The NetBeans E-commerce Tutorial - Adding Entity Classes and Session Beans
This tutorial unit introduces the Enterprise
JavaBeans (EJB) and Java
Persistence (JPA) technologies. In it, you use two of the IDE's wizards that are essential
to Java EE development. These are:
- Entity Classes from Database wizard: Creates a Java Persistence API
entity class for each selected database table, complete with named query annotations,
fields representing columns, and relationships representing foreign keys.
- Session Beans for Entity Classes wizard: Creates an EJB session facade
for each entity class with basic access methods.
These two wizards provide an efficient way for you to quickly set up the model for your
application. If you reexamine the MVC diagram for
the application you are building, you can see where EJB session beans and JPA entity
classes fit into its structure.
In this unit, the entity classes you create form a Java-based representation of the affablebean
database. While each entity class represents a database table, instances of entity classes
correspond to records that can be saved (i.e., persisted) to the database. The business
logic of the application is encapsulated by session beans, which can either be used as facade
classes that enable CRUD (Create-Read-Update-Delete) access to entities (as demonstrated here),
or they can contain code that implements actions specific to your application. (An example of this
is provided in Unit 9: Integrating Transactional Business Logic).
You can view a live demo of the application that you build in this tutorial:
NetBeans E-commerce
Tutorial Demo Application.
Notes:
- The NetBeans IDE requires the Java Development Kit (JDK) to run properly.
If you do not have any of the resources listed above, the JDK should be
the first item that you download and install.
- The NetBeans IDE Java Bundle includes Java Web and EE technologies, which are
required for the application you build in this tutorial.
- The NetBeans IDE Java Bundle also includes the GlassFish server,
which you require for this tutorial. You could
download
the GlassFish server independently, but the version provided with the
NetBeans download has the added benefit of being automatically registered with
the IDE.
- You can follow this tutorial unit without having completed previous units. To
do so, see the setup instructions, which describe how
to prepare the database and establish connectivity between the IDE, GlassFish,
and MySQL.
- Snapshot
4 of the
AffableBean project is available for download and
corresponds to state the project after completing this unit using NetBeans IDE
6.9.
What are EJB and JPA Technologies?
Up until now, the project that you've been developing in this tutorial could be run
in a web server with a servlet container, such as Apache Tomcat. After all, you've
so far only made use of JSTL and servlet technologies, and are connecting to the
database directly using JDBC. In fact, you could theoretically continue to develop
the application using just these technologies, while manually coding for all aspects
of your application, including thread-safety, transactions, and security. However,
using Enterprise beans with JPA entity classes allows you focus on the business logic
of your application while relying on solutions that have already been tried and tested.
The following sections introduce the two technologies and define their role in EE
development.
Enterprise JavaBeans
The official EJB product
page describes EnterPrise JavaBeans technology as a "server-side component
architecture" that "enables rapid and simplified development of distributed,
transactional, secure and portable applications." You can apply EJBs (i.e.,
Enterprise beans) to your projects, and the services provided by the technology
remain transparent to you as a developer, thus eliminating the tedious and often
error-prone task of adding a lot of boiler plate code which would otherwise be
required. If you are new to EE development, you may question the need for EJBs in
your Java web application. The book
EJB 3 In Action, by Debu
Panda, Reza Rahman and Derek Lane, paraphrases the role of EJB technology nicely:
Although many people think EJBs are overkill for developing relatively
simple web applications of moderate size, nothing could be further from
the truth. When you build a house, you don't build everything from scratch.
Instead, you buy materials or even the services of a contractor as you need
it. It isn't too practical to build an enterprise application from scratch
either. Most server-side applications have a lot in common, including churning
business logic, managing application state, storing and retrieving information
from a relational database, managing transactions, implementing security,
performing asynchronous processing, integrating systems, and so on.
As a framework, the EJB container provides these kinds of common functionality
as out-of-the-box services so that your EJB components can use them in your
applications without reinventing the wheel. For instance, let's say that when
you build a credit card module in your web application, you write a lot of
complex and error-prone code to manage transactions and security access control.
