INTERACTIVE WEB SERVICES WITH JAVA

Listeners

- event handlers

Events:

servlet context events (ServletContextListener):
- initialize
- destroy
servlet context attribute events (ServletContextAttributeListener):
- set
- add
- remove
session events (HttpSessionListener):
- create
- invalidate
- time out
session attribute events (HttpSessionAttributeListener):
- set
- add
- remove

Implement the appropriate interface, register your listener (in web.xml).

Example listener and deployment declaration:
public class DataListener implements ServletContextAttributeListener { public void attributeReplaced(ServletContextAttributeEvent event) { if (event.getName().equals("some_data")) updateSomeOtherData(); } ... }

<listener> <listener-class>DataListener</listener-class> </listener>

<listener> <listener-class>DataListener</listener-class> </listener>

Useful for

implementing dependencies between data (but only if data is modified explicitly via setAttribute - and not via some methods/fields in the data)
monitoring the running service