JAVA实现验证码的功能

<form action="LoginCheck" method="post">   <table>   <tr><td><input name="checkCode"  id="checkCode" size="8" ,maxlength="4"/></td></tr>   <tr><td><img onclick="this.src='PictureCheckCode?'+new Date()" src="PictureCheckCode" alt="点击刷新验证码" width="80" height="25"/></td></tr>   <tr><td align="center"><input type="submit" value="登录"></td></tr>   </table>        </form><form action="LoginCheck" method="post">   <table>