We will be focussing on learning about web services with HTTP as the transport protocol and will be deploying web services on web containers as well as application servers . So a decent knowledge about these are good to have.
You can learn more about these @ http://javajee.com/java-ee-web-components-part1-http-and-servlets.
Monitoring and seeing the actual requests and responses sent over wire will give a better understanding of the underlying protocol and message structure.
You can learn about such monitoring and analyzing tools @ http://javajee.com/monitoring-and-analyzing-tools.
Since we are developing web services using Java programming language, decent knowledge about Java is required.
You can start learning about Java @ http://javajee.com/beginning-java-book.
Knowledge of multithreading is good to have, to write good multithreaded web services.
You can learn more about multithreading @ http://javajee.com/java-multithreading-book.
Knowledge of collections is good to have, to write good model classes for web services.
You can learn more about collections @ http://javajee.com/generics-and-collections.