Continuous delivery must not affect user experience

Continuous delivery must not affect user experience. What to do then to support restarting a JEE application? Think about the Front End. Have a rich web front end tier and present the user a message like you have seen before in sites like Gmail:
The above is of course related to the lack of connection but you can easily inform the user that you are retrying for other issues as well. See below a different message Gmail sends back to the user when for example I change my /etc/hosts to point mail.google.com to yahoo.com ;-)

This creates the opportunity to restart the server and catch any 5XX HTTP Errors, then retry until the backend server is back. Needless to say it is expected that your backend server comes back sooner than later.