You can add new Java services to the service layer. The Java services you add can perform any of the functions of a typical Java Web service.
The Java services you add to the Java service layer are used to retrieve data from the vSphere environment and deliver the data to the user interface layer, or to make changes to the vSphere environment in response to actions in the user interface layer.
Service plug-in modules that gather data from the vSphere environment usually extend the native services on the vSphere Client application server, such as the Data Service. You can extend the built-in data service through the Java API from the vSphere Client SDK or create standalone custom Java services for data gathering.
Service plug-in modules that make changes to the vSphere environment are standalone Java services that you create. These services are used when the user starts an action in the vSphere Client user interfaces. If you create an action extension, you must also create the Java service that performs the action operation on the vSphere environment as a service plug-in module.