Java Data Objects JDO Overview and Future

1
Java Data Objects (JDO) Overview and Future

• Michael Vorburger, Verticali
• Java User Group Switzerland
• Zurich, 31.03.05

2
Agenda

• Object Persistence
• O/R Mapping (ORM)
• History
• JDO Introduction
• JDO API
• JDOQL
• JDO 2.0 (JSR 243)
• JDO Implementations (Vendors OSS)

• Using JDO in EJB 2.1
• Using JDO in Web Apps
• Using JDO and/or JDBC
• EJB 3.0 (JSR 220)
• Future

3
About Speaker

• Personal interest in persistence space
• Culprit for an in-house ORM layer
• Somehow picked up JDO in 2001
• Core Java Data Objects co-author
• For past ca. 3 years user of JDO

4
About Audience

• How many of you would consider themselves
  moderately familiar with JDO, or another
  similar persistence framework, anyway?
• How many of you have read the JDO 2.0 and EJB 3.0
  Persistence Specs?

5
Object Persistence Concepts

• Graph of data objects (Domain Model)
• API persist(), Persistence by Reachability
• Transactions
• Query!
• Internal Memory Management, Caching, etc.

6
O/R Mapping (ORM)

• Impedance Mismatch of OOP RBDMS
• Classes vs. Tables, Inheritance mapping
  strategies
• Relationships Single 1-1 or 1-M, Multiple M-N,
  Link with attributes DBA mixed data and
  relationship, Inverses/Bi-directionality, Lists,
  Maps
• Identity Management, Uniqueness
• Class Attribute vs. Column Type Mappings
• More in Core JDO Book Chapter 2, but Solutions
  for
• mismatches exist see also your ORM Vendor Doc!

7
History

• Few things fall from the sky object persistence
  in general and JDO in particular build upon a
  long history
• Have you heard of any of these names before?

• CORBA POS Persistent Object Service
• Project Forest of Sun Labs (OPJ, PJama)
• ODMG API
• Gemstone, O2,
• TopLink (15y)
• (EJB 2.1 Entity Beans)

8
JDO Introduction

• JDO is an interface-based API for selection and
  transformation of transactional persistent
  storage data into native Java programming
  language objects Transparent Object
  Persistence!
• Various implements for this API exist Most as
  ORM for your favourite RDB some for standalone
  or built-in OODBMS.
• JDO 1.0 (JSR 12) in 2003
• JDO 2.0 (JSR 243) Extension, accepted 1m ago

9
JDO API PMF PM

• PersistenceManagerFactory pmf
  JDOHelper.getPersistenceManagerFactory(Properties
  p)
• javax.jdo.PersistenceManager pm
  pmf.getPersistenceManager()
• pm.makePersistent(customer)
• customer pm.getObjectById(oid,true)

10
JDO Metadata Enhancement

• JDO XML metadata describes which classes can be
  persisted (ltclass namegt), with relevant options
  (ltcollection element-typegt, ltextension gt)
• JDO implementation run-time needs to interact
  with persistence instances for lifecycle
  notification, load, store/flush. Generally, this
  is achieved by running a build-time tool, a JDO
  byte code enhancer. (It adds the
  PersistenceCapable interface from SPI API to
  persistent classes. This is no longer strictly
  for JDO compliance,see BinaryCompatibility.)
• Subject of hot debates strange, think AOP etc.

11
JDO API Transactions

• Non-Managed
• javax.jdo.Transaction jtx pm.currentTransaction(
  )
• jtx.begin() / Do Stuff / jtx.commit()
• Managed (usually in J2EE container)
• JTA API
• javax.transaction.UserTransaction ut
• ut lookup somewhere, e.g. JNDIut
  ejbContext.getUserTransaction()
• ut.begin() / Do Stuff / ut.commit()
• Declarative EJB container managed transactions
• Core JDO Book Chapter 11

12
JDO API Query and JDOQL

• Query q pm.newQuery(Customer.class,
  "customerName.startsWith('Tintin')
  lastOrder.price gt 100")
• Collection results (Collection)q.execute()Iter
  ator it results.iterator()while
  (it.hasNext()) Customer cust
  (Customer)it.next())
• Parameters q.declareParameters(String
  variable") and q.execute(variable) etc. etc.
  many many more query facilities!

13
JDO Queries More of v1.0

• JDOQL Filter can contain almost all Java
  language operators, and some methods such as
  Collection.isEmpty() contains(),
  String.startsWith() endsWith()But not
  arbitrary other domain model methods!
• Ordering
• Free Variables
• Compiled Queries
• Namespaces (packages)
• Navigation (dots) in Filters, Ordering, etc.