You could have avoided that by using the declarative transaction and security
services provided by the EJB container. These services as well as many others
are available to EJB components when they are deployed in an EJB container.
This means writing high-quality, feature-rich applications much faster than
you might think.[1]
You can think of EJB both as components, or Java classes that are incorporated in
your project, as well as a framework that provides numerous enterprise-related
services. Some of the services that we take advantage of in this tutorial are described
in EJB 3 In Action as follows:
- Pooling: For each EJB component, the EJB platform creates
a pool of component instances that are shared by clients. At any point in
time, each pooled instance is only allowed to be used by a single client.
As soon as an instance is finished servicing a client, it is returned to
the pool for reuse instead of being frivolously discarded for the garbage
collector to reclaim.
- Thread Safety: EJB makes all components thread-safe and
highly performant in ways athat are completely invisible. This means that
you can write your server components as if you were developing a single-threaded
desktop application. It doesn't matter how complex the component itself is;
EJB will make sure it is thread-safe.
- Transactions: EJB supports declarative transaction management
that helps you add transactional behavior to components using simple
configuration instead of code. In effect, you can designate any component
method to be transactional. If the method completes normally, EJB commits
the transaction and makes the data changes made by the method permanent.
Otherwise the transaction is rolled back. Container-managed EJB transactions
are demonstrated in Unit 9, Integrating Transactional
Business Logic.
- Security: EJB supports integration with the Java Authentication
and Authorization Service (JAAS) API, so it is easy to completely externalize
security and secure an application using simple configuration instead of cluttering
up your application with security code.[2] In Unit 11, Securing
the Application, a demonstration of EJB's @RolesAllowed annotation is provided.
Java Persistence
In the context of Java Enterprise, persistence refers to the act of
automatically storing data contained in Java objects into a relational database.
The Java
Persistence API (JPA) is an object-relational mapping (ORM) technology that
enables applications to manage data between Java objects and a relational database
in a way that is transparent to the developer. This means that you can apply
JPA to your projects by creating and configuring a set of Java classes (entities)
that mirror your data model. Your application can then access these entities
as though it were directly accessing the database.
There are various benefits to using JPA in your projects:
- JPA has its own rich, SQL-like query language for static and dynamic queries.
Using the Java Persistence Query Language (JPQL), your applications remain
portable across different database vendors.
- You can avoid the task of writing low-level, verbose and error-prone JDBC/SQL code.
- JPA transparently provides services for data caching and performance optimization.
What are Session Beans?
Enterprise session beans are invoked by a client in order to perform a specific business
operation. The name session implies that a bean instance is available for the
duration of a "unit of work". The EJB
3.1 specification describes a typical session object as having the following
characteristics:
- Executes on behalf of a single client
- Can be transaction-aware
- Updates shared data in an underlying database
- Does not represent directly shared data in the database, although it may
access and update such data
- Is relatively short-lived
- Is removed when the EJB container crashes. The client has to re-establish
a new session object to continue computation.
EJB provides three types of session beans: stateful, stateless,
and singleton. The following descriptions are adapted from the
Java
EE 6 Tutorial.
- Stateful: The state of the bean is maintained across multiple
method calls. The "state" refers to the values of its instance variables.
Because the client interacts with the bean, this state is often called the
conversational state.
- Stateless: Stateless beans are used for operations that can
occur in a single method call. When the method finishes processing, the
client-specific state of the bean is not retained. A stateless session bean
therefore does not maintain a conversational state with the client.
- Singleton: A singleton session bean is instantiated once
per application, and exists for the lifecycle of the application. Singleton
session beans are designed for circumstances where a single enterprise bean
instance is shared across and concurrently accessed by clients.
For more information on EJB session beans, see the
Java
EE 6 Tutorial: What is a Session Bean?.
For purposes of developing the e-commerce application in this tutorial, we will
only be working with stateless session beans.
About Specifications and Implementations
EJB and JPA technologies are defined by the following specifications:
These specifications define the technologies. To apply a technology to your
project however, you must use an implementation of the specification.
