File upload with a progress bar, or accessing the Html 5 File Api from Google Web Toolkit?

I'm looking for a way to do file uploads, with a custom progress bar, with google web toolkit. Not looking for a plugin which shows its own progress bar, I'm rather looking for something which will call my callback method and notify it of progress, so I can show my own progress bar.

If that's not possible, then I'd like to know how to access the HTML5 File API so I can build my own file uploader widget.

Any ideas?



There are many issues in GWT which makes difficult deal with this:

  • There is no FormData object in GWT, so you have to use JSNI to get one.
  • RequestBuilder does not support sending FormaData but only strings.
  • RequestBuilder does not support upload or download progress notifications.
  • FileUpload does not support the multiple attribute.
  • The Elemental package in GWT only works with webkit browsers, and I had some issues last time I tried to use the File Api.

Fortunately, gwtquery 1.0.0 has a bunch of features which facilitates to solve the problem.

Here you have a working example (with very few lines of code), which works in all browsers supporting the HTML5 file api:

import static*;

final FileUpload fileUpload = new FileUpload();

  // FileUpload does not have a way to set the multiple attribute,
  // we use gQuery instead
  .prop("multiple", true)
  // We use gQuery events mechanism
  .on("change", new Function() {
    public void f() {
      // Use gQuery utils to create a FormData object instead of JSNI
      JavaScriptObject form = JsUtils.runJavascriptFunction(window, "eval", "new FormData()");
      // Get an array with the files selected
      JsArray<JavaScriptObject> files =  $(fileUpload).prop("files");
      // Add each file to the FormData
      for (int i = 0, l = files.length(); i < l; i++) {
        JsUtils.runJavascriptFunction(form, "append", "file-" + i, files.get(i));

      // Use gQuery ajax instead of RequestBuilder because it 
      // supports FormData and progress
        // gQuery ajax returns a promise, making the code more declarative
        .progress(new Function() {
          public void f() {
            double total = arguments(0);
            double done = arguments(1);
            double percent = arguments(2);
            // Here you can update your progress bar
            console.log("Uploaded " + done + "/" + total + " (" + percent + "%)");
        .done(new Function() {
          public void f() {
            console.log("Files uploaded, server response is: " + arguments(0));
        .fail(new Function() {
          public void f() {
            console.log("Something went wrong: " + arguments(0));

Another option is to use gwtupload, which supports any browser, and it comes with some nice widgets and progress bar, even you can plug your own progress and status widgets.

It does not use HTML5 File api yet but an ajax solution based on a server listener. It will though, support html5 in future versions, falling back to ajax mechanism for old browsers. When gwtupload supports this, you wont have to modify anything in your code.

All the building blocks are in Elemental but might not work everywhere (Elemental is "close to the metal", without any support detection or hiding/working around browser bugs/discrepancies).

Or you can use JSNI.

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