JDO (Java Data Objects) What It Is And Why It Matters Ron Hitchens [e-mail] http://www.ronsoft.com

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JDO(Java Data Objects) What It Is And Why It
MattersRon Hitchensron_at_ronsoft.comhttp//www.
ronsoft.com
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Speaker Info

• 25 years industry experience
• 6 years using Java
• Built a website with JDO (www.europeasap.com)
• OReilly author (Java NIO)
• Tech reviewer on JDO book (Russell Jordan)

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What is JDO?

• New Java standard extension
• JSR 12 (http//jcp.org)
• Transparent object persistence
• No code changes to persisted objects
• Standardized API
• Vendor neutral
• Datastore neutral
• Not an object database
• May use conventional RDBMS, OODB or other means
  to store object data

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JVM
POJO
POJO
JDO API
JDO Impl
POJO
PM
Query
POJO
SPI
Persist
Datastore
PM Persistence Manager POJO Plain Old Java
Object API Application Programming
Interface SPI Service Provider Interface
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How JDO Works

• Transparent Persistence
• Persistence by Reachability
• Object Lifecycle
• Inheritance
• Identity
• Queries
• Metadata
• Restrictions

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Transparent Persistence

• Transparent to Persisted Objects
• No source code changes to persistent objects
  needed
• Clients are unaware an object is persistent
• Persisted objects are auto-loaded when referenced
• Not Transparent to Entire Application
• JDO APIs are used to manage and query for objects
• Transaction boundaries affect object state
• Object instances are per-PM collisions are
  possible at commit

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Transparent Data Access

• Objects and object fields are lazy-loaded when
  referenced
• Changes to object state result in eventual
  updates to datastore without explicit saves
  (subject to transaction boundaries)
• PersistenceManagers maintain object caches,
  datastore access is optimized where possible

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PersistenceCapable

• The interface that all persistent objects must
  implement at runtime
• Byte code enhancement
• Source code pre-processing
• Direct implementation by programmer
• StateManager
• Set through PersistenceCapable interface
• Manages objects state while persistent
• Mediates access to object fields
• SPI hook into runtime JDO Implementation

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Mediated Object Access
client
PersistenceManager
myObject PersistenceCapable
implSM StateManager
makePersistent(myObject)
jdoReplaceStateMananger(implSM)
setFoo(12)
setIntField (this, n, foo, 12)
makeTransient(myObject)
jdoReplaceStateMananger(null)
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Persistence By Reachability

• All objects referenced directly or indirectly
  from a PersistenceCapable object are
  automatically persisted at transaction commit.
• Persistence applies to entire object graph
• Referenced non-PersistenceCapable objects are
  serialized to the datastore
• Deletion is done per object, not by reachability
• No datastore garbage collection

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Simple JDO Example
PersistenceManagerFactory factory
JDOHelper.getPersistenceManagerFactory(props) Per
sistenceManager pm JDOFactory.getPersistenceMana
ger() Transaction trans pm.currentTransaction()
User user new User ("ron", "Ron Hitchens",
"ron_at_ronsoft.com") Address addr new Address
(123 Main St., Smallville, CA,
12345) user.setAddress (addr) trans.begin()
pm.makePersistent (user) trans.commit() pm.clos
e()

• Two objects were persisted
• The instance of User explicitly made persistent
• The Address instance reachable from user

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JDO Object Lifecycle (1)
Object retrieved from datastore, instantiated by
JDO
POJO object instantiation
Transient
Hollow
Modify a field
Read a field
makePersistent()
Modify a field
Persistent New
Persistent Clean
Persistent Dirty
deletePersistent()
deletePersistent()
Persistent New Deleted
Persistent Deleted
deletePersistent()
deletePersistent()
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JDO Object Lifecycle (2)
Transaction Completion
Persistent Clean
commit(), rollback()
commit(), rollback()
Hollow
Persistent Dirty
Persistent Deleted
rollback()
commit()
Persistent New
commit()
commit(), rollback()
Transient
Persistent Deleted
rollback()
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Lifecycle Callbacks

• An object may optionally implement the
  InstanceCallbacks interface
• jdoPostLoad(), jdoPreStore(), jdoPreClear(),
  jdoPreDelete()
• May be used to release resources when an object
  is going hollow
• May be used to reconstitute transient values that
  can be recalculated from persisted fields.
• Couples the object to JDO

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Inheritance

• Polymorphism is supported
• Base type must be PersistenceCapable
• Persistent super classes must be listed in
  metadata definition
• Queries may return subclasses, if requested
• Implementation defines table mapping strategy
• Single Table, Class Table, Concrete Table
• Interfaces may not be directly persisted