When a specification becomes finalized, it includes a reference implementation,
which is a free implementation of the technology. If you find this concept
confusing, consider the following analogy: A musical composition (i.e., the notes
on a page) defines a piece of music. When a musician learns the composition and
records her performance, she provides an interpretation of the piece.
In this manner the musical composition is likened to the technical specification,
and the musician's recording corresponds to the specification's implementation.
See What is the Java Community Process?
for an explanation of Java technical specifications, and how they are formally
standardized.
If you examine the download pages for the final releases of the EJB and JPA specifications,
you'll find links to the following reference implementations:
Implementations of the JPA specification are dubbed persistence providers,
and the persistence provider which has been chosen as the reference implementation
for the JPA 2.0 specification is EclipseLink.
If you examine the link for the EJB reference implementation, you'll come to a page
that lists not only the implementation for EJB, but for all reference implementations
provided by Project GlassFish.
The reason for this is that Project GlassFish forms the reference implementation of the
Java EE 6 platform specification (JSR
316). The GlassFish v3 application server (or the Open Source Edition), which you
are using to build the e-commerce project in this tutorial, contains the reference
implementations of all technologies developed under Project GlassFish. As such, it
is referred to as a Java EE 6 container.
A Java EE container contains three essential components: a web (i.e., servlet) container,
an EJB container, and a persistence provider. The deployment scenario for the e-commerce
application is displayed in the diagram below. Entity classes that you create in this
unit are managed by the persistence provider. The session beans that you create in this
unit are managed by the EJB container. Views are rendered in JSP pages, which are
managed by the web container.
Adding Entity Classes
Begin by using the IDE's Entity Classes from Database wizard to generate entity classes
based on the affablebean schema. The wizard relies on the underlying
persistence provider to accomplish this task.
- Open the project
snapshot in the IDE. In the IDE, press Ctrl-Shift-O (�-Shift-O on Mac) and
navigate to the location on your computer where you unzipped the downloaded file.
- Press Ctrl-N (⌘-N on Mac) to open the File wizard.
- Select the Persistence category, then select Entity Classes from Database. Click Next.
- In Step 2: Database Tables, choose
jdbc/affablebean from the Data Source
drop-down list. The drop-down list is populated by data sources registered with the
application server.
When you choose the jdbc/affablebean data source, the IDE scans the
database and lists the database tables in the Available Tables pane.
- Click the Add All button, then click Next.
- Step 3 of the Entity Classes from Database wizard differs slightly between NetBeans
IDE 6.8 and 6.9. Depending on the version IDE you are using, perform the following steps.
NetBeans IDE 6.8
- Type in entity in the Package field. The wizard will create a new
package for the entity classes upon completing.
- Click the Create Persistence Unit button. The Create Persistence Unit dialog opens.
A persistence unit refers to a collection of entity classes that exist in an
application. The above dialog generates a persistence.xml file, which is
used by your persistence provider to specify configuration settings for the persistence
unit. Note that 'EclipseLink (JPA 2.0)' is the default selection for the server associated
with the project. Leave 'Table Generation Strategy' set to 'None'. This
prevents the persistence provider from affecting your database. (For example, if you
want the persistence provider to delete then recreate the database based on the existing
entity classes, you could set the strategy to 'Drop and Create'. This action
would then be taken each time the project is deployed.)
- Click Create.
- Back in Step 3: Entity Classes, note that the class names for the entities are based
on database tables. For example, the
CustomerOrder entity is mapped to
the customer_order database table. Also note that the 'Generate Named
Query Annotations for Persistent Fields' option is selected by default. We will be
using various named queries later in the tutorial.
- Continue to step 7 below.
NetBeans IDE 6.9
- Type in entity in the Package field. The wizard will create a new
package for the entity classes upon completing.
- Note the following:
- The class names for the entities are based on database tables. For example, the
CustomerOrder entity will be mapped to the customer_order
database table.
- The 'Generate Named Query Annotations for Persistent Fields' option is selected by
default. We will be using various named queries later in the tutorial.
- The 'Create Persistence Unit' option is selected by default. A persistence
unit is a collection of entity classes that exist in an application. The
persistence unit is defined by a
persistence.xml configuration file,
which is read by your persistence provider. Enabling this option therefore means
that the wizard will also generate a persistence.xml file and populate
it with default settings.
