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Sharing Information

Web components, like most objects, usually work with other objects to accomplish their tasks. There are several ways they can do this. They can use private helper objects (for example, JavaBeans components), they can share objects that are attributes of a public scope, they can use a database, and they can invoke other Web resources. The Java Servlet technology mechanisms that allow a Web component to invoke other Web resources are described in Invoking Other Web Resources.

Using Scope Objects

Collaborating Web components share information via objects maintained as attributes of four scope objects. These attributes are accessed with the [get|set]Attribute methods of the class representing the scope. Table 3-3 lists the scope objects.

Table 3-3 Scope Objects 
Scope Object
Class
Accessible From
Web context
Web components within a Web context. See Accessing the Web Context .
session
Web components handling a request that belongs to the session. See Maintaining Client State .
request
Web components handling the request.
page
The JSP page that creates the object. See Implicit Objects .

Figure 3-1 shows the scoped attributes maintained by the Duke's Bookstore application.

Duke's Bookstore Scoped Attributes

Figure 3-1 Duke's Bookstore Scoped Attributes

Controlling Concurrent Access to Shared Resources

In a multithreaded server, it is possible for shared resources to be accessed concurrently. Besides scope object attributes, shared resources include in-memory data such as instance or class variables, and external objects such as files, database connections, and network connections. Concurrent access can arise in several situations:


Note: This interface does not prevent synchronization problems that result from Web components accessing shared resources such as static class variables or external objects.


When resources can be accessed concurrently, they can be used in an inconsistent fashion. To prevent this, you must control the access using the synchronization techniques described in the Threads lesson in The Java Tutorial.

In the previous section we showed five scoped attributes shared by more than one servlet: bookDB, cart, currency, hitCounter, and orderCounter. The bookDB attribute is discussed in the next section. The cart, currency, and counters can be set and read by multiple multithreaded servlets. To prevent these objects from being used inconsistently, access is controlled by synchronized methods. For example, here is the util.Counter class:

public class Counter {
  private int counter;
  public Counter() {
    counter = 0;
  }
  public synchronized int getCounter() {
    return counter;
  }
  public synchronized int setCounter(int c) {
    counter = c;
    return counter;
  }
  public synchronized int incCounter() {
    return(++counter);
  }
} 

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 JDBC 2.0 API to access relational databases. The data for the bookstore application is maintained in a database and accessed through the helper class database.BookDB. For example, ReceiptServlet invokes the BookDB.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. To ensure the order is processed in its entirety, the calls to buyBook are wrapped in a single JDBC transaction. The use of the shared database connection is synchronized via the [get|release]Connection methods.

public void buyBooks(ShoppingCart cart) throws OrderException {
  Collection items = cart.getItems();
  Iterator i = items.iterator();
  try { 
    getConnection(); 
    con.setAutoCommit(false); 
    while (i.hasNext()) {
      ShoppingCartItem sci = (ShoppingCartItem)i.next();
      BookDetails bd = (BookDetails)sci.getItem();
      String id = bd.getBookId();
      int quantity = sci.getQuantity();
      buyBook(id, quantity);
    }
    con.commit();
    con.setAutoCommit(true); 
    releaseConnection(); 
  } catch (Exception ex) { 
    try { 
    con.rollback(); 
    releaseConnection(); 
    throw new OrderException("Transaction failed: " +
      ex.getMessage()); 
    } catch (SQLException sqx) { 
      releaseConnection(); 
      throw new OrderException("Rollback failed: " +
        sqx.getMessage()); 
    } 
  } 
} 
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