Execute backing bean action on load?

Question

I would like to build a results page for a report export page. This results page must display the status of the export and offer the download of this export.

The export is done in an action method. I can execute it via a commandButton but it must be executed automatically on load.

How can I accomplish this?

JSF:

<h:commandButton value="Download report" action="#{resultsView.downloadReport}"/>

Backing bean:

  public String downloadReport() {
    ...
    FileDownloadUtil.downloadContent(tmpReport, REPORT_FILENAME);
    // Stay on this page
    return null;
  }

Clarification: Is this feasible with a4j? I thought of a solution that an Ajax request triggers my downloadReport action and its request is the file download.

Answer 1

You can also solve this in JSF 2.0 using component system events, specifically the PreRenderViewEvent.

Just create a download view (/download.xhtml) that fires a download listener before render.

<?xml version="1.0" encoding="UTF-8"?>
<f:view
    xmlns="http://www.w3.org/1999/xhtml"
    xmlns:f="http://java.sun.com/jsf/core">
    <f:event type="preRenderView" listener="#{reportBean.download}"/>
</f:view>

Then, in your report bean (defined using JSR-299), you push the file and mark the response as complete.

public @Named @RequestScoped class ReportBean {

   public void download() throws Exception {
      FacesContext ctx = FacesContext.getCurrentInstance();
      pushFile(
           ctx.getExternalContext(),
           "/path/to/a/pdf/file.pdf",
           "file.pdf"
      ); 
      ctx.responseComplete();
   }

   private void pushFile(ExternalContext extCtx,
         String fileName, String displayName) throws IOException {
      File f = new File(fileName);
      int length = 0; 
      OutputStream os = extCtx.getResponseOutputStream();
      String mimetype = extCtx.getMimeType(fileName);

      extCtx.setResponseContentType(
         (mimetype != null) ? mimetype : "application/octet-stream");
      extCtx.setResponseContentLength((int) f.length());
      extCtx.setResponseHeader("Content-Disposition",
         "attachment; filename=\"" + displayName + "\"");

      // Stream to the requester.
      byte[] bbuf = new byte[1024];
      DataInputStream in = new DataInputStream(new FileInputStream(f));

      while ((in != null) && ((length = in.read(bbuf)) != -1)) {
         os.write(bbuf, 0, length);
      }  

      in.close();
   }
}

That's all there is to it!

You can either link to the download page (/download.jsf), or use a HTML meta tag to redirect to it on a splash page.