- Click Finish. The JPA entity classes are generated, based on the
affablebean
database tables. You can examine the entity classes in the Projects window by expanding
the newly created entity package. Also, note that the new persistence unit
exists under the Configuration Files node.
Note that the wizard generated an additional entity class, OrderedProductPK.
Recall that the data model's ordered_product table uses a composite
primary key that comprises the primary keys of both the customer_order
and product tables. (See Designing
the Data Model - Creating Many-To-Many Relationships.) Because of this, the
persistence provider creates a separate entity class for the composite key, and
embeds it into the OrderedProduct entity. You can open
OrderedProduct in the editor to inspect it. JPA uses the @EmbeddedId
annotation to signify that the embeddable class is a composite primary key.
public class OrderedProduct implements Serializable {
private static final long serialVersionUID = 1L;
@EmbeddedId
protected OrderedProductPK orderedProductPK;
Press Ctrl-Space on the @EmbeddedId annotation to
invoke the API documentation.
- Open the persistence unit (
persistence.xml) in the editor. The
IDE provides a Design view for persistence units, in addition to the XML view. The
Design view provides a convenient way to make configuration changes to the persistence
provider's management of the project.
- Click the XML tab at the top of the
AffableBeanPU persistence unit
to open the XML view. Add the following property to the file.
<persistence-unit name="AffableBeanPU" transaction-type="JTA">
<jta-data-source>jdbc/affablebean</jta-data-source>
<properties>
<property name="eclipselink.logging.level" value="FINEST"/>
</properties>
</persistence-unit>
You set the logging level property to FINEST so that you can view all
possible output produced by the persistence provider when the application runs. This
enables you to see the SQL that the persistence provider is using on the database,
and can facilitate in any required debugging.
See the official EclipseLink documentation for an explanation
of logging and a list of all logging values:
How
To Configure Logging
Adding Session Beans
In this section, we use the IDE's Session Beans for Entity Classes wizard to generate
an EJB session facade for each of the entity classes that you just created.
Each session bean will contain basic access methods for its respective entity class.
A session facade is a design pattern advertised in the
Enterprise
BluePrints program. As stated in the
Core
J2EE Pattern Catalog, it attempts to resolve common problems that arise in a
multi-tiered application environment, such as:
- Tight coupling, which leads to direct dependence between clients and business objects
- Too many method invocations between client and server, leading to network performance problems
- Lack of a uniform client access strategy, exposing business objects to misuse
A session facade abstracts the underlying business object interactions and provides a
service layer that exposes only the required functionality. Thus, it hides from the
client's view the complex interactions between the participants. Thus, the session
bean (representing the session facade) manages the relationships between business
objects. The session bean also manages the life cycle of these participants by
creating, locating, modifying, and deleting them as required by the workflow.
- Press Ctrl-N (⌘-N on Mac) to open the File wizard.
- Select the Persistence category, then select Session Beans for Entity Classes.
- Click Next.
- In Step 2: Entity Classes, note that all entity classes contained in your project
are listed on the left, under Available Entity Classes. Click Add All. All entity
classes are moved to the right, under Selected Entity Classes.
- Click Next.
- In Step 3: Generated Session Beans, type in session into the Package field.
Note: You can use the wizard to generate local and remote
interfaces for the session beans. While there is benefit to programming session beans to
interfaces (For example, hiding business object interactions behind an interface enables
you to further decouple the client from your business logic. This also means that you can
code multiple implementations of the interface for your application, should the need arise.),
this lies outside the scope of the tutorial. Note that EJB versions prior to 3.1 require
that you implement an interface for each session bean.
- Click Finish. The IDE generates session beans for each of the entity classes contained
in your project. In the Projects window, expand the new
session package to
examine the session beans.
| NetBeans 6.8 |
NetBeans 6.9 |
|
|
Note: As shown above, NetBeans IDE 6.9 provides slight
improvements in the way the Session Beans for Entity Classes wizard generates facade
classes. Namely, boiler-plate code that is common to all classes is factored out
into an abstract class named AbstractFacade. If you are working in version
6.9, open any of the facade classes that have been generated (aside from AbstractFacade).
