Java Data Object

Che-Rung Lee

JDO Objectives

Transparent persistence

Range of implementations embedded (J2ME) two tier (J2SE) enterprise (J2EE, EJB)

Datastore independence relational, object, hierarchical databases XML DB, file systems

What’s the difference?

Serialization no database capabilities (transactions, queries)

JDBC cannot storing Java object models. incompatibilities among SQL implementations

can result in a loss of application portability CMP (Container Managed Persistence)

a distributed model of computation, imposing performance degradations

JDO Introduction

JDO APIAn exampleDatastore mappingJDO implementation

JDO API

Two classes I18NHelper JDOHelper

Six interfaces1. PersistenceManagerFactory2. PersistenceManager3. Transaction4. Extent5. Query6. InstanceCallbacks

Persistence Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

1. PersistenceManagerFactory

Create PersistenceManager May implement PersistenceManager

pooling, connection pooling among PersistenceManagers

Datastore configuration Supports JNDI

Persistence Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

2. PersistenceManager

Primary application interface

PersistenceCapable instance management identity life cycle

Transaction factory

Query factory

Extent factory

Persistence Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

3. Transactions

One-one relation to Persistence Manager

Transaction interface for local transactions isActive, begin,

commit, rollback

Provide ACID transaction Atomic, Consistent,

Isolated, Durable

Persistence Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

4. Extents

Defined for PersistenceCapable classes pm.getExtent (Class pc, boolean subclasses);

Used in Query andclass navigation Persistence

Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

5. Query

Can do filtering and ordering

Outer/inner join

Has JDOQL

Persistence Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

6. InstanceCallbacks

Event trigger functions in DBMSPersistenceCapable class that provides

callback methods implements this interfaceMethods

jdoPostLoad(), jdoPreStore(), jdoPreClear(), jdoPreDelete()

Persistence Manager Factory

Persistence Manager

Transaction Query Extent

InstanceCallbacks

JDOHelperI18NHelper

How to use JDO

Write persistent object (Persistence Capable)1. Write normal java objects2. Specify their relation in a XML file3. Enhance classes for persistence

Make transactions (Persistence Aware) Create and connect to a datastore Insert / update / delete Query

Write PersistenceCapable objects

.java

.class

.class

Java Compiler

JDOEnhancer

JDO MetaData(XML)

Byte code enhancement

Address book example

Persistent Object Model Many to many relation

Person Category* *

Classes to Persist

public class Person { Vector category; // element is Category

…public Person() {}

} public class Category { Vector person; // element is Person

…private Category() {}

}

JDO MetaData

<?xml version=“1.0” encoding=UTF-8?><!doctype jdo public http://java.sun.com/dtd/jdo_1_0.dtd><jdo>

<package name=“addressBook”><class name = “Person”> <field name= category>

<collection element-type = “Category”> </field></class><class name = “Category”> <field name= person>

<collection element-type = “Person”> </field></class>

</package></jdo>

JDO Enhancer

Enhance classes to implement the interface PersistenceCapable

run Enhancer with JDO metadata (.xml file) and class files $enhancer$ addressbook.jdo Person.class Cate

gory.class

JDO Runtime Environment

JVMApplication

PersistenceManager

PersistenceCapable

transient

transient

transientPersistenceCapable

PersistenceCapable

Query

Transaction

Extent

Access PersistenceCapable objects

Code template1. PersistenceManagerFactory pmf = JDOHelper.

getPersistenceManagerFactory(properties);2. PersistenceManager pm =

pmf.getPersistenceManager();3. Transaction tx = pm.currentTransaction();4. tx.begin();5. execute(); // transaction code6. tx.commit();7. pm.close();

Create a Datastore

JDO will create database, tables, indexes

Set property ConnectCreate = true Unset this property if not creating a datastore

Do nothing on code void execute() {}

Create persistence object

void execute(){

Person p = new Person(…);

pm.makePersistent(p);

// pm is a PersistenceManager

}

Update persistence object

We need get persistent objects from datastore for update and delete

Assume we already got the object (Person p) from datastore. For update, it’s simple void execute(){

p.setName (newName);

}

Delete persistence object

Assume we get the object (Person p) from datastore, and we want to delete it void execute(){

Vector cat = p.getCategory();

for ( i=0;i<cat.size(); i++) {

Category c = (Category)cat.elementAt(i);

c.getPerson().remove (p);

