Accessing Databases

Data that is shared between web components and is persistent between invocations of a web application is usually maintained by a database. Web components use the Java Persistence API to access relational databases. The data for Duke’s Bookstore is maintained in a database and is accessed through the database access class tut-install/javaeetutorial5/examples/web/bookstore1/src/java/com/sun/bookstore1/database/BookDBAO. For example, ReceiptServlet invokes the BookDBAO.buyBooks method to update the book inventory when a user makes a purchase. The buyBooks method invokes buyBook for each book contained in the shopping cart, as shown in the following code.

public void buyBooks(ShoppingCart cart) throws OrderException{

    Collection items = cart.getItems();
    Iterator i = items.iterator();
    
    try {
        while (i.hasNext()) {
            ShoppingCartItem sci = (ShoppingCartItem)i.next();
            Book bd = (Book)sci.getItem();
            String id = bd.getBookId();
            int quantity = sci.getQuantity();
            buyBook(id, quantity);
        }
    } catch (Exception ex) {
        throw new OrderException("Commit failed: " +
            ex.getMessage());
     }
}

public void buyBook(String bookId, int quantity)
     throws OrderException {

    try {
        Book requestedBook = em.find(Book.class, bookId);
        
        if (requestedBook != null) {
            int inventory = requestedBook.getInventory();
            if ((inventory - quantity) >= 0) {
                int newInventory = inventory - quantity;
                requestedBook.setInventory(newInventory);
            } else{
                throw new OrderException("Not enough of "
                     + bookId + " in stock to complete order.");
            }
        }
    } catch (Exception ex) {
        throw new OrderException("Couldn’t purchase book: "
             + bookId + ex.getMessage());
    }
}

To ensure that the order is processed in its entirety, the call to buyBooks is wrapped in a single transaction. In the following code, the calls to the begin and commit methods of UserTransaction mark the boundaries of the transaction. The call to the rollback method of UserTransaction undoes the effects of all statements in the transaction so as to protect the integrity of the data.

try {
    utx.begin();
    bookDB.buyBooks(cart);
    utx.commit();
} catch (Exception ex) {
    try {
        utx.rollback();
    } catch(Exception e) {
        System.out.println("Rollback failed: "+e.getMessage());
    }
    System.err.println(ex.getMessage());
    orderCompleted = false;}
}