eZ Publish runs on PHP, it is one of the most popular web server platforms, the most known are PHP, Java, .Net, Coldfusion, Phyton and Ruby. '.jsp' is a Java technology, and can not easily be mixed with the other server platforms like PHP.
But, ezfind uses Solr, a Java based search engine, and it works like this:
ezfind is a bunch of PHP code to make handle all updates / indexing / searching in eZ Publish. But in the background these actions are sent using a http (alternatives in your case: Soap,Rest, Post or a simple Get) request to a Java server on the same machine (you can set it up on a different machine as well, but that would increase latency a bit) using a different port ( on ezfind its normally http://localhost:8984 ).
Edit: If your talking about .js (client side JavaScript) on the other hand, then you can easily mix the code with the (x)html in your templates.
There's a way, a tricky one, but a way to do that.
For one of my project, we put JSP code directly in templates between literal markups. We generated a static export to another plateform (Websphere portal) which will run those files and will serve it to the user. We just did that because our client would like to keep his Websphere license. Another point is that when mixing two technologies, you are presuming to face more bugs...