GWT: form post works only on the local server, not with the app engine - google-app-engine

Question

i have a form with a FormPanel, a FileUpload and a Button
final FormPanel formPanel = new FormPanel();
formPanel.setAction("uploadServlet");
formPanel.setMethod(FormPanel.METHOD_POST);
formPanel.setEncoding(FormPanel.ENCODING_MULTIPART);
formPanel.setSize("100%", "100%");
setWidget(formPanel);
AbsolutePanel absolutePanel = new AbsolutePanel();
formPanel.setWidget(absolutePanel);
absolutePanel.setSize("249px", "70px");
final FileUpload fileUpload = new FileUpload();
fileUpload.setName("uploadFormElement");
absolutePanel.add(fileUpload, 0, 0);
Button btnOpen = new Button("Open");
absolutePanel.add(btnOpen, 10, 30);
Button btnCancel = new Button("Cancel");
absolutePanel.add(btnCancel, 63, 30);
this.setText("Open...");
this.setTitle(this.getText());
this.setAnimationEnabled(true);
this.setGlassEnabled(true);
btnOpen.addClickHandler(new ClickHandler() {
public void onClick(ClickEvent event) {
formPanel.submit();
}
});
the servlet gets called but the request contains a error message "error post".
When i try it on the local server it works, the request contains the file, but on the app engine server only the error
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
FileItemFactory factory = new DiskFileItemFactory();
ServletFileUpload upload = new ServletFileUpload(factory);
List<?> items = null;
String json = null;
try {
items = upload.parseRequest(request);
}
catch (FileUploadException e) {
e.printStackTrace();
}
Iterator<?> it = items.iterator();
while (it.hasNext()) {
System.out.println("while (it.hasNext()) {");
FileItem item = (FileItem) it.next();
json = item.getString();
}
response.setContentType("text/html");
ServletOutputStream out = response.getOutputStream();
response.setContentLength(json.length());
out.write(json.getBytes());
out.close();
}

Answer 1

DiskFileItemFactory is the default implementation for the commons-fileupload library, and based in it's javadoc:
This implementation creates FileItem instances which keep their content either in memory, for smaller items, or in a temporary file on disk, for larger items. The size threshold, above which content will be stored on disk, is configurable, as is the directory in which temporary files will be created.
If not otherwise configured, the default configuration values are as follows:
Size threshold is 10KB. Repository is the system default temp directory, as returned by System.getProperty("java.io.tmpdir").
So as you see, this implementation is going to write in filesystem when it does not have enough memory.
In GAE, there are many constrains, like the memory you are allow to use, or the prohibition of writing in the filesystem.
Your code should fail in GAE developing mode, but maybe you have not reached the memory limitation, or whatever since GAE dev tries to emulate the same constrains than production server, but it is not identical.
Said, that I could take a look to gwtupload library, they have a servlet for GAE which can save files in different ways: BlobStore, FileApi and MemCache.