Reviews :

Mapping Java Objects to a Database with Castor-JDO:

The Source of Magic

I was curious about what was going on behind the scenes. How many SQL SELECTs were being issued? One way to find out is to turn on logging. You do this simply by adding these statements in the top of your program:

import org.exolab.castor.util.Logger;
import java.io.*;
. . .
PrintWriter writer = new Logger(System.out).setPrefix("test");      
jdo.setLogWriter(writer);

If we run the program once more we'll see this:

[test] SQL for creating hansen.playground.Movie: INSERT INTO movie ...
(more lines cut out here)
[test] SELECT actor.id,actor.name,actor_movie.id_movie FROM actor 
       LEFT OUTER JOIN actor_movie 
       ON actor.id=actor_movie.id_actor WHERE (actor.name = ?)
[test] Castor: Loading hansen.playground.Actor (23)
[test] Castor: Loading hansen.playground.Movie (12)
[test] Castor: Loading hansen.playground.Media (1)
[test] Castor: Loading hansen.playground.Actor (22)
[test] Castor: Loading hansen.playground.Movie (11)
[test] Castor: Loading hansen.playground.Media (2)
[test] Castor: Loading hansen.playground.Movie (13)
[test] Castor: Loading hansen.playground.Actor (21)

The first lines that are listed are all the SQL statements that Castor will use when reading,  creating, updating, and deleting objects from the tables. I've only shown part of the first create statement above. Following this is the (first) SELECT statement that is the result of the query for Keanu Reeves. Finally there's a line each time an object is created. The keys are listed in parenthesis. Here's a "translation":

[test] Castor: Loading hansen.playground.Actor (Ian Holm)
[test] Castor: Loading hansen.playground.Movie (Lord of the Rings)
[test] Castor: Loading hansen.playground.Media (DVD)
[test] Castor: Loading hansen.playground.Actor (Laurence Fishburne)
[test] Castor: Loading hansen.playground.Movie (The Matrix)
[test] Castor: Loading hansen.playground.Media (VHS)
[test] Castor: Loading hansen.playground.Movie (Chain Reaction)
[test] Castor: Loading hansen.playground.Actor (Keanu Reeves)

The lines seems to come in opposite order, but this might simply reflect the way Castor is coded, e.g. by using recursive methods. You might also wonder why all objects are loaded, even the movie where Keanu Reeves doesn't act. The reason is that if you use the DVD object to get all movies in DVD format then you'll need "Lord of the Rings". And when you get this movie you'll also need Ian Holm. And so on.

Getting more information

If you're familiar with SQL then you'll see that one SELECT is not enough to get all the objects we need. Let's therefore use a tool like P6Spy. It's a utility that sits between a JDBC program and the database and watches and reports on the "traffic". To use it you replace the name of the JDBC driver in Castor's database.xml file with the P6Spy driver, and tell P6Spy to use the real JDBC driver. I won't go into details about installation of P6Spy. Suffice to say, it's very simple to do.

What P6Spy tells you (in its spy.log file) when you run the ReadActor program is this:

SELECT actor.id,actor.name,actor_movie.id_movie 
       FROM actor 
       LEFT OUTER JOIN actor_movie ON actor.id=actor_movie.id_actor 
       WHERE (actor.name = 'Keanu Reeves')
SELECT movie.title,movie.id_media,actor_movie.id_actor 
       FROM movie 
       LEFT OUTER JOIN actor_movie ON movie.id=actor_movie.id_movie 
       WHERE movie.id='11'
SELECT media.type,movie.id 
       FROM media 
       LEFT OUTER JOIN movie ON media.id=movie.id_media 
       WHERE media.id='1'
(5 more follows)
commit

The first select is the same as we've seen before. The others are used to get data for all the objects in the model (3 actors, 3 films, 2 medias).

This brings up a question about performance. What if you don't have a need for all the objects? Couldn't we postpone loading of objects until they're actually needed? The answer is yes, there's a mechanism called "lazy loading", which is one of the topics we'll look into in my next article. So stay tuned on JavaBoutique.com!  

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