You'll see that the class extends AbstractFacade.
- Open a session facade in the editor, for example,
ProductFacade. All of
the generated session facades instantiate an
EntityManager using the
@PersistenceContext annotation.
@PersistenceContext(unitName = "AffableBeanPU")
private EntityManager em;
The @PersistenceContext annotation is used to inject a container-managed
EntityManager into the class. In other words, we rely on GlassFish' EJB
container to open and close EntityManagers as and when needed. The
unitName element specifies the AffableBeanPU persistence unit,
which has been defined in the application's persistence.xml file.
The EntityManager is an integral component of the Java Persistence
API, and is responsible for performing persistence actions on the database.
The book EJB 3 In Action
describes the EntityManager as follows:
The JPA EntityManager interface manages entities in terms
of actually providing persistence services. While entities tell a JPA
provider how they map to the database, they do not persist themselves.
The EntityManager interface reads the ORM metadata for an
entity and performs persistence operations.
Your application now contains a persistence model of the affablebean database in
the form of JPA entity classes. It also contains a session facade consisting of Enterprise
beans that can be used to access the entity classes. The next section demonstrates how you
can access the database using the session beans and entity classes.
Accessing Data with EJBs
In the previous tutorial unit, you learned how to access the
database from the application by configuring a data source on GlassFish, adding a resource
reference to the application's deployment descriptor, and using JSTL <sql>
tags in the application's JSP pages. This is a valuable technique, as it allows you to
quickly set up prototypes that include data from the database. However, this is not a
realistic scenario for medium to large-sized applications, or applications managed by a
team of developers, as it would prove difficult to maintain or scale. Furthermore, if you
are developing the application into multiple tiers or are adhering to the MVC pattern, you
would not want to keep data-access code in your front-end. Using Enterprise beans with a
persistence model enables you better conform to the MVC pattern by effectively decoupling
the presentation and model components.
The following instructions demonstrate how to begin using the session and entity beans in
the AffableBean project. You are going to remove the JSTL data access logic
that you previously set up for the index and category pages. In its place, you'll utilize
the data access methods provided by the session beans, and store the data in scoped
variables so that it can be retrieved from front-end page views. We'll tackle the index
page first, then move on to the more complicated category page.
index page
The index page requires data for the four product categories. In our current setup,
the JSTL <sql> tags query the database for category details each
time the index page is requested. Since this information is rarely modified, it makes
more sense from a performance standpoint to perform the query only once after the
application has been deployed, and store the data in an application-scoped attribute.
We can accomplish this by adding this code to the ControllerServlet's
init method.
- In the Projects window, double-click the Source Packages >
controller
> ControllerServlet node to open it in the editor.
- Declare an instance of
CategoryFacade, and apply the @EJB
annotation to the instance.
public class ControllerServlet extends HttpServlet {
@EJB
private CategoryFacade categoryFacade;
...
}
The @EJB annotation instructs the EJB container to instantiate
the categoryFacade variable with the EJB named CategoryFacade.
- Use the IDE's hints to add import statements for:
javax.ejb.EJBsession.CategoryFacade
Pressing Ctrl-Shift-I (⌘-Shift-I on Mac) automatically adds
required imports to your class.
- Add the following
init method to the class. The web container
initializes the servlet by calling its init method. This occurs
only once, after the servlet is loaded and before it begins servicing requests.
public class ControllerServlet extends HttpServlet {
@EJB
private CategoryFacade categoryFacade;
public void init() throws ServletException {
// store category list in servlet context
getServletContext().setAttribute("categories", categoryFacade.findAll());
}
...
}
Here, you apply the facade class' findAll method to query
the database for all records of Category. You then set the resulting
List of Category objects as an attribute that can be
referenced by the "categories" string. Placing the
reference in the ServletContext means that the reference exists
in a scope that is application-wide.
To quickly determine the method signature of the findAll
method, hover your mouse over the method while holding down the Ctrl key
(⌘ on Mac). (The image below displays the popup that appears using
NetBeans IDE 6.8.)
