What is jetty-env.xml?
A jetty-env.xml file is an optional file that you can place in your webapp's WEB-INF directory to configure JNDI resources specifically for that webapp.
The syntax of the file is the standard jetty xml file syntax.
At startup, jetty will automatically look for a file of this name in the webapp's WEB-INF directory, and set up the webapp's naming environment so that naming references in the WEB-INF/web.xml file can be resolved from the information in the WEB-INF/jetty-env.xml file (along with any global naming resources defined in a jetty.xml startup file.
There is more information and examples in the section on JNDI.