>
????<base?href="<%=basePath%>">
????
????<title>驗(yàn)證碼</title>
<meta?http-equiv="pragma"?content="no-cache">
<meta?http-equiv="cache-control"?content="no-cache">
<meta?http-equiv="expires"?content="0">????
<meta?http-equiv="keywords"?content="keyword1,keyword2,keyword3">
<meta?http-equiv="description"?content="This?is?my?page">
<!--
<link?rel="stylesheet"?type="text/css"?href="styles.css">
-->
<script?type="text/javascript">
function?reloadCode(){
var?time?=?new?Date().getTime();
document.getElementById("picCode").src?=?"<%=request.getContextPath()%>/myServlet/ImageServlet?d="+time;
}
</script>
??</head>
??
??<body>
??<form?action="<%=request.getContextPath()%>/myServlet/LoginServlet"?method="get">
??? ?驗(yàn)證碼：<input?type="text"?name="verifyCode">
????<img?alt="驗(yàn)證碼"?id="picCode"?src="<%=request.getContextPath()%>/myServlet/ImageServlet">
????<a?href="javascript:reloadCode();">看不清楚</a>?<br>
????<input?type="submit"?value="提交"> <input?type="reset"?value="重置">
??</form>
??</body>
</html>第一次執(zhí)行每次都有問(wèn)題，如果先刷新再填寫驗(yàn)證碼就沒(méi)問(wèn)題了，第一次獲取不到session中的值，為何？求大神指導(dǎo)！
2019-03-07
我也是 取不到session的值 都是null?
2016-06-24
我也是同樣的問(wèn)題，第一次總是空指針異常，刷新下就能正常運(yùn)行
2015-09-05
解決了吧
2015-06-25
我執(zhí)行了一下你的代碼，沒(méi)有你說(shuō)的這個(gè)問(wèn)題，一開(kāi)始就成功了