Clicking the hyperlink enables you to navigate directly to the method.
- Use the IDE's hint to add the
@Overrides annotation. The init
method is defined by HttpServlet's superclass, GenericServlet.
Adding the annotation is not required, however it does provide several advantages:
- It enables you to use compiler checking to ensure that you are actually
overriding a method that you assume you are overriding.
- It improves readability, as it becomes clear when methods in your source code
are being overridden.
For more information on annotations, see the
Java
Tutorials: Annotations.
- Now that you have set up an application-scoped attribute that contains a list of
categories, modify the index page to access the newly created attribute.
Double-click the Web Pages > index.jsp node in the Projects window
to open the file in the editor.
- Comment out (or delete) the
<sql:query> statement that is
listed at the top of the file. To comment out code in the editor, highlight
the code, then press Ctrl-/ (⌘-/ on Mac).
- Modify the opening
<c:forEach> tag so that its items
attribute references the new application-scoped categories attribute.
<c:forEach var="category" items="${categories}">
- Open the project's web deployment descriptor. Press Alt-Shift-O (Ctrl-Shift-O on Mac)
and in the Go to File dialog, type '
web', then click OK.
- Comment out (or delete) the
<resource-ref> entry. The entry was required
for the <sql> tags in order to identify the data source registered on the
server. We are now relying on JPA to access the database, and the jdbc/affablebean
data source has already been specified in the persistence unit. (Refer to the Design
view of the project's persistence unit above.)
Highlight the entire <resource-ref> entry, then press Ctrl-/ (⌘-/ on Mac).
<!-- <resource-ref>
<description>Connects to database for AffableBean application</description>
<res-ref-name>jdbc/affablebean</res-ref-name>
<res-type>javax.sql.DataSource</res-type>
<res-auth>Container</res-auth>
<res-sharing-scope>Shareable</res-sharing-scope>
</resource-ref> -->
- Run the project. Click the Run Project (
) button. The project's index page opens in the browser,
and you see that all four category names and images display.
category page
The category page requires three pieces of data
in order to render properly:
- category data: for left column category buttons
- selected category: the selected category is highlighted in
the left column, and the name of the selected category displays above the
product table
- product data for selected category: for products displayed
in the product table
Let's approach each of the three pieces of data individually.
category data
To account for category data, we can reuse the application-scoped categories
attribute that we created for the index page.
- Open
category.jsp in the editor, and comment out (Ctrl-/; ⌘-/ on Mac)
the JSTL <sql> statements that are listed at the top of the file.
- Modify the opening
<c:forEach> tag so that its items
attribute references the application-scoped categories attribute.
(This is identical to what you did above for index.jsp.)
<c:forEach var="category" items="${categories}">
- Run the project to examine the current state of the category page. Click the Run Project (
) button. When the project's index page opens in the browser,
click any of the four categories. The category buttons in the left column display and
function as expected.
selected category
To retrieve the selected category, we can use the categoryFacade
that we already created to find the Category whose ID matches
the request query string.
- Open the
ControllerServlet in the editor. (If already opened, press Ctrl-Tab
and choose from the pop-up list.)
- Start implementing functionality to acquire the selected category. Locate
the
TODO: Implement category request comment, delete it and
add the following code (in bold).
// if category page is requested
if (userPath.equals("/category")) {
// get categoryId from request
String categoryId = request.getQueryString();
if (categoryId != null) {
}
// if cart page is requested
} else if (userPath.equals("/viewCart")) {
You retrieve the requested category ID by calling getQueryString()
on the request.
Note: The logic to determine the selected
category within the left column category buttons is already implemented
in category.jsp using an EL expression, which is comparable
to calling getQueryString() in the servlet. The EL expression
is: pageContext.request.queryString.
- Add the following line of code within the
if statement.
// get categoryId from request
String categoryId = request.getQueryString();
if (categoryId != null) {
// get selected category
selectedCategory = categoryFacade.find(Short.parseShort(categoryId));
}
You use the CategoryFacade's find method to
retrieve the Category object based on the requested category
ID. Note that you must cast categoryId to a Short,
as this is the type used for the id field in the Category
entity class.
