form提交后台上传文件,为什么得不到文件内容?该怎么处理

form提交后台上传文件,为什么得不到文件内容??
版主、各位大侠,小弟在此求教了!

我为了实现客户端导入功能,写了一个jsp页面用来form提交要导入的文件到服务器端,但在后台取读取不到文件的内容。请赐教!!!!
jsp代码如下:

HTML code

<%@ page language="java" import="java.util.*" pageEncoding="GBK"%>
<%
    String path = request.getContextPath();
    String basePath = request.getScheme() + "://"
            + request.getServerName() + ":" + request.getServerPort()
            + path + "/";
    String contextPath = request.getContextPath();
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
    <head>
        <base href="<%=basePath%>">

        <title>文件上传</title>

        <script language="javascript"
            src="<%=basePath%>/pms/platform/pms.js"></script>
        <script type="text/javascript"
            src="<%=basePath%>common/ext/ext-base.js"></script>
        <script type="text/javascript"
            src="<%=basePath%>common/ext/ext-all.js"></script>
        <script type="text/javascript"
            src="<%=basePath%>common/ext/ext-lang-zh_CN.js"></script>
        <link rel="stylesheet" type="text/css"
            href="<%=basePath%>common/ext/resources/css/ext-all.css" />
        <script language="javascript"
            src="<%=basePath%>/common/fileUpload/fileUploadAction.js"></script>
        <meta http-equiv="pragma" content="no-cache">
        <meta http-equiv="cache-control" content="no-cache">
        <meta http-equiv="expires" content="0">
        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
        <meta http-equiv="description" content="This is my page">
        <!--
    <link rel="stylesheet" type="text/css" href="styles.css">
    -->
        <style type="text/css">
        #jtxt{
            color:#666666;
            font-size:12px;
        }
    </style>

        <script type="text/javascript">
    var pbar1;    //定义一个变量用于进度条对象
    var btn1;    //定义一个变量用于进按钮对象
    
    Ext.onReady(function(){
        pbar1=new Ext.ProgressBar({    //实例化进度条
            renderTo:"div1",        //进度条呈现的一个DIV
            width:350,                //进度条的宽度
            text:"上传进度..."    //在进度条里的初始文本
        });
        //btn1=Ext.get("btnUpload");        //按钮对象
        //btn1.on("click",isnull);        //按钮事件,单击时调用f1()函数
        //隐藏进度条
        document.getElementById("div1").style.display='none';
    });
    var items = ["file","btnUpload","btnReset"];
    function isnull(){
        var file = document.getElementById("file").value;
        if(file==null || file==""){
            alert("请选择上传的文件!");
            return ;
        }
        document.getElementById("div1").style.display='';
        f1();
    }
    function f1(){
        //实际文件没有上传完成效果
        if(document.getElementById("hd").value==96){
            document.getElementById("hd").value=95;
        }
        var i = document.getElementById("hd").value++;//取得当前进度和设置进度
        
        pbar1.updateProgress(i/100.0); //让进度条更新到指定的进度。这个值是0-1之间的数,我让i/100.0,以让它的结果是一个小数
        pbar1.updateText("进度:"+i+"%");    //进度条显示的文本
        //当i==100的时候让按钮状态可以使用,并让i=1,再让进度条停止    
        if(i==100){
            //设置相应元件可编辑
            _setDisabled(items,false);
            //设置初始进度为0
            document.getElementById("hd").value=0;
            try{
                clearInterval(x);
            }catch(e){
                return ;
            }
        }
        //设置相应元件状态为不可用
        _setDisabled(items,true);
        setTimeout("f1()",1);//用setTimeout函数递归调用f1()函数,以达到进度条一直跑的效果
    }
    /**
    *用于设置一些元件是否可编辑
    */
    function _setDisabled(items,isTrue){
        for(var i=0;i<items.length;i++){
            document.getElementById(items[i]).disabled=isTrue;
        }
    }
    
    /* 
    *动态结束进度条,使其完成度为100%
    */
    _closeProgress = function ()
    {
        document.getElementById("hd").value=99;
        //f1();
    }
    //隐藏进度条
    _displayNone = function ()
    {
        document.getElementById("div1").style.display='none';
        document.getElementById("file").value = "";
    }
    
