/**
 * 顯示驗(yàn)證碼圖片
 */
public void showCheckCode(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
  // 調(diào)用業(yè)務(wù)邏輯
  String checkCode = getCheckCode();
  //將驗(yàn)證碼字符放入session域?qū)ο笾?
  req.getSession().setAttribute("checkCode", checkCode);

  //圖片寬
  int width = 80;
  //圖片高
  int height = 30;
  //在內(nèi)存中創(chuàng)建一個(gè)圖片
  BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
  //獲取一個(gè)畫筆
  Graphics g = image.getGraphics();
  //設(shè)置畫筆顏色，用灰色做背景
  g.setColor(Color.GRAY);
  //向Image中填充灰色
  g.fillRect(0,0,width,height);

  Random r = new Random();

  //設(shè)置3條干擾線
  for (int i = 0; i < 3; i++) {
    g.setColor(new Color(r.nextInt(255),r.nextInt(255),r.nextInt(255)));
    g.drawLine(r.nextInt(80), r.nextInt(30), r.nextInt(80), r.nextInt(80));
  }

  //設(shè)置驗(yàn)證碼字符串的顏色
  g.setColor(new Color(r.nextInt(255),r.nextInt(255),r.nextInt(255)));
  //設(shè)置字符的大小
  g.setFont(new Font("黑體",Font.BOLD,24));
  //在圖片中寫入驗(yàn)證碼字符串
  g.drawString(checkCode,15,20);
  //將Image對(duì)象以PNG格式輸出給所有的客戶端
  ImageIO.write(image,"PNG",resp.getOutputStream());
}

/**
 * 獲取4位驗(yàn)證碼中的4位隨機(jī)字符串
 */
public static String getCheckCode(){
  //驗(yàn)證碼中的字符由數(shù)字和大小寫字母組成
  String code = "0123456789qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM";
  Random r = new Random();
  StringBuffer sb = new StringBuffer();
  for (int i = 0; i < 4; i++) {
    sb.append(code.charAt(r.nextInt(code.length())));
  }

  return sb.toString();
}

<script type="text/javascript">
  function changeCodeImage(img){
    img.src = "${pageContext.request.contextPath}/UserServlet&#63;method=showCheckCode&time="+new Date().getTime();
  }

</script>

 <div class="form-group">
  <label for="date" class="col-sm-2 control-label">驗(yàn)證碼</label>
  <div class="col-sm-3">
   <input type="text" class="form-control" id="writeCode" onkeyup="checkCodeMethod(this.value)" >

  </div>
  <div class="col-sm-2">
  <img src="${pageContext.request.contextPath}/UserServlet&#63;method=showCheckCode" id="checkCodeImage" title="點(diǎn)擊換一張" onclick="changeCodeImage(this)" />
  </div>
  <span id="checkCodeSpan"></span>
 </div>

/**
 * 驗(yàn)證驗(yàn)證碼
 */
public void checkCode(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

  //獲取從頁面中接收到的驗(yàn)證碼參數(shù)
  String checkCode = req.getParameter("checkCode");
  //從session域?qū)ο笾蝎@取驗(yàn)證碼
  String sessionCode = (String) req.getSession().getAttribute("checkCode");
  //判斷驗(yàn)證碼是否相同
  if (checkCode.equalsIgnoreCase(sessionCode)) {
    resp.getWriter().print(true);
  }else {
    resp.getWriter().print(false);
  }

<script type="text/javascript">
  function changeCodeImage(img){
    img.src = "${pageContext.request.contextPath}/UserServlet&#63;method=showCheckCode&time="+new Date().getTime();
  }

  function checkCodeMethod(code){
    $.get("${pageContext.request.contextPath}/UserServlet&#63;method=checkCode", 
        { checkCode: code}, 
        function(data){
          if (data == 'true') {
            document.getElementById("checkCodeSpan").innerHTML = "<font>驗(yàn)證碼正確!</font>";
          }else {
            document.getElementById("checkCodeSpan").innerHTML = "<font>驗(yàn)證碼錯(cuò)誤!</font>";
          }
        }
      );
  }

</script>

 <div class="form-group">
  <label for="date" class="col-sm-2 control-label">驗(yàn)證碼</label>
  <div class="col-sm-3">
   <input type="text" class="form-control" id="writeCode" onkeyup="checkCodeMethod(this.value)" >

  </div>
  <div class="col-sm-2">
  <img src="${pageContext.request.contextPath}/UserServlet&#63;method=showCheckCode" id="checkCodeImage" title="點(diǎn)擊換一張" onclick="changeCodeImage(this)" />
  </div>
  <span id="checkCodeSpan"></span>
 </div>