4.1Java Basics

Compared to JavaScript, what is different in Java? We list a few observations:

In Java, visibility levels are used to define the possible levels of access:

Table 4.2 Java Visibility Level

Normally, properties are defined as private, such that they can only be assigned with a public setter, which allows to control, and possibly log, any change of the property value. Unfortunately, Java does not allow to protect only write, but not read access. Therefore, we always have to define getters along with setters, even if they are not needed.

A Java bean class (or, simply, bean class) is a Java class with a parameter-free constructor where all properties are serializable and have a get and set method (also called “getter” and “setter”). A Java bean is an object created with the help of a bean class.

A JPA entity class (or, simply, entity class) is a Java bean class with an @Entity annotation, implying that a Java EE runtime environment (such as provided by the TomEE PLUS71 web server) will take care of the persistent storage of its instances.

JPA is a Java API for the management of persistent data in Java applications. It uses the Java Persistence Query Language (JPQL), a platform-independent object-oriented query language based on SQL. JPQL query expressions look very similar to SQL query expressions, but they are executed in the context of JPA entity objects.

JSF is a Java specification for building component-based user interfaces for web applications. Its current version, JSF 2, by default, uses Facelets as its view technology. By contrast, JSF 1 has used JavaServer Pages (JSP) as its default view technology.

A JSF facelet is an XML-based template that combines static UI content items with data variables for generating an XHTML user interface page.

4.2Step 1 – Set up the Folder Structure

In the first step, we set up our folder structure for the application program code. The application name is “Public Library”, so it would be natural to use a corresponding name for the application folder, like “PublicLibrary”, but we prefer using MinimalApp. Then we create the application structure. There are many ways to do this, like for example to use the Eclipse72 development environment (and create and configure a Dynamic Web Project). In this chapter we show how to do it manually, so there is no need to use special tools. For your convenience, we also provide an ANT script (see our guide73 for a download link), allowing to automatically create the folder structure of the web application, compile it to a Web application Archive (WAR) file and then deploy it to a TomEE server for execution. The application structure (which is compatible with the Dynamic Web Project structure of Eclipse, so it can be imported in Eclipse) is the following:

This folder structure has the following parts:

We create a “Main Menu” page for our application, thus we add an index.xhtml file to our views folder with the following content:

The code consists of HTML elements and JSF elements, which are discussed below.

4.3Step 2 – Write the Model Code

In the second step, we create the model classes for our app, using a separate Java source code file (with extension . java) for each model class. In the information design model shown in Figure 4.1 above, there is only one class, representing the object type Book. So, we create a file Book. java in the folder src/pl/m with the following code:

Notice that the model class Book is coded as a JPA entity class, which is a JavaBean class enriched with the following JPA annotations:

1. The annotation @Entity designates a class as an entity class implying that the instances of this class will be stored persistently.
2. The annotation @Table( name="books") specifies the name of the database table to be used for storing the Book entities. This annotation is optional and defaults to a table name being the same as the class name but in lower case (that is, it would be book in our case).
3. The @Id annotation marks the standard identifier attribute, implying that the corresponding column of the underlying SQL database table is designated as the PRIMARY KEY. In our example, isbn is used as the standard identifier attribute, and the corresponding isbn column of the books table stores the primary key values.

In the entity class Book, we also define the following static (class-level) methods:

1. Book. create for creating a new Book instance.
2. Book. retrieveAll for retrieving all Book instances from the persistent data store.
3. Book.retrieve for retrieving a specific Book instance from the persistent data store by means of its standard identifier.
4. Book. update for updating an existing Book instance.
5. Book. delete for deleting a Book instance.
6. Book. generateTestData for creating a few example book records to be used as test data.
7. Book.clearData for clearing the book database table.

The signatures of these methods, which are discussed in more detail in the following subsections, are shown in the following program listing:.

The JPA architecture for data management and object-to-storage mapping is based on the concept of an entity manager, which provides the data management methods persist for saving a newly created or updated entity, find for retrieving an entity, and remove for deleting an entity.

