Getting rid of synchronous XHR - does it affect Vaadin?

Long time ago (in 2012) the browsers deprecated Synchronous XMLHttpRequest (XHR) support: Getting rid of Synchronous XHR. Most recently, Chrome 80 dropped Synchronous XHR in Page Dismissal handler altogether. Do/did these changes affect Vaadin?

The case of Vaadin 8

Vaadin 8 client-side is built with GWT and therefore uses XHR provided by GWT: either the class directly, or the RequestBuilder class (which wraps in a more friendly API). Looking closer, we can see that all of’s open() methods always pass true as the async parameter to JavaScript’s XMLHttpRequest. Therefore, anyone using those classes will always use XHR in async mode. And indeed, Vaadin’s GWT code uses RequestBuilder from its XhrConnection class and XMLHttpRequest in VDragAndDropWrapper and FileDropTargetConnector. So this part of Vaadin is safe.

What about native JavaScript code? Vaadin creates new XMLHttpRequest in vaadinBootstrap.js but then uses async mode - OK.

Atmosphere-JavaScript client is used for Push, which uses async (see atmosphere.js containing the line, url, true); which indicates async mode). However, Vaadin uses its own forked version of atmosphere.js named vaadinPush.js, which modifies the call as follows:, url, request.async);

By default, the request.async comes from _request.async (which is true - OK), however in function _disconnect() at line 461 it comes from _request.closeAync which is false. That’s the only place where Vaadin uses sync request, potentially failing in modern browsers.

This can cause problems when trying to reinitialize the session from Vaadin, for example to prevent the Session Fixation attack as seen at Vaadin bug #7526; the workaround is to use the following code (taken from Tatu’s DemoUtils):

public class DemoUtils {
	public static void sessionFixation() {
		// Chrome 80 does not support synchronous XHR during page dismissal anymore
		// thus Push needs to be disabled during session re-initialization

The case of Vaadin 14+

Vaadin’s Flow is compiled with GWT, and therefore is using the XMLHttpRequest class which is async. All of Vaadin’s XHR-related code (,, ultimately go through Vaadin’s Xhr helper class (which uses GWT’s XMLHttpRequest class), therefore this part is okay.

However, vaadinPush.js uses the same modified call as Vaadin 8 does:, url, request.async);

Therefore, the code suffers from the same issues related to session reinitialization as the Vaadin 8 code does.

Written on November 17, 2020