In this tutorial you will become familiar with the basic tools and development processes under Kodo by creating a simple JDO application.
Imagine that you have decided to create a software toolkit to be used by pet shop operators. This toolkit must provide a number of solutions to common problems encountered at pet shops. Industry analysts indicate that the three most desired features are inventory maintenance, inventory growth simulation, and behavioral analysis. Not one to question the sage advice of experts, you choose to attack these three problems first.
According to the aforementioned experts, most pet shops focus on three types of animals only: dogs, rabbits, and snakes. This ontology suggests the following class hierarchy:
Animal ^ | +--------------------+ | | | Dog Rabbit Snake
We have provided an implementation of Animal
and Dog
classes, plus some helper classes
and files to create the initial schema and populate the database
with some sample dogs. Let's take a closer look at these classes.
tutorial.jdo.AnimalMaintenance
:
Provides some utility methods for examining and
manipulating the animals stored in the database. We will
fill in method definitions in
Section 2.2.3, “Inventory Maintenance”.
tutorial.jdo.Animal
: This
is the superclass of all animals that this pet store
software can handle.
tutorial.jdo.Dog
: Contains data
and methods specific to dogs.
tutorial.jdo.Rabbit
: Contains data and
methods specific to rabbits. It will be used in
Section 2.2.4, “Inventory Growth”.
tutorial.jdo.SeedDatabase
: Populates the tutorial database with
an initial set of values.
tutorial.jdo.Snake
: Contains data and
methods specific to snakes. It will be used in
Section 2.2.5, “Behavioral Analysis”.
package.jdo
: This is a JDO metadata
file that defines which types should be enhanced into
persistence-capable or persistence-aware classes. For
more information on JDO metadata, consult
Chapter 5, Metadata of the JDO Overview.
../../META-INF/jdo.properties
: This
properties file contains Kodo-specific and standard JDO
configuration settings.
The solutions
directory contains the complete solutions to this tutorial,
including finished versions of the
.java
files listed above and a
correct package.jdo
metadata file:
java: Runs main methods in specified Java classes.
javac: Compiles .java
files into .class
files that can be executed by java.
kodoc: Runs the Kodo enhancer against the specified classes. More information is available in Section 5.2, “Enhancement” of the Reference Guide.
mappingtool: A utility that can be used to create and maintain the object-relational mappings and schema of all persistent classes in a JDBC-compliant datastore. This functionality allows the underlying mappings and schema to be easily kept up-to-date with the Java classes in the system. See Chapter 7, Mapping of the Reference Guide for more information.
Let's compile the initial classes and see them in action. To do
so, we must compile the .java
files, as we would
with any Java project, and then pass the resulting classes through
the JDO enhancer:
Note | |
---|---|
Be sure that your |
Change to the tutorial/jdo
directory.
All examples throughout the tutorial assume that you are in this directory.
Examine Animal.java
, Dog.java
, and SeedDatabase.java
These files are good examples of the simplicity JDO engenders.
As noted earlier, persisting an object or manipulating an
object's persistent data requires almost no JDO-specific code.
For a very simple example of creating persistent objects,
please see the main method of SeedDatabase.java
. Note the objects are created with normal Java
constructors. The files Animal.java
and
Dog.java
are also good examples of
how JDO allows you to manipulate persistent data without
writing any specific JDO code.
Compile the .java
files.
javac *.java
You can use any java compiler instead of javac.
Enhance the JDO classes.
kodoc -p jdo.properties package.jdo
This step runs the Kodo enhancer on the
package.jdo
file mentioned above. The
package.jdo
file contains an enumeration
of all the classes that should be JDO enhanced. The Kodo
enhancer will examine the metadata defined in this file and
enhance all classes listed in it appropriately. See
Section 5.2, “Enhancement”
of the Reference Guide for more information on the JDO
enhancer, including how to use Kodo's automatic runtime
enhancement.
