直接上代码，注意引入js

<fieldset> <legend>用户注册</legend> <form method="post" name="form" action="user.do?method=register" onSubmit="return beforeSubmit(this);"> <table border="1" width="100%" cellspacing="0" cellpadding="0"> <tr><td><label>用户名：<input type="text" name="username" value=""></label></td></tr> <tr><td><label>密 码：<input type="password" name="password" value=""></label></td></tr> <tr><td><label>重复密码：<input type="password" name="password2" value=""></label></td></tr> <tr><td><input value="注册" type="submit"> <input type="reset" value="重置"></td></tr> </table> </form></fieldset><script type="text/javascript">function beforeSubmit(form){if(form.username.value==''){alert('用户名不能为空！');form.username.focus();return false;}if(form.password.value==''){alert('密码不能为空！');form.password.focus();return false;}if(form.password.value.length<6){alert('密码至少为6位，请重新输入！');form.password.focus();return false;}if(form.password.value!=form.password2.value) {alert('你两次输入的密码不一致，请重新输入！');form.password2.focus();return false;}return true;}</script>