How it works...

A web application running under HTTP/1.0 sends a request to the server when it finds references for an image file, CSS file, and any other resources needed to render a web page.

With HTTP/2.0 you still can do it, but now you can do better: the server can now push the resources beforehand, avoiding unnecessary new requests, decreasing the server load, and improving performance.

In this recipe, our resources are represented by the following:

        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <link rel="stylesheet" type="text/css" href="resources/style.css">
        <script src="resources/functions.js"></script>

And the push happens at this part of our filter:

        HttpServletRequest httpReq = (HttpServletRequest)request;
        PushBuilder builder = httpReq.newPushBuilder();
        
        if (builder != null){
            builder
                .path("resources/javaee-logo.png")
                .path("resources/style.css")
                .path("resources/functions.js")
                .push(); 
            System.out.println("Resources pushed");
        }

So when the browser needs those resources to render the web page, they are already available.