Note | |
---|---|
The |
Now that we've compiled the source files and enhanced the JDO classes, we're ready to set up the database. Hypersonic SQL, a pure Java relational database, is included in the Kodo distribution. We have included this database because it is simple to set up and has a small memory footprint; however, you can use this tutorial with any of the relational databases that we support. You can also write your own plugin for any database that we do not support. For the sake of simplicity, this tutorial only describes how to set up connectivity to a Hypersonic SQL database. For more information on how to connect to a different database or how to add support for other databases, see Chapter 4, JDBC of the Reference Guide.
Create the object-relational mappings and database schema.
mappingtool -p jdo.properties package.jdo
This command creates object-relational mappings for the
classes listed in package.jdo
, and at
the same time propagates the necessary schema to
the database configured in jdo.properties
. If you are using the default Hypersonic SQL
setup, the first time you run the mapping tool Hypersonic
will create tutorial_database.properties
and tutorial_database.script
database files in your current directory. To
delete the database, just delete these files.
By default, JDO stores object-relational mapping
information in your .jdo
files.
As you will see in the
Reverse Mapping Tool Tutorial, you can also
configure Kodo to store object-relational mappings in
separate files or in a database table.
Chapter 7, Mapping of the Reference Guide
describes your mapping options in detail.
If you'd like to see the mapping information Kodo has
just created, examine the
package.jdo
file.
Chapter 15, Mapping Metadata of the JDO Overview
will help you understand mapping XML,
should the need ever arise. Most Kodo development does not
require any knowledge of mappings.
If you are curious, you can also view view the schema Kodo created for the tutorial classes with Kodo's schema tool:
schematool -p jdo.properties -a reflect -f tmp.schema
This will create a tmp.schema
file
with an XML representation of the database schema. The
XML should be self explanatory; see
Section 4.15, “XML Schema Format”
of the Reference Guide for details. You may delete the
tmp.schema
file before proceeding.
Populate the database with sample data.
java tutorial.jdo.SeedDatabase
Congratulations! You have now created a JDO-accessible persistent store, and seeded it with some sample data.
The most important element of a successful pet store product, say the
experts, is an inventory maintenance mechanism. So, let's work on the
Animal
and Dog
classes a
bit to permit user interaction with the database.
This chapter should familiarize you with some of the basics of the JDO specification and the mechanics of compiling and enhancing persistence-capable objects. You will also become familiar with the mapping tool for propagating the JDO schema into the database.
First, let's add some code to AnimalMaintenance.java
that allows us to examine the animals currently in the
database.
Add code to AnimalMaintenance.java
.
Modify the getAnimals
method of
AnimalMaintenance.java
to look like this:
/** * Return a list of animals that match the specified query filter. * * @param filter the JDO filter to apply to the query * @param cls the class of animal to query on * @param pm the PersistenceManager to obtain the query from */ public static List getAnimals(String filter, Class cls, PersistenceManager pm) { // Execute a query for the specified class and filter. Query query = pm.newQuery(cls, filter); return (List) query.execute(); }
Compile AnimalMaintenance.java
.
javac AnimalMaintenance.java
Take a look at the animals in the database.
java tutorial.jdo.AnimalMaintenance list Animal
Notice that list
optionally takes a
query filter. Let's explore the database some more, this time
using filters:
java tutorial.jdo.AnimalMaintenance list Animal "name == 'Binney'" java tutorial.jdo.AnimalMaintenance list Animal "price <= 50"
The JDO query language is designed to look and behave much
like boolean expressions in Java. The name
and price
fields identified in the above
queries map to the member fields of those names in
tutorial.jdo.Animal
. More details on
JDO query syntax is available in
Chapter 11, Query of the JDO Overview.
Great! Now that we can see the contents of the database, let's add some code that lets us add and remove animals.
As new dogs are born or acquired, the store owner will
need to add new records to the inventory database. In this
section, we'll write the code to handle additions through
the tutorial.jdo.AnimalMaintenance
class.
This section will familiarize you with the mechanism for
storing persistence-capable objects in a JDO persistence
manager. We will create a new dog, obtain a
Transaction
from a
PersistenceManager
, and, within the
transaction, make the new dog object persistent.
tutorial.jdo.AnimalMaintenance
provides a
reflection-based facility for creating any type of animal,
provided that the animal has a two-argument constructor
whose first argument corresponds to the name of the animal
and whose second argument is an implementation-specific
primitive. This reflection-based system is in place to keep this
tutorial short and remove repetitive creation mechanisms. It is
not a required part of the JDO specification.
