Banner architecture

The architecture for our banner ad servlet is very simple. We will use four classes:

• A general purpose Logger class that will write log messages to a text file.
  
  
• A servlet called BannerServlet that will be called every time a banner image is displayed (that is, every time the page loads) and every time a banner image is clicked. This servlet is the heart of our application.
  
  
• A general purpose DBHandler class that our BannerServlet will use to communicate with our MySQL database.
  
  
• A Banner class, which we will use to create objects that contain all the metadata we have for each banner in the database.

The BannerServlet class and the Banner class are specific to our application. They are fairly straightforward and you can easily expand them to add more complex features.

The great part about the DBHandler and Logger classes are that you can reuse them in practically any application you write in which you need to communicate with a database or write to a log file.

We will go into a more detailed discussion about all four of these classes so you can understand how the servlet works and how it uses DBHandler to communicate with our MySQL database.

Logger class

The Logger class is very simple. It has a single field for the File object we are logging to. You can pass a reference to a single Logger object to several classes and have them all write to the same log file. The Logger class allows you to do several things. You can:

• Create a logger
• Add a divider to the log file (a string of "------"'s)
• Add a log entry by passing in the name of the calling method and the log message
• Add a default message for the start of a method
• Add a default message for the end of a method
• Delete the log file
• Return the File object used by the logger

We will use a Logger object in both the DBHandler class and the BannerServlet.

DBHandler class

DBHandler is a very versatile class and can be used to interface with just about any database through JDBC. It requires a string with the name for the JDBC/ODBC driver that we are using to connect to our database, a string with the name of the database for which we set up the DSN, and a Logger parameter. The Logger parameter tells DBHandler where to print output messages while working its magic. The constructor for DBHandler opens a connection to the database. When you are done using DBHandler, you must close it using the close() method.

After you create a DBHandler object, you must create a query to execute. Use the setQueryString() method to pass in a string with your query in it, which can be in the form of a PreparedStatement class.

A PreparedStatement is a great feature of JDBC. It allows you to define a query string, using question mark characters in place of variable criteria in the query. You can then use setter methods on the PreparedStatement class to set the values of the unknown elements in the query. Fortunately, the DBHandler class will take care of all of this for us. We just set the query we want and call one of the DBHandler methods, as shown below:

public Banner getBannerByName(String name) {
        ...
        ...
        dbHandler.setQueryString("SELECT * FROM ADS WHERE NAME=?");
        ResultSet rs = dbHandler.lookup(name);
        dbHandler.close();
        ...
        ...
}

You can perform SELECT queries using the lookup() method, UPDATE queries using the executeUpdate() method, and INSERT queries using the insert() method. There is also an execute() method that takes no parameters and executes any query that doesn't have any PrepareStatement arguments.

Banner class

The Banner class is simply a bunch of setter and getter methods that directly correspond to the column values in the ADS database table.

BannerServlet class

The BannerServlet is the heart of our application. We will break down the different parts of this class for you. By browsing through the code, you will become more familiar with how the DBHandler class is used to connect to the database.

Fields
The BannerServlet uses five fields:

• String _databaseUrl: The name of the database that we will access (jdbc:odbc:\\localhost\BANNER).
  
  
• String _driverName: The name of the driver that we will use to talk to the database. As described above, we will use the MM MySQL JDBC driver. The name of the driver is org.gjt.mm.mysql.Driver.
  
  
• Logger _logger: The name of the Logger class we will use to log all events that occur in our application.
  
  
• HashMap _banners: A HashMap of all the Banner objects. This HashMap will be populated in the init() method of our servlet. Each row in our database table will translate to a single Banner object that is stored in our HashMap. We'll elaborate on this in a moment.
  
  
• int _totalWeight: The sum of all the Banner weights. This value is also set in the init() method; we'll discuss this shortly.

init()
The init(ServletConfig) method of any servlet is called when the servlet is first loaded by the container. The container in our case is Tomcat. Tomcat generates and passes in a ServletConfig object that contains both default configuration information that the container sets and custom configuration information that the developer (you) can set up in the config files for the servlet. For our purposes, we will not need to pass in any configuration information, but you may want to expand the servlet at some point and use this functionality.

The first thing we do in init() after calling super.init() is initialize our HashMap variable _banners and set our _totalWeight to 0. Then, we connect to the database and get all the rows from our ADS table in the form of a ResultSet. We iterate through our ResultSet using a for loop, construct a Banner object out of each row, then add the Banner object into the HashMap using the index of our for-loop as the hash value. (We could have just as easily used a Vector or some other Collection class to accomplish the same thing.)

We now have a HashMap of all of our Banners in memory. If we update our database, we can just call the init() method to reload our HashMap again. We will use this in our increaseImpressions() and decreaseClicksRemaining() methods.

service()
The service() method is defined in the HttpServlet class that our BannerServlet extends and can process any request, whether it be a GET or POST method. Our implementation of the service() method has two core parts. The first part handles what the servlet does when the Web page sends an image request, and the second part handles a link request.

Finally, we look at the type parameter sent in from the client. If the value of type was image, we get a random Banner object from the database, add the Banner object to the user's session, increase the number of impressions for the given banner, and route the user to the image referenced in the image field of the Banner object.

If the value of type was link, we get the Banner object off of the session, decrease the clicks remaining on the banner, and redirect the user to the link specified in the URL field of the Banner object.

Other methods
getRandomBanner(), increaseImpressions(), and decreaseClicksRemaining() are all helper methods called from the service method. getRandomBanner() uses a simple algorithm to select a random banner from our _banners HashMap. Both increaseImpressions() and decreaseClicksRemaining() use DBHandler to connect to the database and update information for the given Banner. At the end of these two methods, we call the init() method to reload the updated Banner information into our HashMap.