None of both is the better approach. Servlets are intended to listen on HTTP events (HTTP requests), not on deployment events (startup/shutdown).
@WebListener
public class Config implements ServletContextListener {
public void contextInitialized(ServletContextEvent event) {
// Do stuff during webapp's startup.
}
public void contextDestroyed(ServletContextEvent event) {
// Do stuff during webapp's shutdown.
}
}
If you're not on Servlet 3.0 yet and can't upgrade, and thus can't use @WebListener annotation, then you need to manually register it in /WEB-INF/web.xml like below:
<listener>
<listener-class>com.example.Config</listener-class>
</listener>
To store and obtain objects in the application scope (so that all servlets can access them), use ServletContext#setAttribute() and #getAttribute().
Here's an example which lets the listener store itself in the application scope:
public void contextInitialized(ServletContextEvent event) {
event.getServletContext().setAttribute("config", this);
// ...
}
and then obtain it in a servlet:
protected void doGet(HttpServletRequest request, HttpServletResponse response) {
Config config = (Config) getServletContext().getAttribute("config");
// ...
}
It's also available in JSP EL by ${config}. So you could make it a simple bean as well.
import javax.enterprise.context.ApplicationScoped;
@ApplicationScoped
public class Config {
public void init(@Observes @Initialized(ApplicationScoped.class) ServletContext context) {
// Do stuff during webapp's startup.
}
public void destroy(@Observes @Destroyed(ApplicationScoped.class) ServletContext context) {
// Do stuff during webapp's shutdown.
}
}
This is available in a servlet via @Inject. Make it if necessary also @Named so it's available via #{config} in EL as well.
Noted should be that this is new since CDI 1.1. If you're still on CDI 1.0 and can't upgrade, then pick another approach.
import javax.faces.bean.ManagedBean
import javax.faces.bean.ApplicationScoped;
@ManagedBean(eager=true)
@ApplicationScoped
public class Config {
@PostConstruct
public void init() {
// Do stuff during webapp's startup.
}
@PreDestroy
public void destroy() {
// Do stuff during webapp's shutdown.
}
}
This is available via #{config} in EL as well.
@Startup
@Singleton
public class Config {
@PostConstruct
public void init() {
// Do stuff during webapp's startup.
}
@PreDestroy
public void destroy() {
// Do stuff during webapp's shutdown.
}
}
This is available in a servlet via @EJB. I'm saying "consider", because you should not abuse an EJB for the sake of a startup hook. Moreover, a @Singleton is by default read/write locked and primarily intented for transactional stuff such as scheduling background jobs.
See also:
