在html头中加入以下3句话，可以防止大部分浏览器缓存
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="Cache-Control" content="no-cache, must-revalidate">
<meta http-equiv="expires" content="Wed, 26 Feb 1997 08:21:57 GMT">
<meta http-equiv="Cache-Control" content="no-cache, must-revalidate">
<meta http-equiv="expires" content="Wed, 26 Feb 1997 08:21:57 GMT">
或者js中加入类似下面的代码，让每次的链接都不一样
var s=Math.random()*10;
????document.getElementById("checkCodeImg").innerHTML="<img src='/checkcode.jsp?rnd="+s+"'/>";
????document.getElementById("checkCodeImg").innerHTML="<img src='/checkcode.jsp?rnd="+s+"'/>";