2004s file download (how to not store files in context?)

Question

Hi,
I want to use FileDownload element in my web dynpro application.
I use the new type resource  and my own input source implementation, which generates files on the fly.
Each download call should result in a new file. These files can be very large (some MB).
For obvious reasons I don't want the server to store the byte data in application context (200 users * 2 MB ....)
How can I do this?
If I use resource as context element and set debug points on my input stream, I can see the data is read just on time if the user wants to download.
I create my data in my inputstream and the download starts. So far so good.
On the second click on the download link the download starts without any read operation. Thats the problem.
Possible reasons:
First: the data is read and stored to the context on the first download. this would be bad! context would get very very large. not working for me.
second: the browser or the server caches the data. how can I avoid this?
thx in advance
btw I read a tutorial about up and downloading. this pointed to an additional article called something like "downloading in web dynpro tables". But I can't find it. There the on demand downloading should be explained. any help would be great.

Answer 1

Hallo Thomas,
ok, the "default" on-demand streaming solution caches a Web Resource but does not re-calculate it again afterwards.
I wrote a Wiki Code Tutorial which demonstrates how to download an Excel File from a Web Dynpro table UI element on-demand. In this scenario the resource must also be re-created on every request. The solution is based on re-initializing the context with a new  0-byte resource objects within the calculated context attribute getter method.
See <a href="https://wiki.sdn.sap.com/wiki/display/Snippets/ExportingTableDatainWebDynproJava-SAPNetWeaver04s">Exporting Table Data in Web Dynpro Java - SAP NetWeaver 04s</a>
For you the most important line of code in
//@@begin javadoc:getExcelResourceOnDemStreamCalc(IPrivateExcelExportComp.IExcelElement)
   *  Declared getter method for attribute ResourceOnDemStreamCalc of node ExcelResource
   *  @param element the element requested for the value
   *  @return the calculated value for attribute ResourceOnDemStreamCalc
  //@@end
  public com.sap.tc.webdynpro.progmodel.api.IWDInputStream getExcelResourceOnDemStreamCalc(IPrivateExcelExportComp.IExcelElement element)
    //@@begin getExcelResourceOnDemStreamCalc(IPrivateExcelExportComp.IExcelElement)
    IWDInputStream excelOnDemandStream = null;
    try {
      excelOnDemandStream =
        WDResourceFactory.createInputStream(toExcel(tableDataNode, tableColumnInfos).getBytes("UTF-8"));
    } catch (UnsupportedEncodingException e) {
      wdComponentAPI.getMessageManager().reportException(e.getLocalizedMessage(), true);
    // Reset context attribute 'Excel.Resource' again by creating a new 0-byte resource object.
    // Otherwise the on-demand Excel stream gets calculated only once as the resource object,
    // cached in the Web Dynpro Binary Cache on server-side, keeps the same.
    initializeOnDemandExcelResource();
    // Return excel on-demand input stream
    return excelOnDemandStream;
    //@@end
is the method <b>call initializedOnDemandExcelResource()</b> re-initializing the context attribute with a new 0-byte resource object.
//@@begin others
  //======= Private Methods ===========================
   * Create new on-demand resource instance for excel file. This method must be
   * invoked initially and in the calculated context attribute getter method (calculating)
   * the excel resource input stream. Otherwise the resource is streamed to the client only
   * once but not afterwards so that later updates of the table data are not reflected in the
   * streamed Excel file.
   * URL is stored in context so context in client component can map to it.
  private void initializeOnDemandExcelResource() {
    excel0ByteResource =
      WDResourceFactory.createResource(calcAttrPointer, "Table.xls", WDWebResourceType.XLS);
    excelResourceElement.setResource(excel0ByteResource);
    excelResourceElement.setResourceURL(
      excel0ByteResource.getUrl(WDFileDownloadBehaviour.OPEN_INPLACE.ordinal()));
Regards, Bertram