Since the database access operations of an entity manager are executed in the context of a transaction, our data management methods have a parameter ut of type UserTransaction. Before the entity manager can invoke the database write method persist, a transaction needs to be started with ut.begin(). After all write (and state change) operations have been performed, the transaction is completed (and all changes are committed) with ut.commit().

4.3.1Storing Book objects in a database table books

The instances of our entity class Book are Java objects representing “entities” (or business objects), which can be serialized, or, in other words, converted to records (or rows) of a database table, as shown in Table 4.1.

SQL

The Structured Query Language (SQL), created in the early 1970s by Donald D. Chamberlin and Raymond F. Boyce, is an ISO standard that is based on the Relational Database model of Edgar F. Codd and defines

1. a Data Definition Language (based on CREATE TABLE statements) for creating databases consisting of tables containing primitive data;
2. a Data Manipulation Language (based on INSERT, UPDATE and DELETE statements) for creating, updating and deleting the contents of database tables;
3. a Query Language (based on SELECT statements) for retrieving the information stored in database tables.

SQL is widely used in Database Management Systems (DBMSs) and in programming languages for persistent data storage. Despite the existence of the SQL standard, each SQL DBMS has its own dialect of SQL. Consequently, SQL code is in practice not completely portable among different DBMSs without adjustments.

The data storage technology used in our example app is MySQL74, and the SQL code used to create the schema for the database table books is the following:

While it is also possible to create the database schema manually (with the help of CREATE TABLE statements such as the one above), we show below how the database schema can be automatically generated by JPA. In both cases, the database setup, including a user account and the associated rights (create, update, etc), must be done manually before the JPA application can connect to it.

4.3.2Creating a new Book instance and storing it

The Book. create method takes care of creating a new Book instance and saving it to a database with the help of an ’entity manager’:

To store the new object, the persist method of the given ’entity manager’ is invoked. It is responsible for creating the corresponding SQL INSERT statement and executing it.

4.3.3Retrieving all Book instances

The instances of an entity class, such as Book, are retrieved from the database with the help of a corresponding query expressed in the Java Persistence Query Language (JPQL75). These queries are similar to SQL queries. They use class names Instead of table names, property names instead of column names, and object variables instead of row variables.

In the Book. retrieveAll method, first a query asking for all Book instances is created, and then this query is executed with query. getResultList() assigning its answer set to the list variable books:

4.3.4Updating a Book instance

To update an existing Book instance, first we need to retrieve it from the database, by using em. find, and then set those attributes the value of which has changed:

Notice that, when invoking the find method for retrieving an entity, the first argument must be a reference to the entity class concerned (here: Book. class), so the JPA runtime environment can identify the database table from which to retrieve the entity’s data. The second argument must be the value of the entity’s primary key.

Notice that in the update case, we do not have to use persist for saving the changes. Saving is automatically managed by the JPA runtime environment when we complete the transaction with ut.commit().

4.3.5Deleting a Book instance

A book entity can be deleted from the database as shown in the following example code:

To delete an entity from the database, we first need to retrieve it with the help of the find method as in the update case. Then, the remove method has to be invoked by the ’entity manager’, and finally the transaction is completed with ut.commit().

4.3.6Creating test data

For being able to test our code, we may create some test data and save it in our database. We can use the following procedure for this:

After clearing the database, we successively create 3 instances of the Book entity class and save them with the help of persist.

4.3.7Clearing all data

The following procedure clears our database by deleting all rows:

JPA does not provide a direct method to drop the entire population of a specific class from the database. However, this can be easily obtained by using a JPQL statement as shown in the above code. The JPQL code can be read as: delete all rows from the database table associated with the entity class Book.

4.4Step 3 – Configure the App

In this section we show how to

4.4.1Create the EntityManager and UserTransaction objects

A controller class contains the code that glues the views to the model, as well as all methods that do neither belong to the model nor to the view, like getting a connection with the database server. In our example app, this class is pl.c.BookController in the src/pl/c folder.

