public class Counter{
 private int count; 
 public Counter(){
  this(0);
  }
 public Counter(int count){
  this.count=count;
  }
 public void setCount(int count){
  this.count=count;
  }
 public int getCount(){
  return count;
  }
 public void add(int step){
  count+=step;
  }
 }

 /**
 * 统计页面访问的次数，并在关闭应用时将其保存到文件，待下次启应用时读取次数。
 * @author Joanna.Yan
 *
 */
public class MyServletContextListener implements ServletContextListener{
 public void contextInitialized(ServletContextEvent sce){
  System.out.println("====================helloapp application is Initialized.==========");
  ServletContext context=sce.getServletContext();
  try{
   BufferedReader reader = new BufferedReader(
     new InputStreamReader(context.getResourceAsStream("/count/count.txt")));
   int count = Integer.parseInt(reader.readLine());
   reader.close();
   Counter counter = new Counter(count);
   context.setAttribute("counter",counter);
  }catch(IOException e){e.printStackTrace();}
  }
 public void contextDestroyed(ServletContextEvent sce){
  System.out.println("helloapp application is Destroyed.");
  ServletContext context=sce.getServletContext();
  Counter counter=(Counter)context.getAttribute("counter");
  if(counter != null){
   try{
    String filepath = context.getRealPath("/count");
    filepath = filepath+"/count.txt";
    PrintWriter pw= new PrintWriter(filepath);
    pw.println(counter.getCount());
    pw.close();
   }catch(IOException e){e.printStackTrace();};
   }
  }
 }

<listener>
 <listener-class>joanna.yan.listener.MyServletContextListener</listener-class>
 </listener>

<servlet>
 <servlet-name>QRCodeServlet</servlet-name>
 <servlet-class>joanna.yan.servlet.QRCodeServlet</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>QRCodeServlet</servlet-name>
 <url-pattern>/QRCode</url-pattern>
 </servlet-mapping>

public class QRCodeServlet extends HttpServlet{
 @Override
 protected void doGet(HttpServletRequest req, HttpServletResponse resp)
   throws ServletException, IOException {
  doPost(req, resp);
 }
 @Override
 protected void doPost(HttpServletRequest req, HttpServletResponse resp)
   throws ServletException, IOException {
  System.out.println("QRCodeServlet被访问了！");
  ServletContext context=getServletContext();
  Counter counter=(Counter) context.getAttribute("counter");
  if(counter==null){
   counter=new Counter(1);
   context.setAttribute("counter", counter);
  }
  counter.add(1);
  System.out.println("被扫描的次数："+counter.getCount());
  resp.sendRedirect(""+req.getContextPath()+"/apkdownload.jsp");
 }
}

<style type="text/css">
  #weixin-tip{display:none;position:fixed;left:0;top:0;background:rgba(0,0,0,0.8);filter:alpha(opacity=80);width:100%;height:100%;z-index:100;}
  #weixin-tip p{text-align:center;margin-top:10%;padding:0 5%;position:relative;}
  #weixin-tip .close{color:#fff;padding:5px;font:bold 20px/24px simsun;text-shadow:0 1px 0 #ddd;position:absolute;top:0;left:5%;}
</style>
 <script type="text/javascript">
  var is_weixin = (function(){return navigator.userAgent.toLowerCase().indexOf('micromessenger') !== -1;})();
  window.onload = function() {
  var winHeight = typeof window.innerHeight != 'undefined' ? window.innerHeight : document.documentElement.clientHeight; //兼容IOS，不需要的可以去掉
  var btn = document.getElementById('J_weixin');
  var tip = document.getElementById('weixin-tip');
  var close = document.getElementById('close');
  if (is_weixin) {
   btn.onclick = function(e) {
    tip.style.height = winHeight + 'px'; //兼容IOS弹窗整屏
    tip.style.display = 'block';
    return false;
   };
   close.onclick = function() {
    tip.style.display = 'none';
   };
  }
 }; 
 </script>

....你的网页代码......
 <div id="weixin-tip">
   <p>
   <img alt="微信打开" src="img/warn.png">
   <span id="close" title="关闭" class="close">X</span>
   </p>
 </div>