ObjectDB for Java/JDO Demo The JDO Directory - A Brief Tutorial

Introduction

The JDO Directory demo shows how to use ObjectDB and JDO to manage a hierarchical directory of links, like Google Directory or Yahoo Directory. The demo consists of four steps, each of them is a separate program. Steps 1-3 are console programs and step 4, whose screenshot is shown below, is a GUI program. Step 1 shows how to create a database file and how to store objects using JDO. Step 2 shows how to use JDO to update and delete database objects. Step 3 demonstrates how to retrieve objects from the database using JDO extents and JDOQL queries. Step 4 combines all these operations into a complete GUI application.

Demo File Structure

The classes are organized in the directory package and its sub packages. Package directory.pc contains the four persistence capable classes implementing the JDO Directory domain object model, as well as a package.jdo XML file that defines these classes as persistence capable. Persistence capable classes in JDO replace tables in RDBMS and JDBC. Instances of these persistence capable classes replace RDBMS's records. The items in the directory database are represented by Item and Book, and the categories are represented by Category. All these classes are sub classes of the Element abstract class. Each Category instance keeps an ArrayList containing references to its sub categories and items. In addition, every element keeps a reverse reference to its parent category.

JDO Directory Demo - Files JDO Directory Demo - UML Diagram

Each demo step has its own package, containing two java files. The eMain class is a program entry point, demonstrating "on the fly" JDO enhancement. First it enhances persistence capable classes when necessary, and then it invokes the other class in the package, containing the "real" code. This technique is very useful at development time as a replacement to the traditional JDO enhancement (no need to use IDE plugins or tools like ANT, project can be built just normally). Common code shared by all the steps is included in the directory.Utilities class.

Running the Demo (using Embedded Database Mode)

You can run the JDO Directory demo with any ObjectDB for Java/JDO edition. If you haven't done so yet, you can download now the Free Database Edition which also includes this demo. Use the following links for instructions on running the JDO Directory demo in your Java environment:

Java 2 SDK Instructions
Java 2 SDK for Windows
Java 2 SDK for Unix

IDE Instructions
Borland JBuilder
IBM Eclipse
NetBeans / Forte / Sun One Studio
JCreator

Running the Demo in Client-Server Mode

By default, the demo runs in embedded mode, and the database file is accessed directly by the application (no database server is required to be running in the background). The server database edition supports also client server mode. Using the server edition, you can run the demo in client server mode, by editing the properties.jdo file in the directory directory, to use a remote url connection instead of a local url connection (see instructions in that file). An ObjectDB Server can be started, for instance, by the [Tools => Start Database Server] command in the Explorer. After the server is started, you can use the Thread card to watch or to stop its thread:

Consult the developer's guide for more details on using ObjectDB in client-server mode.