Hawk Chen

400 bad request response status code indicates that the server cannot or will not process the request due to something that is perceived to be a client error (for example, malformed request syntax, invalid request message framing, or deceptive request routing).

It could be a browser problem. Could you try another browser? is there any server-side or client-side error? Please open browser developer tool > Network tab to see the request and response.
Please create an issue at https://github.com/keikai/dev-ref/issues and attach related technical details.

I use
* IntelliJ IDEA 2021.1.3 (community edition)

* java 11

* use maven jetty plugin or tomcat7 plugin configured in the project's pom.xml

* Maven 3.6.3

I suppose you use spring-web higher than 3.1.4.RELEASE. We might need to update AssetsBean for the newer spring framework. If you don't need that spring bean example, you can remove AssetsBean first or use spring 3.1.4. We will upgrade that spring example later.

Only when you enable polling server push (it's disabled by default), ZK sends requests periodically to get an update from a server. You can choose comet server push so that ZK just sends one long-live request pending for a response. If you don't enable server push, then there is no such request sent.

ZK optimizes the event sending to a server, so it doesn't always send a request for every user action. Like Filip said, ZK doesn't send an event if there is no corresponding server listener. For some unimportant events, ZK will queue events at the client-side and send them once with an important event.

I'm glad that you like the article.