jsp 中的 问题

jsp 中如何拿到 title 、cont 的值并判断是否为空，若空，如何弹出提示框为空

<form name="reply" action="ReplyOK.jsp" method="post" >
    <input type="hidden" name="id" value="<%=id %>">
    <input type="hidden" name="rootid" value="<%=rootId %>">
   
    <table  Border="1">
        <tr>
            <td>
                <input type="text" name="title"  size="80" >        
            </td>
        </tr>
        <tr>
            <td>
                <textarea cols="80" rows="12" name="cont"></textarea>
            </td>
        </tr>
        <tr>
            <td>
            
                <input type="submit" value="提交">
               
            </td>
        </tr>
    </table>
</form>
----------------解决方案--------------------------------------------------------
用Javascript可以吗？
----------------解决方案--------------------------------------------------------
我学着用了，也不行啊，不知道为什么，我提交的时候它就没有反应啊
<form name="reply" action="ReplyOK.jsp" method="post" onsubmit="return check()">
    <input type="hidden" name="id" value="<%=id %>">
    <input type="hidden" name="rootid" value="<%=rootId %>">
   
    <table  Border="1">
        <tr>
            <td>
                <input type="text" name="title"  size="80" >        
            </td>
        </tr>
        <tr>
            <td>
                <textarea cols="80" rows="12" name="cont"></textarea>
            </td>
        </tr>
        <tr>
            <td>
            
                <input type="submit" value="提交">
               
            </td>
        </tr>
    </table>
</form>

<script language="javascript">
<!--
    //javascript去掉空格函数
    function Ltrim(str) {  //去掉字符串左边的空格
        var i;
        for(i=0; i<str.length; i++) {
            if(str.chatAt(i) != " ") break;
        }
        str = str.substring(i, str.length);
        return str;
    }
   
    function Rtrim(str) {  //去掉字符串右边的空格
        var i;
        for(i=str.length; i>=0; i--) {
            if(str.chatAt(i) != " ") break;
        }
        str = str.substring(0, i + 1);
        return str;
    }
   
    function Trim(str) {
        return LTrim(RTrim(str));
    }
   
    function check() {
        if(Trim(document.r.reply.title.value) == "") {
            alert("Please input the title!");
            document.reply.title.focus();
            return false;
        }
        if(Trim(document.reply.cont.value) == "") {
            alert("Please input the title!");
            document.reply.cont.focus();
            return false;
        }
        
        return true;
    }
-->
</script>

----------------解决方案--------------------------------------------------------
不用这么复杂
<script language="javascript">
function check() {
        if(reply.title.value== "") {
            alert("Please input the title!");
            reply.title.focus();
            return false;
        }
        if(reply.cont.value == "") {
            alert("Please input the title!");
            reply.cont.focus();
            return false;
        }        
        return true;
}
</script>

----------------解决方案--------------------------------------------------------