Why to use Tomcat web server behind IIS?

It’s possible that many of the websites are “hosted in Tomcat” and yet “fronted by IIS” and vice-versa. 

The primary reason for this is that normally IIS can not execute Servlets and Java Server Pages (JSPs), configuring IIS to use the JK ISAPI redirector plugin will let IIS send servlet and JSP requests to Tomcat (and this way, serve them to clients).

Reference:

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p>The Apache Tomcat Connector – Webserver HowTo – IIS HowTo