Running Tomcat Behind Apache with mod_rewrite and mod_proxy

Key words first

Apache is the program that serves most web pages on port 80 and 443.
Tomcat is a program that runs Java web applications. It usually listens on port 8080.
mod_proxy is an Apache add-on that forwards requests to another program. Think of a receptionist who passes your call to the right person.
mod_rewrite is an Apache add-on that changes web addresses by rules.

Visitors then use a clean address like https://example.com/, without :8080 at the end.

Warning: Back up your Apache settings before you change them. A typing mistake can stop your sites.

Log in to your server by SSH as root.
Check that Tomcat is running and answers on port 8080.
```
curl -I http://127.0.0.1:8080/
```
This asks Tomcat for its page headers. A reply starting with HTTP means it works.
Make sure the proxy modules are on. On many systems:
```
a2enmod proxy proxy_http rewrite
```
This turns on the modules. On servers with WHM and EasyApache, install the modules from EasyApache 4 instead.
Open the settings (virtual host) file for your site.
Add these lines inside the virtual host block:
```
ProxyPreserveHost On
ProxyPass / http://127.0.0.1:8080/
ProxyPassReverse / http://127.0.0.1:8080/
```
ProxyPass sends requests to Tomcat. ProxyPassReverse fixes addresses in the replies.
To send only one path to Tomcat, use it like this:
```
ProxyPass /app http://127.0.0.1:8080/app
ProxyPassReverse /app http://127.0.0.1:8080/app
```
This sends only addresses starting with /app.
To use mod_rewrite instead, add:
```
RewriteEngine On
RewriteRule ^/(.*)$ http://127.0.0.1:8080/$1 [P,L]
```
The P flag means proxy. This sends all requests to Tomcat.
Test the settings.
```
apachectl configtest
```
It should say Syntax OK.
Reload Apache.
```
systemctl reload httpd
```
On some systems the service is named apache2.

Tip: Keep port 8080 closed to the outside world in your firewall. Then visitors can only reach Tomcat through Apache.