为什么从服务器上下载下来的文件 是空的？

这段代码 可以将文件下载下来 可是在解析 下载下拉的文件的时候 确报 下面的异常，将同路径下的 另一个 xml 文件放入解析的 路径文件里面就可以解析出来
[code="java"]07-02 07:22:24.068: W/System.err(11862): java.lang.IllegalArgumentException: InputStream cannot be null[/code]
[code="java"]
serviceVersion = MainActivity.serviceVersion;

        StrictMode.setThreadPolicy(new StrictMode.ThreadPolicy.Builder()
        .detectDiskReads()
        .detectDiskWrites()
        .detectNetwork()   // or .detectAll() for all detectable problems
        .penaltyLog()
        .build());
        StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder()
        .detectLeakedSqlLiteObjects()
        .detectLeakedClosableObjects()
        .penaltyLog()
        .penaltyDeath()
        .build());

        downloadpath = "http://www.kskjkf.com/version/update/"+1+".xml";

        //建立下载文件的保存路径
        String dirName= Environment.getExternalStorageDirectory()+"/update/";
        System.out.println("下载文件的保存路径"+dirName);
        File file = new File(dirName);
        if(file.exists()){
            file.mkdir();
        }
        //准备拼接新的文件名
        newFileName = "update.xml";
        newFileName = dirName+newFileName;
        System.out.println("新文件名字是："+newFileName);
        File f = new File(newFileName);
        if(f.exists()){
            System.out.println("如果存在就删除");
            f.delete(); 
        }
        try {
            URL url = new URL(downloadpath);
            URLConnection conn = url.openConnection();
            System.out.println("连接文件路径"+url);
            int contentLength = conn.getContentLength();

            System.out.println("文件长度===="+contentLength);
            InputStream is = conn.getInputStream();
            byte[] bs = new byte[1024];
            int len;
            OutputStream os = new FileOutputStream(newFileName);
            System.out.println("OS的长度"+os);
            while((len=is.read())!=-1){
                os.write(bs,0, len);
            }
            System.out.println("下载完毕。。。。。");
            Toast.makeText(UpdateActivity.this, "下载成功", Toast.LENGTH_LONG).show();

            os.close();
            is.close();
            new Thread(new ResolveUpdateXml()).start();
        } catch (Exception e) {
            e.printStackTrace();
        }
        //new Thread(new ResolveUpdateXml()).start();
    }
    //new Thread(new ResolveUpdateXml()).start();

[/code]