The web app communicates with the server side by using GWT-RPC (like what Peter said above) or many of us are using REST on the backend (Spring Boot ...).
Of course you can also just deploy the JS + HTML + resources in Spring Boot "static" directory if you don't want to make an extra web server.
If you have any special idea with Azure, just tell me, I could try to make a simple example ;-)
Question: Can we access AzureFunction with REST API? Or how do you access AzureFunction?