14
JDO Queries More in v2.0

• Single-String JDOQL Form
• String.matches(), toLowerCase(), toUpperCase(),
  indexOf() x2, substring() x2, Map.containsKey()
  containsValue(), Math.abs() sqrt().
• Paging Query Results with Query.setRange() or
  range start to end and java.util.List results
• User-Defined Result Class, single results, Unique
• Projections, Aggregates functions
• Standardized SQL pass-through

15
JDO 2.0 News

• Disconnected Object Graphs for multi-tier
• Standardized RDBMS Mapping Descriptors (just
  the agreed upon XML language many
  implementations already had possibilities)
• Can get e.g. a java.sql.Connection from
  javax.jdo.PersistenceManager
• More newNamedQuery, FetchPlan, Sequence, delete
  by query, Multiple User Objects on PM, etc.

16
JDO Implementations DISCLAIMER

• Open Source
• JPOX
• ObjectWeb Speedo
• TJDO
• XORM (?)
• Apache JDO 2.0 RI (?)
• Orient (ODBMS, OSS is new)
• Commercial JDO ORM Vendors
• Solarmetric Kodo
• Xcalia (formerly Libelis LiDO) .FR
• Signsoft intellBO .DE
• Versant Open Access (formerly JDO Genie)
• Others, e.g. Exadel, ObjectFrontier,
  ObjectMatter, Spadesoft XJDO,

• Commercial non-ORM
• Versant (incl. Poet)
• ObjectDB (simple inexpensive)
• Non-JDO ORMs
• JBoss Hibernate
• Oracle Toplink
• Castor JDO
• OJB (?)

17
JDO EJB 2.1

• EJB 2.1 CMP Entity Beans mixed persistence and
  remoting, with a complicated API and development
  model (container) sorry, but consider that
  dead, essentially.
• Session and Message-Driven Beans however
  certainly have their place. So JDO inside the
  implementation logic of those makes a lot of
  sense!

18
JDO EJB 2.1

• When using JDO inside a RPC-like (SOA) EJB
  Session Bean (or RMI, Spring Remoting..
  your-favourite-here)
• Syntax aspects About how to get, when to
  etc. For full code details see Core JDO book
  chapter 9 and/or Vendor Doc.
• Architectural aspects How to write your DTOs
  (AKA VO) here or do you write them at all?!

19
JDO EJB DTO (AKA VO)

• Either, traditionally write dedicated
  Serializable DTO/VO in addition to PC, maybe per
  service/use case. Nothing new see Core J2EE
  Patterns etc.
• Or, mark domain model PCs as Serializable
  (although JDO will never serialize) and use
  makeTransient (1.0) or detachCopy (2.0)

20
JDO EJB detachCopy

• New JDO 2.0 state detached-clean/dirty. API
• Object PersistenceManager.detachCopy(Object pc)
• Object PersistenceManager.attachCopy(Object
  detached, boolean makeTransactional)
• Scenario detach/disconnect, close PM, serialize
  across tier, modify disconnected, send back. New
  PM, new Tx, re-attach.
• Keeps original identity and version!
• Simplifies makes safer the similar manual
  makeTransient() approach from Core JDO book
  chapter 9.

21
JDO in the Web Tier (directly)

• If your project does not need EJB-like remoting
  and physical separation of tiers, directly using
  JDO in the Web Tier (Struts, JSF, etc.) can be
  very nice, and lead to a simple and efficient
  development model!
• Recommended Architecture One shared PMF
  (application context) and one PM and Tx per
  request (context), managed by a Servlet Filter.
  Better Enforce Web MVC with Model 2 View,
  allowing pull view (e.g. JSP) but read-only
  allow write changes in Controller only.

22
JDO and/or JDBC?

• Using JDO (any ORM) does not necessarily mean no
  JDBC mixing is possible and may make sense. (Tx
  safe) VI LOB
• Core JDO Book chapter 12 has analysis but
  outdated already because JDO 2.0 addresses some
  cases.
• Still, of course, for data-centric, database
  processing intensive, performance sensitive e.g.
  batch kind applications, as well as for very
  complex existing fixed RDB schemas, ORM and thus
  JDO may not be the appropriate road to go.

23
EJB 3.0

• EJB 3.0 EG decided to abandon EJB 2.1 Entity
  Beans, and create new and separate Persistence
  Doc and API
• This will be an ORM not a transparent Persistence
  API. Some other differences. However, will also
  be usable in J2SE, outside J2EE container.

24
EJB 3.0

• Also a POJO-style approach. Many real world
  domain models will probably be usable with both
  APIs!
• Both a similar API, conceptually close for
  practical purposes, e.g. life cycle aligned.
  EntityManager.persist(), EntityManager.createQuer
  y(), javax.persistence.Query.getResultList()
• Query Languages (JDOQL EJBQL) differ, but maybe
  JDO 2.1 and EJB 3.1 allow cross-spec query
  language? (A la SQL in JDO 2.0)

25
Future of Java Persistence

• Today, the choice is JDO why wait? A proven JCP
  standard with many implementations.
• EJB 3 Final Release supposedly summer 2005 (?).
  Requires JDK 5. Slow adoption?
• Longer Term (2-3 years?) who knows?
• Note All major JDO implementations very likely
  also EJB 3 "persistence engines".

26
Q A

• Thanks for listening!
• To play with running code after
  thishttp//www.jpox.org great tutorials!
• Questions?
• Drinks.