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Mapping Java Objects to a Database with Castor-JDO:

Transient and Persistent Objects 

A key to understanding how Castor works is to know the difference between a transient and a persistent object. An object becomes persistent in one of two ways: either it is the result of a query or it is created in a transaction. All persistent objects are stored in the database when we use commit. After commit every object is transient.

If we therefore get an object into a transaction through a query, we may modify the object, and commit, will then store it in the database. Like this:

      db.begin(); 
      Media m = (Media)db.load(Media.class,new Integer(1));
      m.setType("DVD-X");
      // Commit the transaction
      db.commit(); 

No need to tell Castor that "m" should be updated in the database. It's persistent, so if it is changed it'll be written back to the database automatically. This is very nice. But it gets even better as we'll see later on! If "m" has relations to other objects, and these are updated in the transaction, then they'll also be written to the database when we commit.

The database configuration file    

We'll of course have to tell Castor some details about the database we're using. Magic doesn't come from nothing. In Listing 2 we had the statement

jdo.setConfiguration("database.xml"); 

which points to the database configuration file. It must be placed in the classpath, and it tells which database system we're using:   

<database name="mydb" engine="mysql" >
  <driver url="jdbc:mysql://localhost/test" 
          class-name="org.gjt.mm.mysql.Driver">
    <param name="user" value="root"/>
    <param name="password" value="" />
  </driver>
  <mapping href="mapping.xml" />
</database>

The <driver> element is database specific, and you may see some of the various formats here. The driver element makes it possible for Castor to connect to the database, but nothing more. The file also contains a reference to a "mapping file", which is the primary key to understanding how Castor works. This is where the magic comes from.

The mapping file

The mapping file is where you specify how your objects should be mapped to a relational database. So on one side we have Java classes with their set of properties, and on the other side we have a relation database system, which consists of tables containing fields (columns) and relations between tables. Since relations are also implemented through fields, the challenge is this: how do we map Java class properties to table fields?

Let's look at the Media class that's used in Listing 2:

package hansen.playground;

import java.util.*;

public class Media {

  private int id;
  private String type;

  public Media() {}
  public Media(int id, String type) { this.id = id; 
                                      this.type = type;}
  
  public int getId() { return id; }
  public void setId(int id) { this.id = id; }

  public String getType() { return type; }
  public void setType(String type) { this.type = type; }

}

- Listing 3: The Media class -

It's not very hard to map this to a database table with two fields: id and type. In the MySQL installation section above, we created a table like this by:

create table media (id int primary key, type char(10) not null);

A Castor mapping file that describes this mapping looks like this:

<mapping>

  <class name="hansen.playground.Media" identity="id">
    <map-to table="media" />
    <field name="id" type="integer">
      <sql name="id" type="integer"/>
    </field>
    <field name="type" type="string">
      <sql name="type" type="char" />
    </field>
  </class>

</mapping>

Here's an explanation to the elements in the mapping:

This is straightforward, but note, that Castor doesn't offer defaults for this mapping. On the Castor web-site (or in the documentation you've downloaded) you'll find a description of the possible values for type. 

Until now this has been very elementary. Let's make it more interesting by adding another class, Movie, with a relation to Media.

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