JPA requires an EntityManager object for executing JPQL queries (with SELECT) and data manipulation statements (with INSERT, UPDATE and DELETE). Also, in order to perform database write operations, a UserTransaction object is required for starting and completing transactions. In a standalone application, the programmer has to create an entity manager and a transaction manually, using a factory pattern as shown in the following code fragment:

A JPA-enabled Java web application normally runs in an environment called “container” (in our case this is TomEE), which takes care of creating an EntityManager and a UserTransaction object if the right annotations are used. The code responsible for this is part of the controller class ( e. g., pl.c.BookController) since the controller is responsible for managing the database connections.

A closer look at this code shows that it is sufficient to use the @PersistenceContext annotation and provide a unitName (see the next section) for obtaining an EntityManager instance at runtime. Also, obtaining a UserTransaction instance at runtime is as simple as using the @Resource annotation for the user transaction reference property ut. Not only that the required code is short and simple, but if the database type is changed (e. g. when we switch from MySQL to an Oracle database), this code remains the same.

4.4.2Configure the JPA database connection

In the previous section, discussing the BookController class, we have shown how to obtain the EntityManager and UserTransaction objects required for performing database operations. The @PersistenceContext annotation of the EntityManager reference property requires a unitName, which is just a name used for identifying the storage management configuration defined in the src/ META-INF/persistence.xml file. In our example app this file has the following content:

The configuration name (“MinimalApp”) is defined by the name attribute of the persistence-unit element. This is the value we have to use for the unitName property of the @PersistenceContext annotation.

The persistence-unit element has three content parts:

1. One or more class elements, each one containing the fully qualified name of an entity class of the app (like pl.m.Book in our example app).
2. A set of configuration property elements used for providing further configuration settings.
3. A jta-data-source element for specifying the configuration block in the config/TomEE.xml web server configuration file in the web server installation folder.

In our persistence.xml file, two configuration properties have been set:

–javax.persistence.schema-generation.database.action, with the possible values: none (default), create, drop-and-create and drop. It specifies if the database schema is to be automatically created and additionally allows to drop the existing tables before creating the new ones (with drop or drop-and-create)..

–javax.persistence.schema-generation.create-source, with the possible values metadata (default), script, metadata-then-script and script-then-metadata. It specifies the source of information used to create the database schema. The value metadata enforces using the JPA annotations while the value script allows using an external script for defining the schema.

The jta-data-source element of our persistence.xml file refers to the Resource element with id value “MinimalApp” in the config/TomEE.xml file, which has the following content:

The Resource element contains the information required to connect with the database (user name, password, access URL and the Java class name of the connection driver). Notice that in order to use a connection driver, one needs to download the jar file for that specific technology and copy it into the lib subfolder of the TomEE installation folder. Finally, TomEE needs to be restarted in order to load the new driver. See also our instructions page76, for additional details related to JDBC drivers, and in particular how to use the official MySQL JDBC driver “Connector/J”.

4.4.3Create the main template

The main template, called page.xhtml, is shown below. It has two sub-templates:

1. header.xhtml defines the general header information items (such as the application name)
2. footer.xhtml defines the general footer information items (such as a copyrights notice)

Both sub-templates are included in the main template with the help of a ui:include element. We add all three template files to the WebContent/WEB-INF/ templates folder.

The content of our HTML5-compliant main template page.xhtml is the following:

In the code, one can see that some HTML elements are used (e. g., title, header, main and footer) while others like h:head and ui:insert are not HTML elements, but have been defined by JSF. For instance, JSF defines its own head element h:head for injecting special HTML code such as script elements that load JavaScript code for HTTP messaging between the web browser and the back-end Java program by invoking methods of the JavaScript API XML HTTP Request (XHR).

Our main template defines three content regions: header, main and footer. The header and footer regions are defined by sub-templates included with the help of the ui:include element.

The header.xhtml sub-template contains the application menu, corresponding to the CRUD operations:

The footer.xhtml sub-template contains the “Back to main menu” link, which we like to show on each application page, except the index page:

Notice that the h namespace must be defined once again in footer.xhtml even if it has already been defined in page.xhtml. In the footer template component, we define the “Back to main menu” operation, which is going to be injected into each page that uses this template. The main region is dynamic, and will be replaced with the content generated by a facelet template.

