Within the Maven build the beans are generated using an Ant task. This is all automatic.
However, if you wish to build by hand, see the scomp utility available with the XMLBeans distribution.
Here is an example of a scomp command:
scomp -nopvr -d ../target/classes -src ../target/src/java -out ../target/lib/adqlxmlbeans.jar ../contracts/schema/adql ../config/adql.xsdconfig
Both the Ant task and the scomp utility require a schema and a configuration file.