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JDO Identity

• Each Persistent Object has a unique JDO Identity
• Not the same as Java identity
• JDO Identity is encapsulated as an Object
• One instance per identity per PersistenceManager
• Datastore Identity vs. Application Identity
• Datastore Identity assigned automatically
• Application Identity defined by programmer
• Persisted objects are retrieved by their identity

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Queries

• Three ways of retrieving objects
• Single object, by identity
• Objects of a particular type Extents
• Objects whose fields contain specific values
  Filters

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Queries Single Object By ID

• Customer getCustomerByIdString (String idStr,
• PersistenceManager pm)
• Object id pm.newObjectIdInstance (
• Customer.class, idStr)
• Object customer pm.getObjectById (id, true)
• return ((Customer) customer)

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Queries Extent

• A collection-like object representing a set of
  persistent objects, of a specified type, in the
  datastore
• May contain subclasses, if requested

Extent e pm.getExtent (Customer.class,
true) Iterator it e.iterator() while
(it.hasNext()) Customer customer (Customer)
it.next() customer.computeDailyInterest()
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Queries Filters

• Filters run against Extents
• Objects filtered are always of the type in the
  extent, possibly subclasses
• JDOQL JDO Query Language
• Java-like syntax
• Parameters and variables may be supplied
• Datastore agnostic, references Java fields
• Filters are applied by Query class
• Returns a collection of matched objects

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Queries Filter Example

• Collection getCustomersByCity (City city,
• PersistenceManager pm)
• Extent extent pm.getExtent (Customer.class,
  true)
• String filter address.city city
• Query query pm.newQuery (extent, filter)
• query.declareParameters (City city)
• query.setOrdering (name ascending)
• return (query.execute (city))

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JDO Metadata

• Provides mapping information to JDO
  implementation about classes and fields
• Standardized JDO descriptor (XML)
• Provides information that cannot be determined by
  reflection
• Allows for override of defaults
• Provides for vendor extensions
• Can be used to generate a schema

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Object World
Database World
Object Types and Relationships
How and where to store object data
JDO Impl
JDO API
POJO
POJO
POJO
Datastore
POJO
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JDO Restrictions

• Not all objects are persistable
• Streams, Sockets, many system classes, etc
• Collections must be homogenous
• Maps may have restrictions on keys
• List ordering may not be preserved
• Objects cannot migrate between PersistenceManager
  instances
• Persisted objects cannot outlive their owning
  PersistenceManager

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Why JDO Matters 1

• The Object Model IS the Data Model
• Datastore is one component in the system, not the
  center of the universe
• Promotes datastore independence at design,
  development and deploy times
• End-to-end OO design is possible One system
  architecture
• More agile Datastore is an implementation detail

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Why JDO Matters 2

• Separation of Concerns
• Java Guy and DBA Guy do separate jobs
• No SQL strings buried in the Java code
• Cost
• Standard API Leverage developers
• Lightweight No special container needed
• Competition among compliant vendors
• Legacy databases can be wrapped by JDO objects
• Less work to do overall

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JDO and EJB

• Can JDO and EJB Co-exist?
• JDO can be used as a BMP strategy
• Suns SunOne App Server does this
• JDO can plugin to any JCA compliant App Server
  and participate in managed transactions
• Layered architecture
• One app may use JDO objects directly
• Another may use the same objects within EJBs to
  leverage J2EE container services
• Using JDO/BMP may be more cost-effective than
  paying for full CMP capability

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Whats Similar to JDO?

• CMP
• Proprietary O/R tools
• Toplink
• CocoBase
• Many others
• Open Source O/R tools
• Hibernate
• Torque
• OJB

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Where Can I Get JDO?

• JDO Vendors
• Solarmetric (www.solarmetric.com)
• Libelis (www.libelis.com)
• JDO Genie (www.hemtech.co.za/jdo/)
• Poet FastObjects (www.fastobjects.com)
• Open Source Options
• Apache OJB (db.apache.org/ojb/)
• JORM (www.objectweb.com/
• See www.jdocentral.com for more

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Where Can I Get More Info?

• Web Resources
• http//access1.sun.com/jdo/
• http//www.jdocentral.org/
• http//jdo-tools.sourceforge.net/
• http//groups.yahoo.com/JavaDataObjects
• http//onjava.com (search for JDO)
• Google Java Data Objects
• Publications
• Java Data Objects (Russell Jordan)
• http//www.oreilly.com/catalog/jvadtaobj/
• Java Data Objects (Roos)

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Questions?
Ron Hitchensron_at_ronsoft.comhttp//www.ronsoft.c
om
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