Add the following code to
AnimalMaintenance.java
.
Modify the persistObject
method of
AnimalMaintenance.java
to look like
this:
/** * Performs the actual JDO work of putting <code>object</code> * into the datastore. * * @param object the object to persist in the datastore */ public static void persistObject(Object object) { // Get a PersistenceManagerFactory and PersistenceManager. PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory ("META-INF/jdo.properties"); PersistenceManager pm = pmf.getPersistenceManager(); // Obtain a transaction and mark the beginning // of the unit of work boundary. Transaction transaction = pm.currentTransaction(); transaction.begin(); pm.makePersistent(object); // Mark the end of the unit of work boundary, // and record all inserts in the database. transaction.commit(); System.out.println("Added " + object); // Close the PersistenceManager and PersistenceManagerFactory. pm.close(); pmf.close(); }
Recompile AnimalMaintenance.java
.
javac AnimalMaintenance.java
You now have a mechanism for adding new dogs to the database. Go ahead and add some by running java tutorial.jdo.AnimalMaintenance add Dog <name> <price> For example:
java tutorial.jdo.AnimalMaintenance add Dog Fluffy 35
You can view the contents of the database with:
java tutorial.jdo.AnimalMaintenance list Dog
What if someone decides to buy one of the dogs? The store owner will need to remove that animal from the database, since it is no longer in the inventory.
This section demonstrates how to remove data from the datastore.
Add the following code to AnimalMaintenance.java
.
Modify the deleteObjects
method of
AnimalMaintenance.java
to look like
this:
/** * Performs the actual JDO work of removing * <code>objects</code> from the datastore. * * @param objects the objects to persist in the datastore * @param pm the PersistenceManager to delete with */ public static void deleteObjects(Collection objects, PersistenceManager pm) { // Obtain a transaction and mark the beginning of the // unit of work boundary. Transaction transaction = pm.currentTransaction(); transaction.begin(); for (Iterator iter = objects.iterator(); iter.hasNext(); ) System.out.println("Removed animal: " + iter.next()); // This method removes the objects in 'objects' from the datastore. pm.deletePersistentAll(objects); // Mark the end of the unit of work boundary, and record all // deletes in the database. transaction.commit(); }
Recompile AnimalMaintenance.java
.
javac AnimalMaintenance.java
Remove some animals from the database.
java tutorial.jdo.AnimalMaintenance remove Animal <query>
Where <query>
is a query
string like those used for listing animals above.
All right. We now have a basic pet shop inventory management system. From this base, we will add some of the more advanced features suggested by our industry experts.
Now that we have the basic pet store framework in place, let's add support for the next pet in our list: the rabbit. The rabbit is a bit different than the dog; pet stores sell them all for the same price, but gender is critically important since rabbits reproduce rather easily and quickly. Let's put together a class representing a rabbit.
In this chapter, you will see some more queries and write a bidirectional relation between objects.
Provided with this tutorial is a file called
Rabbit.java
which contains a sample
Rabbit
implementation. Let's get it compiled
and loaded:
Examine and compile Rabbit.java
.
javac Rabbit.java
Add an entry for Rabbit
to
package.jdo
.
The Rabbit class above contains a bidirectional
relationship between parents and children. From the Java side
of things, a bidirectional relationship is simply a
pair of fields that are conceptually linked. There is no
special Java work necessary to express bidirectionality.
However, you must identify the relationship as bidirectional in
the JDO metadata for the
mapping tool to create the most efficient schema. Insert the
snippet below into the package.jdo
file. It identifies both the type of data in the collection
(the element-type
attribute) and the name
of the other side of the relation. The
mapped-by
attribute for identifying
bidirectional relations is part of JDOR, a
subset of the JDO specification for relational databases.
For more information on JDOR, consult
Chapter 14, JDOR of the JDO Overview.
Add the following code immediately
before the "</package>" line in the
package.jdo
file.
