Obtaining Access to an Entity Manager

The BookDBAO object of the Duke’s Bookstore application includes methods for getting the book data from the database and updating the inventory in the database when books are sold. In order to perform database queries, the BookDBAO object needs to obtain an EntityManager instance.

The Java Persistence API allows developers to use annotations to identify a resource so that the container can transparently inject it into an object. You can give an object access to an EntityManager instance by using the @PersistenceUnit annotation to inject an EntityManagerFactory, from which you can obtain an EntityManager instance.

Unfortunately for the web application developer, resource injection using annotations can only be used with classes that are managed by a Java EE compliant container. Because the web container does not manage JavaBeans components, you cannot inject resources into them. One exception is a request-scoped JavaServer Faces managed bean. These beans are managed by the container and therefore support resource injection. This is only helpful if your application is a JavaServer Faces application.

You can still use resource injection in a web application that is not a JavaServer Faces application if you can do it in an object that is managed by the container. These objects include servlets and ServletContextListener objects. These objects can then give the application’s beans access to the resources.

In the case of Duke’s Bookstore, the ContextListener object creates the BookDBAO object and puts it into application scope. In the process, it passes to the BookDBAO object the EntityManagerFactory object that was injected into ContextListener:

public final class ContextListener implements SerlvetContextListener {
...
@PersistenceUnit
private EntityManagerFactory emf;

public void contextInitialized(ServletContexEvent event) {
    context = event.getServletContext();
    ...
    try {
        BookDBAO bookDB = new BookDBAO(emf);
        context.setAttribute("bookDB", bookDB);
    } catch (Exception ex) {
        System.out.println(
            "Couldn’t create bookstore database bean: "
                 + ex.getMessage());
    }
}
}

The BookDBAO object can then obtain an EntityManager from the EntityManagerFactory that the ContextListener object passes to it:

private EntityManager em;

public BookDBAO (EntityManagerFactory emf) throws Exception {
    em = emf.getEntityManager();
    ...
}

The JavaServer Faces version of Duke’s Bookstore gets access to the EntityManager instance a little differently. Because managed beans allow resource injection, you can inject the EntityManagerFactory instance into BookDBAO.

In fact, you can bypass injecting EntityManagerFactory and instead inject the EntityManager directly into BookDBAO. This is because thread safety is not an issue with request-scoped beans. Conversely, developers need to be concerned with thread safety when working with servlets and listeners. Therefore, a servlet or listener needs to inject an EntityManagerFactory instance, which is thread-safe, whereas a persistence context is not thread-safe. The following code shows part of the BookDBAO object included in the JavaServer Faces version of Duke’s Bookstore:

import javax.ejb.*;
import javax.persistence.*;
import javax.transaction.NotSupportedException;
public class BookDBAO {

    @PersistenceContext
    private EntityManager em;
...

As shown in the preceding code, an EntityManager instance is injected into an object using the @PersistenceContext annotation. An EntityManager instance is associated with a persistence context, which is a set of entity instances that the entity manager is tasked with managing.

The annotation may specify the name of the persistence unit with which it is associated. This name must match a persistence unit defined in the application’s persistence.xml file.

The next section explains how the BookDBAO object uses the entity manager instance to query the database.