26.3.1 The Producer Field for the producerfields Example

The most important component of the producerfields example is the smallest, the db.UserDatabaseEntityManager class, which isolates the generation of the EntityManager object so it can easily be used by other components in the application. The class uses a producer field to inject an EntityManager annotated with the @UserDatabase qualifier, also defined in the db package:

@Singleton
public class UserDatabaseEntityManager {

    @Produces
    @PersistenceContext
    @UserDatabase
    private EntityManager em;
    ...
}

The class does not explicitly produce a persistence unit field, but the application has a persistence.xml file that specifies a persistence unit. The class is annotated javax.inject.Singleton to specify that the injector should instantiate it only once.

The db.UserDatabaseEntityManager class also contains commented-out code that uses create and close methods to generate and remove the producer field:

 /* @PersistenceContext
    private EntityManager em;

    @Produces
    @UserDatabase
    public EntityManager create() {
        return em;
    } */

    public void close(@Disposes @UserDatabase EntityManager em) {
        em.close();
    }

You can remove the comment indicators from this code and place them around the field declaration to test how the methods work. The behavior of the application is the same with either mechanism.

The advantage of producing the EntityManager in a separate class rather than simply injecting it into an enterprise bean is that the object can easily be reused in a typesafe way. Also, a more complex application can create multiple entity managers using multiple persistence units, and this mechanism isolates this code for easy maintenance, as in the following example:

@Singleton
public class JPAResourceProducer {
    @Produces 
    @PersistenceUnit(unitName="pu3")
    @TestDatabase 
    EntityManagerFactory customerDatabasePersistenceUnit;

    @Produces 
    @PersistenceContext(unitName="pu3")
    @TestDatabase 
    EntityManager customerDatabasePersistenceContext;
    
    @Produces 
    @PersistenceUnit(unitName="pu4")
    @Documents
    EntityManagerFactory customerDatabasePersistenceUnit;

    @Produces 
    @PersistenceContext(unitName="pu4")
    @Documents 
    EntityManager docDatabaseEntityManager;"
}

The EntityManagerFactory declarations also allow applications to use an application-managed entity manager.

26.3.2 The producerfields Entity and Session Bean

The producerfields example contains a simple entity class, entity.ToDo, and a stateful session bean, ejb.RequestBean, that uses it.

The entity class contains three fields: an autogenerated id field, a string specifying the task, and a timestamp. The timestamp field, timeCreated, is annotated with @Temporal, which is required for persistent Date fields.

@Entity
public class ToDo implements Serializable {

    ...
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    protected String taskText;
    @Temporal(TIMESTAMP)
    protected Date timeCreated;

    public ToDo() {
    }

    public ToDo(Long id, String taskText, Date timeCreated) {
        this.id = id;
        this.taskText = taskText;
        this.timeCreated = timeCreated;
    }
    ...

The remainder of the ToDo class contains the usual getters, setters, and other entity methods.

The RequestBean class injects the EntityManager generated by the producer method, annotated with the @UserDatabase qualifier:

@ConversationScoped
@Stateful
public class RequestBean {

    @Inject
    @UserDatabase
    EntityManager em;

It then defines two methods, one that creates and persists a single ToDo list item, and another that retrieves all the ToDo items created so far by creating a query:

    public ToDo createToDo(String inputString) {
        ToDo toDo = null;
        Date currentTime = Calendar.getInstance().getTime();

        try {
            toDo = new ToDo();
            toDo.setTaskText(inputString);
            toDo.setTimeCreated(currentTime);
            em.persist(toDo);
            return toDo;
        } catch (Exception e) {
            throw new EJBException(e.getMessage());
        }
    }

    public List<ToDo> getToDos() {
        try {
             List<ToDo> toDos =
                    (List<ToDo>) em.createQuery(
                    "SELECT t FROM ToDo t ORDER BY t.timeCreated")
                    .getResultList();
            return toDos;
        } catch (Exception e) {
            throw new EJBException(e.getMessage());
        }
    }
}

26.3.3 The producerfields Facelets Pages and Managed Bean

The producerfields example has two Facelets pages, index.xhtml and todolist.xhtml. The simple form on the index.xhtml page asks the user only for the task. When the user clicks the Submit button, the listBean.createTask method is called. When the user clicks the Show Items button, the action specifies that the todolist.xhtml file should be displayed:

    <h:body>
        <h2>To Do List</h2>
        <p>Enter a task to be completed.</p>
        <h:form id="todolist">
            <p><h:outputLabel value="Enter a string: " for="inputString"/>
                <h:inputText id="inputString"
                             value="#{listBean.inputString}"/></p>
            <p><h:commandButton value="Submit"
                                action="#{listBean.createTask()}"/></p>
            <p><h:commandButton value="Show Items"
                                action="todolist"/></p>
        </h:form>
        ...
    </h:body>

The managed bean, web.ListBean, injects the ejb.RequestBean session bean. It declares the entity.ToDo entity and a list of the entity along with the input string that it passes to the session bean. The inputString is annotated with the @NotNull Bean Validation constraint, so an attempt to submit an empty string results in an error.

@Named
@ConversationScoped
public class ListBean implements Serializable {

    ...
    @EJB
    private RequestBean request;
    @NotNull
    private String inputString;
    private ToDo toDo;
    private List<ToDo> toDos;

The createTask method called by the Submit button calls the createToDo method of RequestBean:

    public void createTask() {
        this.toDo = request.createToDo(inputString);
    }

The getToDos method, which is called by the todolist.xhtml page, calls the getToDos method of RequestBean:

public List<ToDo> getToDos() {
        return request.getToDos();
    }

To force the Facelets page to recognize an empty string as a null value and return an error, the web.xml file sets the context parameter javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL to true:

<context-param>
  <param-name>javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL</param-name>
  <param-value>true</param-value>
</context-param>

The todolist.xhtml page is a little more complicated than the index.html page. It contains a dataTable element that displays the contents of the ToDo list. The body of the page looks like this:

    <body>
        <h2>To Do List</h2>
        <h:form id="showlist">
            <h:dataTable var="toDo"
                         value="#{listBean.toDos}"
                         rules="all"
                         border="1"
                         cellpadding="5">
                <h:column>
                    <f:facet name="header">
                        <h:outputText value="Time Stamp" />
                    </f:facet>
                    <h:outputText value="#{toDo.timeCreated}" />
                </h:column>
                <h:column>
                    <f:facet name="header">
                        <h:outputText value="Task" />
                    </f:facet>
                    <h:outputText value="#{toDo.taskText}" />
                </h:column>
            </h:dataTable>
            <p><h:commandButton id="back" value="Back" action="index" /></p>
        </h:form>
    </body>

The value of the dataTable is listBean.toDos, the list returned by the managed bean's getToDos method, which in turn calls the session bean's getToDos method. Each row of the table displays the timeCreated and taskText fields of the individual task. Finally, a Back button returns the user to the index.xhtml page.

26.3.4 Running the producerfields Example

You can use either NetBeans IDE or Maven to build, package, deploy, and run the producerfields application.