how do i get apache to serve a web application as the root document?

One way is to force your webapp to be at the root context. In WEB-INF/jboss-web.xml you can put:

<?xml version="1.0" encoding="UTF-8"?>
<jboss-web version="10.0"
           xmlns="http://www.jboss.com/xml/ns/javaee"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://www.jboss.com/xml/ns/javaee http://www.jboss.org/j2ee/schema/jboss-web_10_0.xsd">

    <context-root>/</context-root>
</jboss-web>

This means that your webapp is now at http://example.com:8080/, not at /abc as you’re showing so your Apache config will be:

ProxyPass        / http://example.com:8080/ 
ProxyPassReverse / http://example.com:8080/