3.4. Using the Finder

The reversetutorial.Finder class lets you run queries in JDOQL (JDO's Java-centric query syntax) against the existing database:

java reversetutorial.Finder <jdoql-query>

JDOQL is discussed in Chapter 11, Query of the JDO Overview. JDOQL looks exactly like Java boolean expressions. To find magazines matching a set of criteria in JDOQL, just specify conditions on the reversetutorial.Magazine class' persistent fields. Some examples of valid JDOQL queries for magazines include:

java reversetutorial.Finder "true"  // use this to list all the magazines
java reversetutorial.Finder "price < 5.0"
java reversetutorial.Finder "name == 'Vogue' || issue > 1000"
java reversetutorial.Finder "name.startsWith ('V')"

To traverse object relations, just use Java's dot syntax:

java reversetutorial.Finder "publisher.name == 'Adventure' || publisher.revenue > 1000000"

To traverse collection relations, you have to use JDOQL variables. Variables are just placeholders for any member of a collection. For example, in the following query, art is a variable:

java reversetutorial.Finder "articles.contains (art) && art.title.startsWith ('JDO')"

The above query is equivalent to "find all magazines that have an article whose title starts with 'JDO'". The reversetutorial.Finder pre-defines two variables you can use: Article art; Magazine mag;. With these, you can create very complex queries. For example, to find all magazines whose publisher published an article about "Kodo" in any of its magazines:

java reversetutorial.Finder "publisher.magazines.contains (mag) && mag.articles.contains (art) && art.articleSubtitles.contains ('Kodo')"

Have fun experimenting with additional queries.


If you plan on using Kodo JDO to modify data in a schema with non-deferred foreign key constraints, make sure to add the following line to kodo.properties:

kodo.jdbc.ForeignKeyConstraints: true

This line tells Kodo JDO to order its SQL to meet the foreign key constraints of your schema. Note that though this tutorial uses foreign keys with the included Hypersonic database, SolarMetric recommends that you do not use foreign keys with Hypersonic in general use. In fact, in their default configurations the Kodo tools will refuse to create foreign keys in a Hypersonic database. This is because foreign keys seem to destabalize the database under heavy load.

Also, make sure to erase the kodo.jdbc.MappingFactory setting in kodo.properties before continuing with your own JDO experimentation, unless you'd like to always store your mapping information in your JDO metadata files rather than the database.