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Once you've configured CAS, you can run "mvn package" to generate the WAR file.
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WAR Overlay
Deployers do concern concerned about the management of local customizations , upon CAS 3.1.1, Maven 2 has been employed as the major management tool in the project. It is suggested Maven can also offer a help in maintaining local customizations. Here is a page for the detailscan use the Maven2 WAR Overlay method for keeping their local changes separate from the main distribution. Details on that can be found here.