How to show nice error page when your Spring Boot web app is down

When your web application is starting, it's best to let your clients know that it's temporary and soon you will be up and runnning again:

To make it possible you must have Apache HTTPD up-and-running and it must be an intermediate between your web application and your client. This is best practice anyway. To make it happen, you need in /etc/apache2/sites-available a file named YOUR_APP_NAME.conf:

<VirtualHost *:80>
    ServerAdmin YOUR_EMAIL
    ServerName YOUR_APP_HOST_NAME.com
    ServerAlias *.YOUR_APP_HOST_NAME.com

    # where's customized error message
    DocumentRoot /var/www/html
    ProxyPreserveHost On
    # this URLs won't be served to your web application
    ProxyPass /error-500-YOUR_APP_NAME.html !
    ProxyPass /maintenance.jpg !
    # In case of 503 HTTP error show the error page
    ErrorDocument 503 /error-500-YOUR_APP_NAME.html

    # on which port your web application runs
    ProxyPass           /   http://localhost:8080/
    ProxyPassReverse    /   http://localhost:8080/
    ProxyPassReverseCookiePath  /    /
    ErrorLog /var/log/apache2/YOUR_APP_HOST_NAME.com-error.log
    LogLevel warn
</VirtualHost>

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