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> ## The Example Servlets

> This chapter uses the Duke's Bookstore application to illustrate the tasks involved in programming servlets. Table 3-1 lists the servlets that handle each bookstore function. Each programming task is illustrated by one or more servlets. For example, `BookDetailsServlet` illustrates how to handle HTTP `GET` requests, `BookDetailsServlet` and `CatalogServlet` show how to construct responses, and `CatalogServlet` illustrates how to track session information.

> Table 3-1 Duke's Bookstore Example Servlets
> Function | Servlet
> --- | ---
> Enter the bookstore | `BookStoreServlet`
> Create the bookstore banner | `BannerServlet`
> Browse the bookstore catalog | `CatalogServlet`
> Put a book in a shopping cart | `CatalogServlet`, `BookDetailsServlet`
> Get detailed information on a specific book | `BookDetailsServlet`
> Display the shopping cart | `ShowCartServlet`
> Remove one or more books from the shopping cart | `ShowCartServlet`
> Buy the books in the shopping cart | `CashierServlet`
> Send an acknowledgment of the purchase | `ReceiptServlet`

> The data for the bookstore application is maintained in a database and accessed through the database access class `database.BookDBAO`. The `database` package also contains the class `Book` which represents a book. The shopping cart and shopping cart items are represented by the classes `cart.ShoppingCart` and `cart.ShoppingCartItem`, respectively.

> The source code for the bookstore application is located in the `<``INSTALL``>/javaeetutorial5javaeetutorial5/examples/web/bookstore1/` directory, which is created when you unzip the tutorial bundle (see Building the Examples, page xxxi). To build the application, follow these steps:

> 1. In a terminal window, go to`<``INSTALL``>/javaeetutorial5/examples/web/bookstore1/`.
> 2. Run `ant`. This target will spawn any necessary compilations, copy files to the `<``INSTALL``>/javaeetutorial5/examples/web/bookstore1/build/` directory, and create a WAR file and copy it to the `<``INSTALL``>/javaeetutorial5/examples/web/bookstore1/dist/` directory.
> 3. Start the Application Server.
> 4. Perform all the operations described in Accessing Databases from Web Applications (page 54).

> To deploy the example, run `ant deploy`. The deploy target outputs a URL for running the application. Ignore this URL, and instead use the one shown at the end of this section.

> To learn how to configure the example, refer to the deployment descriptor (the `web.xml` file), which includes the following configurations:

> - A `display-name` element that specifies the name that tools use to identify the application.
> - A `context-param` element that specifies the JSTL resource bundle base name.
> - A set of `filter` elements that identify servlet filters contained in the application.
> - A set of `filter-mapping` elements that identify which servlets will have their requests or responses filtered by the filters identified by the `filter` elements. A `filter-mapping` element can define more than one servlet mapping and more than one URL pattern for a particular filter.
> - A `listener` element that identifies the `ContextListener` class used to create and remove the database access.
> - A set of `servlet` elements that identify all the servlet instances of the application.
> - A set of `servlet-mapping` elements that map the servlets to URL patterns. More than one URL pattern can be defined for a particular servlet.
> - A set of error-page mappings that map exception types to an HTML page, so that the HTML page opens when an exception of that type is thrown by the application.
> - A `resource-ref` element that declares the resource reference for the database used in Duke's Bookstore.

> To run the application, open the bookstore URL `http://localhost:8080/bookstore1/bookstore`.

> ### Troubleshooting

> The Duke's Bookstore database access object returns the following exceptions:

> - `BookNotFoundException`: Returned if a book can't be located in the bookstore database. This will occur if you haven't loaded the bookstore database with data or the server has not been started or has crashed. You can populate the database by running `ant` `create-tables`.
> - `BooksNotFoundException`: Returned if the bookstore data can't be retrieved. This will occur if you haven't loaded the bookstore database with data or if the database server hasn't been started or it has crashed.
> - `UnavailableException`: Returned if a servlet can't retrieve the web context attribute representing the bookstore. This will occur if the database server hasn't been started.

> Because we have specified an error page, you will see the message

>     The application is unavailable. Please try later.

> If you don't specify an error page, the web container generates a default page containing the message

>     A Servlet Exception Has Occurred

> and a stack trace that can help you diagnose the cause of the exception. If you use `errorpage.html`, you will have to look in the server log to determine the cause of the exception.