Reviews :

Mapping Java Objects to a Database with Castor-JDO:

Reading from the database

Having data in the media, movie, and actor tables we'll now see how easy it is to create objects with data from the database. Assume that we want to find all movies where Keanu Reeves is an actor. The only thing we need to get started is to query his name, and magic happens: all objects are ready to use. The next program lists the movies and the medias where Keanu Reeves appears:

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

public class ReadActor {
  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(); 
      
      OQLQuery query; 
      QueryResults results; 
      query = db.getOQLQuery(
        "SELECT a FROM hansen.playground.Actor a WHERE a.name=$1");
      query.bind("Keanu Reeves");  
      results = query.execute(Database.ReadOnly);
      Actor actor = (Actor)results.next();
      
      System.out.println("Actor: " + actor.getName());
      Collection movies = actor.getMovies();
      for (Iterator it = movies.iterator(); it.hasNext();) {
        Movie movie = (Movie)it.next();
        System.out.println("Movie: " + movie.getTitle());
        Media media = movie.getMedia();
        System.out.println("Media: " + media.getType());
      }  

      // Commit the transaction
      db.commit(); 
      db.close(); 

    } catch (PersistenceException e) {
      e.printStackTrace(); 
    }  
  }
}

- Listing 8: Reading from the database -

The output from the program is this:

Actor: Keanu Reeves
Movie: The Matrix
Media: DVD
Movie: Chain Reaction
Media: VHS

The execute method specifies "readonly", since we're not updating any data. There might be a performance gain in doing this.

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