JSP验证码问题

杯具了。今天找这个错误找了一个多小时了。我服了。。还是找不出来。救救我吧。。。
当我点击更换验证码的时候没有效果。。刷新的话就可以显示新的验证码。
大家麻烦看一下代码。。。Java code:

public class CodeServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doPost(request, response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setHeader("pragma", "no-cache");
response.setHeader("cache-control", "no-cache");
response.setHeader("expires", "0");

String checkcode="";
char[] c=new char[62];
Random r=new Random();
for(int i=0;i<c.length;i++){
if(i<=9){
c[i]=(char)(i+48);
}
if(i>=10&&i<=35){
c[i]=(char)(i+'A'-10);
}
if(i>=36){
c[i]=(char)(i+'a'-36);
}
}
for(int i=0;i<5;i++){
checkcode+=c[r.nextInt(62)];
}
System.out.println(checkcode);

int width=120,height=40;
BufferedIm