You are using the Exec plugin. You need to post the entire definition of that plugin from your pom.xml file (Maven - The Definitive Guide, page 54, doesn't show the Exec plugin being used). Apparently, you didn't set the classpath, or you used the wrong target (
you should be using exec:java, not exec:exec).
Also, "The Definitive Guide" has be superceeded by "The Complete Reference":
http://www.sonatype.com/books/mvnref-book/reference/