Question

From the server Code (in an HttpServlet) I’m throwing an exception if the file is too large:

 public void doPost(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {
 ...
 // Check if the blob has correct size, otherwise delete it
 final BlobInfo blobInfo = new BlobInfoFactory().loadBlobInfo(blobKey);
 long size = blobInfo.getSize();
 if(size > 0 && size <= BasicConstants.maxImageSize){
    res.sendRedirect("/download?blob-key=" + blobKey.getKeyString());
 } else { // size not allowed
    bs.delete(blobKey);
    throw new RuntimeException(BasicConstants.fileTooLarge);
 }

From the client code I’m missing to successfully catch the exception with this snippet:

try {
    uploadForm.submit(); // send file to BlobStore, where the doPost method is executed
} catch (Exception ex) {
    GWT.log(ex.toString());
}

However, from this other client code snippet I’m somehow detecting when the exception was thrown with an ugly workaround that I don’t trust at all:

uploadForm.addSubmitCompleteHandler(new FormPanel.SubmitCompleteHandler() {

    @Override
public void onSubmitComplete(SubmitCompleteEvent event) {
// This is what gets the result back - the content-type *must* be
// text-html
String imageUrl =event.getResults();

    // This ugly workaround apparently manages to detect when the server threw the exception
if (imageUrl.length() == 0) { // file is too large
  uploadFooter.setText(BasicConstants.fileTooLarge);
} else { // file was successfully uploaded
       ...
    }

The Development Mode view in Eclipse reports an error of type “uncaught exception”, which suggests that I’m really doing a bad job at detecting it.

Can anyone tell me how to properly catch the exception, and if the workaround I’m using makes any sense?

Thanks!

Answer 1

Your first attempt

try {
    uploadForm.submit(); // send file to BlobStore, where the doPost method is executed
} catch (Exception ex) {
    GWT.log(ex.toString());
}

doesn’t work, because submit() doesn’t wait until the browser receives the response (it’s an asynchronous call).

uploadForm.addSubmitCompleteHandler(new FormPanel.SubmitCompleteHandler() {

  @Override
  public void onSubmitComplete(SubmitCompleteEvent event) {
    ...

Here you actually receive the response from the server. But it’s a form submission, not a GWT-RPC call, so the result is just plain text, not GWT Java Objects.

When you throw a RuntimeException in a Servlet, the server will simply send a response with an error code (probably ‘500’, but ideally use the “Network” tab in Firebug or Chrome Developer Tools to see the actual response and response code.) So in the success case you’ll get the URL, otherwise the response is empty.

Possible solution

You can catch Exceptions on the server side, and send better descriptions explicitly:

public void doPost(HttpServletRequest req, HttpServletResponse res)
    throws ServletException, IOException {

  try {

      ...
      if (...) {
        throw new MyTooLargeException();
      } else {
          ...
        res.getWriter().write("ok " + ...);
      }

  } catch (MyTooLargeException e) {
     res.getWriter().write("upload_size_exceeded"); // just an example string 
                                                    // (use your own)

     res.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE);
  }
}

Then, on the client, check for

"upload_size_exceeded".equals(event.getResults()).