Re: Java persistence

On Sun, 26 Sep 2010, Lew wrote:


I'd never even heard of Google Persist. Here we go:

http://code.google.com/p/persist/

It seems its goal is to be an as-simple-as-possible ORM. Of course,
eliminate the complexity of Hibernate-style ORMs means eliminating some of
their features: notably missing, from what i can see, are relationships
between objects, in that persistent objects can only have fields of simple
types (primitives, strings, and so on).


Good advice. I'm using OpenJPA at the moment and, once i realised what i
was doing wrong with the schema generator, it's an absolute doddle to use.
Classes looks like (and this is standard JPA; this will be identical with
Hibernate, EclipseLink, or any other JPA provider):

@Entity
public class Customer {
  @Id
  private int customerNumber;
  private String name;
  @OneToMany(mappedBy="customer")
  private List<Order> orders;
  // constructor, methods, etc
}

@Entity
public class Order {
  @Id
  private orderNumber;
  @ManyToOne
  private Customer customer;
  // constructor, methods, etc
}

You compile that as normal.

You put a persistence.xml file in the classpath root with these contents
(again, this is standard JPA):

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence">
     <persistence-unit name="example" transaction-type="RESOURCE_LOCAL" />
</persistence>

You then set up properties describing the database conenction; i do this
as an environment variable in a shell script, because i think it's runtime
configuration rather than part of the code, but you can also put this in
the persistence.xml file. Mine look like:

DB_ARGS="-Dopenjpa.ConnectionDriverName=$DB_DRIVER -Dopenjpa.ConnectionURL=$DB_URL -Dopenjpa.ConnectionUserName=$DB_USER -Dopenjpa.ConnectionPassword=$DB_PASS"

That's not standard JPA; i think JPA 2.0 defines standard property names
for the database connection details, but OpenJPA doesn't support them yet.

You then generate SQL for the necessary tables like this (having defined
the classpath variables appropriately):

java -cp "$PROJECT_CLASSPATH:$OPENJPA_CLASSPATH:$DB_CLASSPATH" $DB_ARGS org.apache.openjpa.jdbc.meta.MappingTool -schemaAction build -sql schema.sql

You then run that schema.sql against your database. You can use the
mapping tool to set up the tables directly, but i prefer to make SQL and
then run that, because i have a record of exactly what gets done. This bit
is entirely OpenJPA-specific, i'm afraid. I think the resulting SQL files
would work with any JPA provider, though - i think the default mapping is
standardised.

You then write code to manipulate the objects like this (again, this is
standard JPA):

EntityManagerFactory emf = Persistence.createEntityManagerFactory("example"); // name matches the one in persistence.xml
EntityManager em = emf.createEntityManager();
EntityTransaction tx = em.getTransaction();
try {
  tx.begin();
  Customer c = new Customer(100, "Harry Brown");
  Order o = new Order(83479);
  c.addOrder(o);
  em.persist(c);
  tx.commit();
}
finally {
  if (tx.isActive()) {
  tx.rollback();
  }
  em.close();
}

You compile that as normal, and run it with:

java -cp "$PROJECT_CLASSPATH:$OPENJPA_CLASSPATH:$DB_CLASSPATH" $DB_ARGS MainClass

That's it.

Now, there are three things i really should have done but didn't, because
they in fact aren't truly necessary.

The first was to give my classes no-args constructors. The JPA spec
requires that entity classes have public or protected no-args
constructors. As it happens, the OpenJPA entity enhancer adds these if
they're needed. For portability and correctness, you should add them
explicitly.

The second was to list the entity classes in the persistence.xml. I'm not
sure the spec requires this, but i think it's a good idea even if it
doesn't - OpenJPA warns that i haven't done it, but deals with it. I have
a step in my build script which adds them automatically, so there's no
manual work involved in this for me.

The third is to enhance my classes manually. For OpenJPA, class files need
to be modified before they are loaded to allow transparent persistence.
You can do this with a command-line tool at build time, but if you don't,
OpenJPA can usually load an agent at runtime which will do it on the fly.
It's safer to do it at build time, though. Again, i have a line in my
script which does this, so there's no manual work.

So, even doing it properly, JPA is pretty simple.

tom

--
Only the bagel has the correct aspect ratio.