    /*
     * 导入操作
     */
    function OperExport()
    {
        if (_isExporting)
        {
            alert("正在导入过程中,请稍候再操作...");
            return;
        }
        
        var flowid = GetUrlQuery("flowid");
        var patternid = GetUrlQuery("patternid");
        var formworkname = GetUrlQuery("formworkname");
        var startrownum = GetUrlQuery("startrownum");
        var flag = GetUrlQuery("flag");
        
        var list = parent.defaultForm.getCellsByType("EditGrid")[0];
        var fileObj = document.getElementById("file");
        var file = fileObj.value;
        if(file != "")
        {
            var option = confirm("确定导入文件 " + file + " ?");
            if(option)
            {debugger;
                //file = file.replace("\", "\\");
                isnull();//启动进度条                
                mainform = document.getElementById("myform");
                  mainform.action = parent.$appRoot+ "/servlet/com.sdjxd.pms.platform.serviceBreak.Invoke?_sender=page&_c=zyws.TJGLMethod&_m=impExcelData&_p0=\""+flowid+"\"&_p1=\""+patternid+"\"&_p2=\""+formworkname+"\"&_p3=\""+startrownum+"\"&_p4="+JSON.stringify(parent._fileExParam)+"&_p5=\""+file+"\"&_p6=\""+flag+"\"";
                mainform.submit();
                _isExporting = true;
            }
        }
        else
        {
            alert("请先选择文件!");
        }
    }
    
    /*
     * 重置文件选择内容
     */
    resetFileObj = function ()
    {
        var file = document.getElementById("file");
        file.value = "";
    }
    //控制上传文件格式
    var fileFormat = function(){
        var file = document.getElementById("file").value;
        if(file==null || file==""){
            return ;
        }
        //截取扩展名并转换为小写
        var str = file.substring((file.length-4),file.length).toLowerCase();
        if(str==null || str != ".xls" && str != "xlsx"){
            document.getElementById("myform").reset();
            alert("请上传正确的电子表格文件!");
            return ;
        }
    }
    /* 
     * 回调函数
     */
    var impResultBack = function (result, sMsg)
    {
        _isExporting = false;
        _closeProgress();//结束进度条
        setTimeout("sleep('"+sMsg+"')",1000);
    }
    //暂停1秒执行函数,使上传进度效果更好
    var sleep = function(sMsg){
        var list = parent.defaultForm.getCellsByType("EditGrid")[0];
        alert(sMsg)
           _displayNone();//隐藏进度条
           if (parent._fileUploadWin)
            parent._fileUploadWin.hide();
        list.refresh();
    }
    </script>
    </head>

    <body
        style="font-size:12px; width:100%; margin:0px; border-width:0px; overflow:auto; text-align:center; ; background-color:#E4EEEF;">
        <br>
        <input id="hd" type="hidden" value="0">
        <div id="div1"></div>
        <iframe name="upload_iframe" id="upload_iframe" style="width: 400px; height: 100px;display:none" src="about:blank"></iframe>
        <form id="myform" name="myform" target="upload_iframe" enctype="multipart/form-data"
            action="" method="post">
            <p>
                &nbsp;
            </p>
            <table width="413">
                <tr>
                    <td width="60" align="right"
                        style="word-break:keep-all;white-space:nowrap;">
                        <span id="jtxt">文件名:&nbsp;</span>
                    </td>
                    <td width="353" align="left">
                        <input name="file" type="file" id="file"
                            style="border:1px solid #92C1ED; background-color:#FFF;"
                            onkeydown="return false;" size="40" height="39px"
                            onpaste="return false;" onchange="fileFormat();"/>
                    </td>
                </tr>
                <tr>
                    <td width="60">
                        &nbsp;
                    </td>
                    <td width="353">
                        &nbsp;
                    </td>
                </tr>
                <tr>
                    <td width="60">
                        &nbsp;
                    </td>
                    <td align="right" width="353">
                        <input id="btnUpload" type="button"
                            onclick="OperExport();"
                            style="border:1px solid #92C1ED;" value="上&nbsp;传" />
                        <input id="btnReset" type="reset" onclick="resetFileObj();"
                            style="border:1px solid #92C1ED;" value="重&nbsp;置" />
                        &nbsp;&nbsp;
                    </td>
                </tr>
            </table>
        </form>
    </body>
</html>