<class name="Rabbit"> <field name="parents"> <collection element-type="Rabbit"/> </field> <field name="children" mapped-by="parents"> <collection element-type="Rabbit"/> </field> </class>
Enhance the Rabbit
class.
kodoc -p jdo.properties Rabbit.java
Refresh the object-relational mappings and database schema.
mappingtool -p jdo.properties Rabbit.java
Now that we have a Rabbit class, let's get some preliminary rabbit data into the database.
Create some rabbits.
Run the following commands a few times to add some male and female rabbits to the database:
java tutorial.jdo.AnimalMaintenance add Rabbit <name> false java tutorial.jdo.AnimalMaintenance add Rabbit <name> true
Now run some breeding iterations.
java tutorial.jdo.Rabbit breed 2
Look at your new rabbits.
java tutorial.jdo.AnimalMaintenance list Rabbit java tutorial.jdo.AnimalMaintenance details Rabbit ""
Often, pet stores sell snakes as well as rabbits and dogs. Pet stores are primarily concerned with a snake's length; much like rabbits, pet store operators usually sell them all for a flat rate.
This chapter demonstrates more queries, schema manipulation, and additional relation types.
Provided with this tutorial is a file called Snake.java
which contains a sample Snake
implementation. Let's get it compiled and loaded:
Examine and compile Snake.java
.
javac Snake.java
Add tutorial.jdo.Snake
to
package.jdo
.
<class name="Snake"/>
Enhance the class.
kodoc -p jdo.properties Snake.java
Refresh the mappings and database.
As we have created a new persistence-capable class, we must map it to the database and change the schema to match. So run the mapping tool:
mappingtool -p jdo.properties Snake.java
Once you have compiled everything, add a few snakes to the database using:
java tutorial.jdo.AnimalMaintenance add Snake <name> <length>
Where <name>
is the name
and <length>
is the length in feet
for the new snake. To see the new snakes in the database, run:
java tutorial.jdo.AnimalMaintenance list Snake
Unfortunately for the massively developing rabbit population, snakes
often eat rabbits. Any good inventory system should be able to capture
this behavior. So, let's add some code to Snake.java
to support the snake's eating behavior.
First, let's modify Snake.java
to contain a
list of eaten rabbits.
Add the following code snippet to Snake.java
.
// *** Add this member variable declaration. *** private Set giTract = new HashSet(); ... // *** Modify toString(boolean) to output the giTract list. *** public String toString(boolean detailed) { StringBuffer buf = new StringBuffer(1024); buf.append("Snake ").append(getName()); if (detailed) { buf.append(" (").append(length).append(" feet long) sells for "); buf.append(getPrice()).append(" dollars."); buf.append(" Its gastrointestinal tract contains:\n"); for (Iterator iter = giTract.iterator(); iter.hasNext();) buf.append("\t").append(iter.next()).append("\n"); } else buf.append("; ate " + giTract.size() + " rabbits."); return buf.toString(); } ... // *** Add these methods. *** /** * Kills the specified rabbit and eats it. */ public void eat(Rabbit dinner) { // Consume the rabbit. dinner.kill(); dinner.eater = this; giTract.add(dinner); System.out.println("Snake " + getName() + " ate rabbit " + dinner.getName() + "."); } /** * Locates the specified snake and tells it to eat a rabbit. */ public static void eat(String filter) { PersistenceManagerFactory pmf = JDOHelper. getPersistenceManagerFactory("META-INF/jdo.properties"); PersistenceManager pm = pmf.getPersistenceManager(); Transaction transaction = pm.currentTransaction(); transaction.begin(); // Find the desired snake(s) in the datastore. Query query = pm.newQuery(Snake.class, filter); List results = (List) query.execute(); if (results.isEmpty()) { System.out.println("No snakes matching '" + filter + "' found"); return; } Query uneatenQuery = pm.newQuery(Rabbit.class, "isDead == false"); Random random = new Random(); for (Iterator iter = results.iterator(); iter.hasNext();) { // Run a query for a rabbit whose 'isDead' field indicates // that it is alive. List menu = (List) uneatenQuery.execute(); if (menu.isEmpty()) { System.out.println("No live rabbits in DB."); break; } // Select a random rabbit from the list. Rabbit dinner = (Rabbit) menu.get(random.nextInt(menu.size())); // Perform the eating. Snake snake = (Snake) iter.next(); System.out.println(snake + " is eating:"); snake.eat(dinner); } transaction.commit(); pm.close(); pmf.close(); } public static void main(String [] args) { if (args.length == 2 && args[0].equals("eat")) { eat(args[1]); return; } // If we get here, something went wrong. System.out.println("Usage:"); System.out.println(" java tutorial.jdo.Snake eat 'snakequery'"); }
Add an eater
field to
Rabbit.java
.
