Apache Persistence Layers

Cayenne, OpenJPA, iBatis Apache Persistence Layers ApacheCon EU 2008 Henning Schmiedehausen [email_address]

The O/R impedance mismatch Objects (Java Knowledge) RDBMS ?

In a nutshell…
“ Object-Relational mapping (O/RM),
is a programming technique that links
databases to object-oriented language
concepts, creating (in effect) a ‘virtual
object database’.”
– Wikipedia

Selection criterias
Data access (CRUD)
Developers knowledge
Database languages (SQL, DDL)

Can we stop here? I just need to query data for my (web) application. I could use the “industry standard”, right?

Yeah, right
JDO 1.0, 2.0, 2.1 ?
EJB 2 (Ieek!) or 3 ?
JPA ?
J2SE ? J2EE ?
And wait, there is more…
… Legacy code? Optimized SQL?
… Transparent or explicit persistence?

Meanwhile, outside Apache…
Hibernate
„ de facto standard“
(L)GPL licensed
driven by a single company
TopLink Essentials
JSR 220 Reference Implementation
CDDL + GPLv2
driven by a single company

Ok, ok, ok… So, why Apache?

Because we are at ApacheCon…

Apache Software License V2
Open Source
Commercial friendly
Non discriminatory

Strong community
Many contributors
Meritocracy
Commercial support available

Apache Persistence Layers
Apache Cayenne
Apache OpenJPA
Apache iBatis
not in this talk:
Apache Torque (http://db.apache.org/torque/)
Apache OJB (http://db.apache.org/ojb/)

Common Ground
JDBC, JTA, JNDI
Many popular DBs supported:
MySQL, PostgreSQL, Oracle, HSQLDB, Apache Derby
XML mapping definition files
1:1, 1:n, m:n mappings
native and generated primary keys

Where are we?
Apache Cayenne
2.0.4 (Oct 12th, 2007)
3.0M3
Apache OpenJPA
1.0.2 (Feb 18th, 2008)
1.1.0-SNAPSHOT
Apache iBatis
2.3.1 Beta (Mar 25th, 2008)
3.0 in planning state

Swing GUI tool for modeling
Ant support, maven through ant tasks
concepts related to WebObjects EOF

Cayenne
Pros:
GUI modeler included
Good documentation
Cons:
all data objects inherit DataObject (3.0 will have POJO support)
no standards compliant API (3.0 will be JPA compliant)

implements JPA (JSR-220)
Command line tools included
based on Kodo, donated by BEA

Pros:
Standards based (JPA)
POJO support
Good documentation
Cons:
Some learning effort required
Only command line tools included

Example code
This talk can only scratch the concepts
Example code is available from my homepage, along with examples for other O/R tools:
http://henning.schmiedehausen.org/or-mappers/
http://svn.softwareforge.de/svn/opensource/talks/or-mappers/

A sample relation

Cayenne property
class People extends CayenneDataObject {
void setFirstName(String firstName ) { writeProperty ( "firstName" , firstName );
}
String getFirstName() { return (String) readProperty ( "firstName" );
}

Cayenne relation
class People extends CayenneDataObject {
void addToContacts(Contact contact ) { addToManyTarget ( "contacts" , contact , true);
}
void removeFromContacts(Contact contact ) { removeToManyTarget ( "contacts" , contact , true);
}
List getContacts() { return (List) readProperty ( "contacts" );
}

Cayenne mapping
<obj-entity name="People" className=" om.People " dbEntityName=" people ">
<obj-attribute name=" firstName " type="java.lang.String" db-attribute-path=" people_firstname "/>
</obj-entity>
<obj-relationship name="people" source=" Contact " target=" People " db-relationship-path="people"/>
Class/DB Map Prop. / Col Map Obj Relation

OpenJPA property
@Entity @Table(name="people")
public class People {
private String firstName ;
@Basic @Column(name="people_firstname")
public String getFirstName() {
return firstName ;
}
public void setFirstName(String firstName) {
this. firstName = firstName;
}
Class/DB Map Prop. / Col Map

OpenJPA relation
@Entity @Table(name="people")
public class People {
private Collection<Contact> contacts ;
@OneToMany(mappedBy = "people", cascade={CascadeType.PERSIST, CascadeType.REMOVE})
public Collection<Contact> getContacts() {
return contacts ;
}
public void setContacts( Collection<Contact> contacts) {
this. contacts = contacts;
}
Class/DB Map Obj Relation

Accessing the Mapper
Cayenne:
DataContext dataContext = DataContext.createDataContext();
OpenJPA:
EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory();
EntityManager em = entityManagerFactory .createEntityManager();

Retrieving an Object by PK
Cayenne:
People user = (People) DataObjectUtils .objectForPK( dataContext , People.class, 2);
OpenJPA:
People user = (People) em .find(People.class, 2L);

Changing an Object
Cayenne:
People people = retrieve();
people.setMember(true);
dataContext. commitChanges() ;
OpenJPA:
Transaction tx = em .getTransaction();
tx.begin() ;
People people = retrieve();
people.setMember(true);
tx.commit() ;

And Now… …for Something Completely Different

iBATIS
Data mapper framework
couples SQL queries to Java objects
Abator tool / Eclipse plugin

iBATIS
Pro:
fast, lightweight, unintrusive
Other Languages: Ruby, .NET
POJO support
Eclipse plugin
Cons:
SQL knowledge required
„ not mainstream“ concept

A mapped query
<select id=" getPeople " parameterClass="people"
resultMap="peopleResult" >
select * from people
<dynamic prepend="where">
<isNotNull prepend="and" property="id">
people_id = #id#
</isNotNull>
</dynamic>
</select>

Query examples
Querying a single object
People select = new People ();
select.setId(2L);
People user = (People) sqlMap .queryForObject(" getPeople ", select);
Querying a list
List select = sqlMap .queryForList(" getPeople ", null);

Caveat
iBatis is not an O/R mapper, it is a data mapper!
calling retrieve() twice returns:
Cayenne, OpenJPA: The same object
iBatis: Two different objects

Which one to use?
This table is highly subjective! YMMV!
… when standards compliance is a concern OpenJPA … if you need J2EE integration All of them … if your SQL is better than your Java / .NET iBATIS … if your Java is better than your SQL Cayenne, OpenJPA When What

Where to go from here?
http://henning.schmiedehausen.org/or-mappers/
Talk slides and Example code
O/R Mappers Homepages:
http://cayenne.apache.org/
http://openjpa.apache.org/
http://ibatis.apache.org/
Other ASF persistence mappers:
http://db.apache.org/torque /
http://db.apache.org/ojb /

Thanks for your attention!

Apache Persistence Layers

by Henning Schmiedehausen

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