- Click the badge (
) in the left margin to use the editor's hint to declare
selectedCategory as a local variable within the doGet
method.
Because selectedCategory is of type Category, which
hasn't yet been imported into the class, the IDE automatically adds an import
statement for entity.Category to the top of the file.
- Add the following line to place the retrieved
Category object
in the request scope.
// get categoryId from request
String categoryId = request.getQueryString();
if (categoryId != null) {
// get selected category
selectedCategory = categoryFacade.find(Short.parseShort(categoryId));
// place selected category in request scope
request.setAttribute("selectedCategory", selectedCategory);
}
- In the editor, switch to
category.jsp. (Press Ctrl-Tab and choose from
the pop-up list.)
- Locate
<p id="categoryTitle"> and make the following
change.
<p id="categoryTitle">
<span style="background-color: #f5eabe; padding: 7px;">${selectedCategory.name}</span>
</p>
You are now using the selectedCategory attribute, which you just placed
in the request scope from the ControllerServlet. Using '.name'
within the EL expression calls the getName method on the given
Category object.
- Switch back to the browser and refresh the category page. The name of the selected
category now displays in the page.
product data for selected category
In order to retrieve all products for a selected category, we'll make use of the
Category entity's getProductCollection() method. Start
by calling this method on selectedCategory to get a collection of
all Products associated with the selectedCategory.
Then store the collection of products as an attribute in the request scope, and
finally reference the scoped attribute from the category.jsp page
view.
- In the
ControllerServlet, add the following statement to the code that
manages the category request.
// if category page is requested
if (userPath.equals("/category")) {
// get categoryId from request
String categoryId = request.getQueryString();
if (categoryId != null) {
// get selected category
selectedCategory = categoryFacade.find(Short.parseShort(categoryId));
// place selected category in request scope
request.setAttribute("selectedCategory", selectedCategory);
// get all products for selected category
categoryProducts = selectedCategory.getProductCollection();
}
Calling getProductCollection() here enables us to get a collection
of all Products associated with the selectedCategory.
- Use the editor's hint to define
categoryProducts as a local variable
for the doGet method.
- Place the collection of
Products in the request scope so that it
can be retrieved from the application's front-end.
// if category page is requested
if (userPath.equals("/category")) {
// get categoryId from request
String categoryId = request.getQueryString();
if (categoryId != null) {
// get selected category
selectedCategory = categoryFacade.find(Short.parseShort(categoryId));
// place selected category in request scope
request.setAttribute("selectedCategory", selectedCategory);
// get all products for selected category
categoryProducts = selectedCategory.getProductCollection();
// place category products in request scope
request.setAttribute("categoryProducts", categoryProducts);
}
- Open the
category.jsp file in the editor and make the following
change to the product table.
<table id="productTable">
<c:forEach var="product" items="${categoryProducts}" varStatus="iter">
The <c:forEach> tag now references the categoryProducts
collection. The c:forEach loop will now iterate over each Product
object contained in the collection, and extract data accordingly.
- Press F6 (fn-F6 on Mac) to run the project. Navigate to the category
page in the browser and note that all products now display for each category.
This tutorial unit provided a brief introduction to JPA and EJB technologies. It also
described the role of Java specifications, and how their reference implementations are
used by the GlassFish application server. It then demonstrated how to create a set of
JPA entity classes that provide a Java implementation of the project database. Then,
following the session facade pattern, it showed how to create a set of EJB
session beans that exist on top of the entity classes and enable convenient access to
them. Finally, you modified the AffableBean project to utilize the new
session beans and entities for database access required in the index and category pages.
You can download
snapshot
4 of the AffableBean project, which corresponds to state the project
after completing this unit using NetBeans IDE 6.9.
In the next unit you explore session management, and how to enable the application to
remember a user's actions as he or she clicks through the site. This is key to implementing
a shopping cart mechanism in an e-commerce application.
See Also
NetBeans Resources
EJB Resources
JPA Resources
GlassFish Resources
Technical Articles
Books
References