Notice that we are making this field
protected
, and that we set it
from the Snake
class. This
demonstrates that it is possible to directly access public or
protected persistent fields in other classes.
This is not recommended practice, however, because
it means that all classes that access this field directly must
be JDO enhanced, even if they are not persistence-capable.
Add the following member variable to Rabbit.java
:
protected Snake eater;
Add metadata to package.jdo
.
Notice the giTract
declaration in
Snake.java
: it is
a simple Java collection declaration. As with the
parents
and children
sets in
Rabbit.java
, we augment this
declaration with JDO metadata.
<class name="Snake"> ... <field name="giTract" mapped-by="eater"> <collection element-type="Rabbit"/> </field> </class>
Note that we specified a mapped-by
attribute in this example. This is because the relation is
bidirectional; that is, the rabbit has knowledge of which
snake ate it. We could have left out the
eater
field and instead created a
standard unidirectional relation. The metadata might have
looked like this:
<class name="Snake"> <field name="giTract"> <collection element-type="Rabbit"/> </field> </class>
For more information on types of relations, see Section 15.11, “Field Mapping” of the JDO Overview.
Compile Snake.java
and
Rabbit.java
and enhance the classes.
javac Snake.java Rabbit.java kodoc -p jdo.properties Snake.java Rabbit.java
Refresh the mappings and database.
mappingtool -p jdo.properties Snake.java Rabbit.java
Now, experiment with the following commands:
java tutorial.jdo.Snake eat "" java tutorial.jdo.AnimalMaintenance details Snake ""
Imagine that one of the snakes in the database was named Killer. To find out which rabbits Killer ate, we could run either of the following two queries:
java tutorial.jdo.AnimalMaintenance details Snake "name == 'Killer'" java tutorial.jdo.AnimalMaintenance list Rabbit "eater.name == 'Killer'"
The first query is snake-centric - the query runs against the
Snake
class, looking for all snakes named
Killer and providing a detailed listing of them. The second is
rabbit-centric - it examines the rabbits in the database for
instances whose eater
is named Killer.
This second query demonstrates the that simple Java 'dot' syntax
is used when traversing a relation field in a query.
It is also possible to traverse collection fields. Imagine that there was a rabbit called Roger in the datastore and that one of the snakes ate it. In order to determine who ate Roger Rabbit, you could run a query like this:
java tutorial.jdo.AnimalMaintenance details Snake "giTract.contains(rabbit) && rabbit.name == 'Roger'"
Note the use of the rabbit
variable above.
Variables are tokens that represent any element of the collection
that contains them. So in our query, the rabbit
variable stands for any rabbit in the snake's giTract
collection. We could have called the variable anything;
just as in Java, JDO query variables do not have to follow a
set naming pattern.
Congratulations! You are now the proud author of a pet store inventory suite. Now that you have all the major features of the pet store software implemented, it's time to add some extra features. You're on your own; think of some features that you think a pet store should have, or just explore the features of JDO.
Here are a couple of suggestions to get you started:
Animal pricing.
Modify Animal
to contain an inventory
cost and a resale price. Calculate the real dollar amount
eaten by the snakes (the sum of the inventory costs of all the
consumed rabbits), and the cost assuming that all the eaten
rabbits would have been sold had they been alive. Ignore the
fact that the rabbits, had they lived, would have created more
rabbits, and the implications of the reduced food costs due to
the not-quite-as-hungry snakes and the smaller number of
rabbits.
Dog categorization.
Modify Dog
to have a
relation to a new class called Breed
,
which contains a name identifying the breed of the dog and a
description of the breed. Put together an admin tool for
breeds and for associating dogs and breeds.