Web App State Management

• CE212 Web Application Programming Part 6

Handling State in Web Applications

• Web shopping transactions require tracking data across multiple pages.
• Customer data must be kept separate on the server.
• The web uses a connectionless model where each request is a separate activity.
• State handling involves identifying with which transaction a request is associated.

Cookies for State Management

• Cookies can uniquely identify a transaction.
• The server sends a cookie with the first HTTP request's response.
• The browser sends the cookie back with subsequent requests.
• The servlet API uses the Session class for this purpose.
• The getSession() method returns a Session object associated with the cookie.
• Calling getSession(true) creates a new session object and sends its cookie if none exists.
• Calling getSession(false) returns null if no cookie has been sent.

Creating a Transaction Object

• When logging in, user and password values can be retrieved from the request.
• Code can be used to create a transaction object and store it in a session object.
• Example code:

if (validPassword(user, password)) {
    Session session = request.getSession(true);
    session.putValue("trans", new Transaction(user));
}

• The putValue method's first argument must be a string, the second can be any object.
• Transaction is a written class.

Retrieving Transaction Data

• Subsequent requests can retrieve the transaction from the session object.
• The object must be downcast, as getValue returns a generic Object type.
• Example code:

Session session = request.getSession(false);
if (session != null) {
    Transaction t = (Transaction) session.getValue("trans");
    // Update t with request data
} else {
    // Handle case where user is not logged in
}

Disadvantages of Cookies and Alternatives

• Cookies are stored on the user's machine.
• Users can modify, delete, or disable cookies.
• Modern sites use alternative methods to handle state.
• One approach is to include transaction identity material in a hidden control within each page.
• When the form is submitted, the invisible transaction identity is sent to the server.

Hidden Form Control Example in JSP

• A hidden control can be placed on a form in a JSP page.
• Example HTML:

<form ...>
    <input type="hidden" name="trans" value="<%=transId%>">
</form>

• The value in the hidden control cannot be seen or changed by the user.
• The transId string can be a key for a map with the transaction object associated with the session.

Shopping Basket Applications and JavaBeans

• In shopping basket applications, users may return to previously-visited pages.
• The hidden form data approach will not work in such scenarios.
• The preferred approach is to use a JavaBean with session scope.
• The bean represents a shopping basket and is initialized to be empty upon the first request.
• Subsequent requests use the same bean; the shopping basket is not reinitialized.

Shopping Basket Example: Implementation Details

• A Product class with instance variables should correspond to the product table in the shop database.
• A class can be created to communicate with the database, shared by all bean objects.
• A static method creates an object only if one does not already exist, otherwise returns the existing object.
• A single connection object can be created when the object is created.
• Creating a connection pool is better for a shop with a large number of customers

Order Handling in the Shopping Basket

• Each order needs a unique id.
• The customer ID can be stored within the row containing the ordered item.
• A more realistic database would use a separate table mapping order IDs to customer IDs.
• The order method returns a boolean indicating if the order was successfully stored.

Java Code for Database Interaction

• The ShopDB class is responsible for database communication.
• It uses JDBC to connect to the database.
• It has a singleton pattern to ensure only one instance exists.
• It includes methods to retrieve product information from the database.

Retrieving Products

• The getProduct method retrieves product information from the database.
• It uses a SQL query to select the product based on its ID.
• The getProductCollection method executes the query and returns a list of Product objects.

Implementation of getProductCollection

• The getProductCollection method queries the database.
• CreatesProduct objects from the row results.
• Returns Java LinkedList product objects.

Java Bean: Shopping Basket

• The shopping basket is a Java Bean.
• The items in the basket are stored as a collection of Product objects.
• The price member of the Product class is of type double.
• The shopping basket class includes methods to:
  • Add items
  • Get the total value of the basket
  • Clear the basket

JSP Pages for Shopping Cart

• basket.jsp adds an item to the basket and displays the updated basket.
• Employs a non-generic iterator to avoid referencing the Product class in the page.
• Contains two forms
  • Clear the basket, which submits to the same JSP.
  • Place order which submits to order.jsp
• Should include a link back to the product listing.

JSP page code

• Page imports collections and iterates over the Basket object.

<%@ page import="java.util.Collection, java.util.Iterator"%>
<jsp:useBean id='basket' scope='session' class='shop.Basket'/>

• The basket adds products with the following method.

basket.addItem(request.getParameter("addItem"));

Adding items to the basket

• Users can "empty basket" or "place order"
• Includes basic html and form objects

Generating the "order" page

• Generates the order with custName to persist users details
• Uses beans to order the database

<jsp:useBean id='basket' scope='session' class='shop.Basket'/>
<jsp:useBean id='db' scope='page' class='shop.ShopDB' />