}

pm.deletePersistent(p);

}

Get objects from datastore

Navigation

Query

From PersistenceManager Object getObjectById(Object oid) Object getTransactionalInstance(Object pco)

Navigate whole class

void execute(){

Extent ext = pm.getExtent(Person.class);

Iterator itor = ext.iterator();

while (itor.hasNext()) {

Person p = (Person) itor.next();

}

}

Query with Filters

void execute(){

Query qry = pm.newQuery(Person.class);

qry.setFilter(filters); // filter is a String

qry.setOrdering(“name ascending; birth descending”);

Collection result = (Collection) qry.execute();

for(Iterator i = result.iterator();i.hasNext();)

Person p = (Person) i.next();

}

Query examples

Find the person named “Roger” qry.setFilter (“name == \”Roger\”“);

Find the person by parameter (String who) qry.declareParameters (“String who");

qry.setFilter (“name == who");

result = qry.execute (who)

JDO Query Language

JDO defines a set of functions for querying isEmpty(): null singleton or collection or empty

collection. contains(): collection startsWith(), endWith(): String matching

Find persons in “CS*” Category qry.declareParameters (“Category cat");

qry.setFilter(“category.contains(cat) && cat.name.startWith(\”CS\”)”);

Inside JDO

PersistenceCapable Object identity Object life cycle

Data mapping Inheritance Collection

Other issues

Primary key of PersistenceCapable

In datastore assigned by the datastore and is not based on

field values in the object

In application uses values of object state to form a unique

identity

Nondurable not uniquely identifiable

How to compare o1 and o2?

Java identity implemented by JVM o1 == o2

Object equality implemented by class developer o1.equals (o2)

JDO identity implemented by JDO vendor o1.jdoGetObjectId().equals(o2.jdoGetObjectId())

Life Cycle of PersistenceCapable

Life Cycle States (1)

Persistent-new. During the current transaction, the instance has

been placed under the control of JDO and a JDO identity has been assigned.

Persistent-clean. The instance’s fields have been read but not

modified in the current transaction. Persistent-dirty.

The instance’s fields have been modified in the current transaction.

Life Cycle States (2)

Hollow. Only the JDO identity is loaded. Non-key fields

are reread from the datastore in subsequent transactions.

Provides uniqueness of persistent instances across transactions.

Hollow objects are subject to garbage collection if the application does not hold strong references to them.

Persistent-clean, persistent-dirty and persistent-new instances all become hollow after commit.

Life Cycle States (3)

Persistent-deleted. Deleted in the current transaction. Access to

non-key fields will throw a JDOUserException. Persistent-new-deleted.

Made persistent-new and then deleted in the current transaction.

Transient, Transient-clean, Transient-dirty The instance has not been placed under the

control of JDO.

Data Mapping

Issues Database normalization: multi-table per class… Performance: number of joins, update… Field mapping: data types, read only… Relation: delete policy…

Implementation dependence Some implementations can configure data

mapping in JDO meta file

Data Mapping

Inheritance Map all hierarchy to base class Discriminator is required

Collection Collection objects are second order object

Not have a JDO identity Its owner can aware of its change

Difference among Set, List and Map

Other issues

Object instantiation during query execution Invocation of object methods

Persistence by reachability Garbage collector in database

JDOQL to SQL compilation Allowing SQL in JDOQL

Cache management Search caches for uncommitted objects

More …

Some JDO Implementations

Free Sun Reference Implementation Object Relational Bridge (OJB)

Commercial Fast Objects Frontier Suit for JDO (Object Frontier) JDO Genie LIBeLIS LiDO SolarMetric Kodo JDO

Next version ?

Managed relationship support Inter-PersistenceManager references JDOQL to support projections API for specification of pre-read policy Enhancer invocation API Read-only fields Generation of sequence numbers for fields. Aggregation functions for JDOQL

Summery

References

http://access1.sun.com/jdo/ http://www.JDOcentral.com/index.html http://www.oreilly.com/catalog/jvadtaobj/chapter/c

h01.pdf

http://www.cognitivemachines.com/JDOchat.html http://java.sun.com/products/jdo/ http://db.apache.org/ojb/ http://servlet.java.sun.com/javaone/javaone2000