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Mapping Java Objects to a Database with Castor-JDO:

Installation of Castor-JDO

To get a head start we'll also start by downloading Castor from this address: http://castor.exolab.org/download.html. Select the link "The Castor JARs, docs, DTDs, command line tools and examples". It'll give you the jar-files for both Castor-JDO and Castor-XML, documentation (including javadoc) and a few examples. Unzip the files, locate the Castor-JDO jar file (mine is called castor-0.9.4.3.jar), put it in you classpath, and then compile this program:

package hansen.playground;
import org.exolab.castor.jdo.*;

public class CreateMedia {
  public static void main(String[] args) {
    try {
      // Define the JDO object
      JDO jdo = new JDO("mydb"); 
      jdo.setConfiguration("database.xml"); 

      // Open a connection to the database
      Database db = jdo.getDatabase();
      
      // Begin a transaction
      db.begin(); 
      Media m = new Media(2,"VHS");
      db.create(m); 
      // Commit the transaction
      db.commit(); 
      db.close(); 
    } catch (PersistenceException e) {
      e.printStackTrace(); 
    }  
  }
}

- Listing 2: The first Castor program -

You don't get it shorter than that. Nevertheless it illustrates the basic layout of a Castor program. The program creates an instance of object Media, and stores it in a database. The Media class will be shown below.

Let's look at the program structure:

• The JDO object is the one that creates the connections to the database. The name "mydb" refers to a database definition found in the configuration file "database.xml". We'll look into this file in a second.
• Just like we're used to it when using JDBC we need to have a connection to the database--this is what getDatabase gives us.  
• In order for us to query or store data in the database we must work in an open transaction. We use the begin method to start a transaction.
• To tell Castor that an object should be stored in the database (when we commit) we must use the create method.
• commit writes data to the database. After commit you may start a new transaction, or use...
• close to close the database connection.

My error handling is of course only sufficient for a small test program like this. In a properly designed program you'd have to be able to close connections and maybe rollback transactions if severe errors occur. The small program above may actually throw four different exceptions.

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