Create a file

Question

Hi,
I just would like to create a file, which the user can download.
Scenario:
The user choose some options on the JSF-page. Then a database query will execute. The user should be able to download that data of the query as a .csv file.
So how should I handle that scenario?
The created file should only exists for the user-request. So how and when should I delete these temporary files?
It would be great if someone could help me!

Answer 1

So I've tested. Unfortunately it doesn't work :-/
I've just received a NullPointerException. Something with the "ResponseWriter" is wrong. I can't write anything in that writer.
After the NullPointerException I received the "test.csv "-File. But the content is just the whole page (where the CommandLink was pressed) with all HTML-Tags.
May someone could help me again please?
My implementation:
In the JSP-page i've got a commandLink with the action:
<h:form>
    <h:commandLink value="download" action="#{bean.actionDownload}" />
</h:form>In the bean i've got the methode actionDownload:
public String actionDownload()
    FacesContext ctx = FacesContext.getCurrentInstance();
    if( !ctx.getResponseComplete() )
      try
        String strMIME = "text/plain";
        HttpServletResponse response = (HttpServletResponse) ctx.getExternalContext().getResponse();
        response.setContentType( strMIME );
        response.setHeader( "Content-Disposition", "attachment;filename=\"" + "test.csv" + "\"" );
        ResponseWriter writer = ctx.getResponseWriter();
        writer.writeText( "test", null );  // <-- java.lang.NullPointerException
        writer.flush();
        ctx.responseComplete();
      catch (IOException e)
        System.out.println("IO-Fehler bei createCSV...");
        e.printStackTrace();
      catch (Exception e)
        System.out.println("Allg. Fehler bei createCSV...");
        e.printStackTrace();
    return null;
}