JSF is using the following namespaces:

–xmlns:ui=" http://java.sun.com/jsf/facelets" for the JSF Facelets Tag Library providing templating elements (like ui:insert for specifying the region of a template where to inject the facelet content).

–xmlns:h=" http://java.sun.com/jsf/html" for the JSF HTML Tag Library providing JSF versions of HTML elements, which are then mapped to HTML elements. For example h: inputText, which is mapped to an HTML input element.

–xmlns:f=" http://java.sun.com/jsf/core" for the JSF Core Tag Library providing custom actions or elements that are independent of any particular render kit. For example, f:actionListener can be used to define a Java method which is executed when the user clicks a button.

–xmlns:p=" http://xmlns.jcp.org/jsf/passthrough" for using HTML attributes in JSF HTML elements and passing them through to the generated HTML. For example, with p:type in, <h:inputText p:type="number"> an HTML5 input type attribute can be created in the generated HTML: <input type="number">.

–xmlns:c=" http://java.sun.com/jsp/jstl/core" for the JSTL Core Tag Library providing all kinds of features, like dealing with loops and defining variables. For example, we can use <c:set var="isbn" value="#{book. isbn}"/> to create a variable named isbn which can then be used in the view code for conditional expressions.

–xmlns:fn=" http://java.sun.com/jsp/jstl/functions" for the JSTL Functions Tag Library providing various utility functions, such as string converters. For example, we can use fn:toUpperCase to convert a string to its uppercase representation.

4.4.4Define the managed beans needed in facelets

JavaBean classes, including entity classes, can be used for creating ’managed beans’ with the help of the @ManagedBean annotation, which allows defining the name of a variable for accessing the created bean in the view code, typically in an EL expression. In our example app, we want to access a Book bean as well as a BookController bean, therefore both classes have to be annotated as follows:

Notice how a lifetime scope can be specified for a managed bean with a scope annotation. In our example the book bean is @RequestScoped, this means the instance exists as long as the HTTP request and the associated response are being processed. The bookCtrl bean is @ SessionScoped, which means it is created when the session starts, and destroyed when the session is closed. Other scopes are available, but in our example we only need these two.

4.5Step 4 – Implement the Create Use Case

The CRUD use case Create involves creating a new object in main memory and then saving it to persistent storage with the help of the create method.

The corresponding create action method code from the src/pl/c/BookController.java is shown below:

The BookController::create action method invokes the Book.create model class method for creating and saving a Book instance. It returns the name of the view file found in the same folder as the view that triggered the action. This file (create. xhtml in our case) will be displayed after executing the action. Using the FacesContext object, the form is cleared after creating a Book instance. The code of the create method in src/pl/m/Book.java is the following:

Now we need to create the facelet template for the view of the Create use case, WebContent/views/books/create.xhtml. Such a facelet template essentially defines an HTML form with data binding and action binding.

Data binding refers to the binding of model class properties to form (input or output) fields. For instance, in the following facelet code fragment, the entity property book. isbn is bound to the form input field “isbn”:

In JSF, for the inputText elements of a form, the id attribute is used with a given value, e. g., id="isbn". The rendered HTML5 input elements have both, the id and the name attributes, and their values are obtained by using the form id and element id values separated by a colon, i. e., id="createBookForm:isbn" and name="createBookForm:isbn".

We can also see a first example of an expression in the Java EE Expression Language (EL), where an expression starts with # and is enclosed in curly brackets, like # {expression}. Such an expression allows reading the value of a property of, or invoking a method on, a Java bean or a context object. The value of the expression is assigned to the value attribute of the generated HTML input element. The example in our JSF code above is the expression #{book.isbn}, which retrieves the value of the isbn property of the book bean.

Action binding refers to the binding of method invocation expressions to actionable UI elements, where the invoked methods typically are controller action methods, and the actionable UI elements typically are form buttons. For instance, in the following facelet code fragment, the method invocation expression bookCtrl. create(…) is bound to the form’s submit button:

After discussing data binding and action binding, it’s time to look at the complete code of the facelet:

This facelet replaces the main region of the template defined in page.xhtml, because the name attribute of the ui:define element has been set to “main”, but it also replaces the headerTitle region of the template, which is part of the header and displays the current operations allowed by the page.

h:outputLabel elements can be used for creating form field labels, while h: inputText elements are used for creating HTML input elements. It is possible to specify an HTML5 type of an input element by using a special namespace prefix (xmlns:p = "http://xmlns.jcp.org/jsf/passthrough") for the type attribute, enforcing it to be ’passed through’. In this way the year input field can be defined with type number, so it’s rendered by the corresponding number widget in the browser.

