Java(TM) Boutique - Mapping Java Objects to a Database with Castor-JDO

Reviews :

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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