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Rowsets make it easy to send tabular data over a network. They can also be used to provide scrollable result sets or updatable result sets when the underlying JDBC driver does not support them. These are the main uses for a rowset, and this tutorial will walk you through examples of these uses. Because there can be so many variations in rowsets, the first part of this chapter gives a conceptual description of rowsets and their uses. The next part walks you through creating and using a rowset. The last part shows you how a rowset can be used in a distributed Enterprise JavaBeans (EJB) application.
5.1 Types and Uses of Rowsets
A RowSet object contains a set of rows from a result set or some other source of tabular data, like a file or spreadsheet. Because a RowSet object follows the JavaBeans model for properties and event notification, it is a JavaBeans component that can be combined with other components in an application. As is true with other Beans, application programmers will probably use a development tool to create a RowSet object and set its properties.
Rowsets may have many different implementations to fill different needs. These implementations fall into two broad categories, rowsets that are connected and those that are disconnected. A disconnected rowset gets a connection to a data source in order to fill itself with data or to propagate changes in data back to the data source, but most of the time it does not have a connection open. While it is disconnected, it does not need a JDBC driver or the full JDBC API, so its footprint is very small. Thus a rowset is an ideal format for sending data over a network to a thin client.
Because it is not continually connected to its data source, a disconnected rowset stores its data in memory. It needs to maintain metadata about the columns it contains and information about its internal state. It also needs a facility for making connections, for executing commands, and for reading and writing data to and from the data source. A connected rowset, by contrast, opens a connection and keeps it open for as long as the rowset is in use.
Although anyone can implement a rowset, most implementations will probably be provided by vendors offering RowSet classes designed for fairly specific purposes. To make writing an implementation easier, the Java Software division of Sun Microsystems, Inc., plans to provide reference implementations for three different styles of rowsets in the future. The following list of planned implementations gives you an idea of some of the possibilities.
5.2 Using a Rowset
As the conceptual description of rowsets pointed out, what you can do with a rowset depends on how it has been implemented. It can also depend on which properties have been set. The example rowsets used in this chapter are based on the CachedRowSet implementation, but because they are used for different purposes, one has several properties set whereas the other has none. Among other things, this tutorial will show you which properties to use and when to use them.
Getting back to our owner of The Coffee Break chain, he has had one of his developers write an application that lets him project the effects of changing different coffee prices. To create this application, the developer hooked together various JavaBeans components, setting their properties to customize them for his application. The first JavaBeans component, called Projector, was one that the owner bought from an economic forecasting firm. This Bean takes all kinds of factors into account to project future revenues. Given the price and past sales performance of a coffee, it predicts the revenue the coffee is likely to generate and displays the results as a bar chart.
The second JavaBeans component is a CachedRowSet. The owner wants to be able to look at different coffee pricing scenarios using his laptop, so the application is set up such that it creates a rowset that can be copied to the laptop's disc. The owner can later fire up the application on his laptop so that he can make updates to the rowset to test out various pricing strategies.
The third Bean is a form for displaying and updating ResultSet objects. The form can be used for displaying and updating our CachedRowSet because CachedRowSet is simply a specialized implementation of ResultSet.
The application has a graphical user interface that includes buttons for opening and closing the application. These buttons are themselves JavaBeans components that the programmer assembled to make the GUI for his application.
While he is at work, the owner can click on the form's New Data button to get a rowset filled with data. This is the work that requires the rowset to get a connection to the data source, execute its query, get a result set, and populate itself with the result set data. When this work is done, the rowset disconnects itself. The owner can now click on the Close button to save the disconnected rowset to his laptop's disc. At home or on a plane, the owner can open the application on his laptop and click the button Open to copy the rowset from disc and start making updates using the form. The form displays the rowset, and he simply uses arrow keys or tabs to highlight the piece of data he wants to update. He uses the editing component of the form to type in new values, and the Projector Bean shows the effects of the new values in its bar chart. When he gets back to headquarters, the owner can copy his updated rowset to his office computer if he wants to propagate the updates back to the database.
As part of the implementation, the application programmer will do the following:
To put this all together, the application programmer will probably use a visual Bean development tool, which means that he will use very little RowSet API directly. Of course, the owner will use the application without writing any RowSet code himself. The upshot of all of this is that generally tools will generate the RowSet code you see in this tutorial. Also, remember that the code shown here is for illustrative purposes only because it uses the CachedRowSet class, for which there is no implementation currently available. Although the JDBC Standard Extension specification gives a preliminary outline of its functionality, some details in its implementation may be different when it is completed....