The h:commandButton element allows creating submit buttons rendered as a input elements with type="submit", and binding them to an action to be performed when the button is clicked. The value of the action attribute is a method invocation expression. In our Create use case we want that, when the button is clicked, a Book instance with the property values provided by corresponding form fields is created and saved.

4.6Step 5 – Implement the Retrieve/List All Use Case

For the Retrieve use case we have to add a method in the controller class (src/pl/ c/BookController.java file), which reads all Book records from the books database table and then delivers this information to the view. The controller action method code is shown below:

The getBooks method returns a list of Book instances which are obtained by calling the static retrieveAll method of the Book model class. The code of the Book.retrieveAll method is:

The code is simple, and as already discussed in Section 3.3, it uses a JPQL statement to retrieve the Book records from the books table and create the corresponding Book instances. The EntityManager object required for being able to perform the JPQL query is passed to Book.retrieveAll from the BookController object as discussed in Section 4.1 section.

Now it is the time to define the facelet template for displaying a table with all book records found in the database. The view template files corresponding to the model classes of our app are located in model class subfolders of the WebContent/views/ folder. For the Retrieve/list all books use case, we create a file named retrieveAndListAll.xhtml in WebContent/views/books/ with the following content:

The ui:composition element specifies which template is applied (i. e. template="/WEB-INF/templates/page.xhtml") and which view block (<ui: define name="main">) is replaced by this facelet at render time.

The h:dataTable element defines a table view for a set of records, which is then rendered as an HTML table. Its value attribute defines a data binding to a record collection, while the var attribute defines a variable name for iteratively accessing records from this collection. The expression provided in the value attribute normally specifies a collection-valued property (here: books) which is accessed via a corresponding getter (here: getBooks) as defined in the controller class BookController. In this particular case it is just sufficient to define the getBooks method since there is no need of a books property in the controller class. In any case, value does not allow to invoke a method, so we cannot call getBooks directly. Instead we have to use the (possibly virtual) property books,which internally evaluates to a call of getBooks without checking if a books property really exists.

The h:button element allows to create redirect buttons. The value of the outcome attribute specifies a name of a facelet file (omitting the .xhtml extension, such that, in the example, the file name is index.xhtml).

4.7Step 6 – Implement the Update Use Case

Like for Create, for the Update use case also a controller action method is defined in the BookController class:

The Book. update takes care of saving property value changes for a book object identified by its isbn value as shown below:

Now, we create the view where a Book can be selected so the user can edit the title and year properties, and then save the changes. The code for this view is stored in the WebContent/views/books/update.xhtml file which has the following content:

In this facelet template, a single selection list (that is, a single-select HTML element) is created with the help of the JSF element h: selectOneMenu, where the selection list items (the HTML option elements) are defined by the JSF elements f: selectItem or f:selectItems. The value attribute of h:selectOneMenu binds book. isbn to the value of the selected item (or option element). The selection list is populated with book records with the help of a f:selectItems element bound to bookCtrl.books. The attributes itemLabel and itemValue define the option elements’ text and value.

In the update view, when the user selects a book from the selection list, the form fields are filled with the (ISBN, title and year) property values of the selected book. While the ISBN is immediately available in the view (on the front-end) as the value of the selected option element, the values of the title and year properties have to be fetched from the back-end database. This can be done with the help of the JSF element f:ajax,which sends an HTTP request message for invoking a remote method, bookCtrl.refreshObject, on the back-end using XHR. This method takes the managed book bean, and updates its title and year properties with the current values retrieved from the database. Its code is the following:

To enforce a refresh of the HTML form after the user’s selection, such that it displays the values of isbn, title and year, the f:ajax element allows specifying form fields to be updated with the render attribute like, in our case, render="isbn title year".

