Using special auto start servlet to initialize on startup and share application data

Question

I need to get some configuration and connect to external resources/objects/systems somewhere and store it in application scope.

I can see two ways to setup my applic

Answer 1

None of both is the better approach. Servlets are intended to listen on HTTP events (HTTP requests), not on deployment events (startup/shutdown).

---

CDI/JSF/EJB unavailable? Use ServletContextListener

@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.

---

CDI available? Use @Observes on ApplicationScoped.class

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.

---

JSF available? Use @ManagedBean(eager=true)

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.

---

EJB available? Consider @Startup@Singleton

@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:

• How to run a background task in a servlet based web application?
• ServletContainerInitializer vs ServletContextListener
• How do I force an application-scoped bean to instantiate at application startup?