Step 3: Add a Context Listener Class

The Guest entity objects will be stored in an ObjectDB database, which will be represented by a JPA's EntityManagerFactoryjakarta.persistence.EntityManagerFactory - JPA Interface Interface used to interact with the persistence unit, and to create new instances of EntityManager. instance with a global application scope.

We have to register a ServletContextListener to perform initialization and cleanup operations:

• The database will be opened (or created if not existing yet) when the web application starts, by instantiating an EntityManagerFactory.
• The database will be closed when the web application stops (or when the web server shuts down), by closing the EntityManagerFactory.

To register a ServletContextListener:

• Open the [New File] dialog box by right clicking the guest package node (in the [Projects] window) and selecting New > Other...
• Select Web > Web Application Listener and click Next.

• Enter GuestListener as the class name - use exactly that case sensitive class name.
• The Java package name should be guest.

• Click Finish to create the new listener class.

Now replace the content of the new source file with the following code:

package guest;
 
import javax.persistence.*;
import javax.servlet.*;
 
@WebListener
public class GuestListener implements ServletContextListener {

    // Prepare the EntityManagerFactory & Enhance:
    public void contextInitialized(ServletContextEvent e) {
        com.objectdb.Enhancer.enhance("guest.*");
        EntityManagerFactory emf =
            Persistence.createEntityManagerFactory("$objectdb/db/guest.odb");
        e.getServletContext().setAttribute("emf", emf);
    }
 
    // Release the EntityManagerFactory:
    public void contextDestroyed(ServletContextEvent e) {
        EntityManagerFactory emf =
            (EntityManagerFactory)e.getServletContext().getAttribute("emf");
        emf.close();
    }
}

The code above:

• Invokes the Enhancer to enhance the entity class and creates an EntityManagerFactory instance and stores it as an application scope attribute in the servlet context - when the web application starts (contextInitialized).
• Retrieves the EntityManagerFactory from the application scope attribute and closes the EntityManagerFactory and the database - when the application stops (contextDestroyed).

The next step is adding a servlet class that will manage the guestbook page.