Finally, the h:commandButton element is used for invoking the update action method of the BookController with the parameters isbn, title and year, for making the changes persistent.

4.8Step 7 – Implement the Delete Use Case

For the Delete use case, the delete action method of the BookController invokes the Book. delete method by providing the ISBN of the Book object to be deleted:

The Book. delete method first retrieves the Book object to be deleted, and then invokes the entity manager’s remove method on it:

The view for the Delete action provides a selection list for selecting the book to be deleted. A “Delete” button allows performing the deletion of the selected book. The code of the view in WebContent/views/books/delete.xhtml is as follows:

As in the Update use case, a h:selectOneMenu element is used to create and populate a selection list containing all the books to choose from. Clicking on the “Delete” command button results in invoking the delete action method of the controller with the isbn value of the selected book, thus resulting in the deletion of the Book object from the database.

4.9Style the User Interface with CSS

We style the UI with the help of the HTML5 Boilerplate81 CSS and the browser style normalization file normalize.css82. The two CSS files main.css and normalize. min.css are located in the WebContent/resources/css folder.

Adding the CSS to the HTML pages requires to edit the WebContent/WEB-INF/ templates/ page.xhtml file and add the style elements referencing the CSS files:

Notice that we also need to add the CSS to the WebContent/views/books/ index.xhtml file, which is not generated with the page.xhtml template. The facesContext.externalContext.requestContextPath expression allows to get the path to WebContent folder. We then need to append the rest of the path, including the CSS file name, for each of the CSS files.

With JSF, there are two main ways to apply CSS styles to specific elements. For the elements created with JSF, we need to use the @styleClass attribute, e. g.:

For the HTML elements, we simply use the CSS classes, as in any normal HTML document, e.g.:

For creating advanced user interfaces, additional block elements that act as containers are used for defining paddings, margins, various background colors and other styles. Thus, in various HTML pages you may find DOM structures as shown below, where the div element containing the @class attribute is used as container for styling purposes:

4.10Run the App and Get the Code

You can run the minimal app83(or run the minimal app with CSS84) on our server or download the code85 as a ZIP archive file.

Follow our instructions86 to get your environment prepared for running back-end Java EE web apps with JPA and JSF.

4.11Possible Variations and Extensions

4.12Points of Attention

The code of this app should be extended by adding constraint validation. We show how to do this in Chapter 9.

We briefly discuss three further issues.

4.13Practice Projects

In most parts of the following projects you can follow, or even copy, the code of the book data management app discussed above.

For developing the apps, simply follow the sequence of seven steps described above:

Make sure that international characters are supported by using UTF-8 encoding for all HTML files.

4.13.1Project 1 – Managing information about movies

The purpose of the app is managing information about movies. The app deals with just one object type: Movie, as depicted in the following class diagram:

Notice that in most parts of this project you can follow, or even copy, the code of the book data management app, except that in the Movie class there is an attribute with range Date, so you have to find out how to handle such an attribute.

You can use the sample data shown in Table 3.2 for testing your app.

4.13.2Project 2 – Managing information about countries

The purpose of the app to be developed is managing information about countries. The app deals with just one object type: Country, as depicted in the following class diagram:

You can use the sample data shown in Table 3.3 for testing your app.

4.14Quiz Questions

If you would like to look up the answers for the following quiz questions, you can check our discussion forum88. If you don’t find an answer in the forum, you may create a post asking for an answer to a particular question.

4.14.1Question 1: Casting types in Java

Which of the following are valid type casts in Java?

4.14.2Question 2: JPA standard identifier property

Complete the following code, so that the Book JPA entity class contains a property named isbn, with String type and playing the role of the standard identifier:

4.14.3Question 3: Valid Java Bean classes

Which of the following classes are valid JavaBean classes:

4.14.4Question 4: HTML5 attributes with JSF forms

Complete the following code with the JSF construct which results in using date as value of the @type attribute of the rendered input HTML5 element.:

4.14.5Question 5: JPQL query

Consider a JPA entity class named Customer.Write down the JPQL query to extract all its instances managed by the EntityManager (e. g., stored in DB or memory storage):