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Java Data Objects (JDO)
Overview and Future
Michael Vorburger, Vertical*i
Java User Group SwitzerlandZurich, 31.03.05
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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
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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
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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 JDO2.0 and EJB 3.0 Persistence Specs?
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Object Persistence Concepts Graph of data objects (Domain Model)
API: persist(), Persistence by Reachability
Transactions
Query!
Internal: Memory Management, Caching, etc.
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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, Linkwith 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 Chapter2, but: Solutions for
mismatches exist; see also your ORM Vendor Doc!
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History Few things fall from the sky
object persistence in general
and JDO in particular buildupon 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)
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JDO Introduction JDO is an interface-based API for selection and
transformation of transactional persistent storagedata into native Java programming language
objects Transparent Object Persistence!
Various implements for this API exist: Most as ORMfor 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
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JDOA
PI: PMF & PM PersistenceManagerFactory pmf =
JDOHelper.getPersistenceManagerFactory(Properties p);
javax.jdo.PersistenceManager pm =pmf.getPersistenceManager();
pm.makePersistent(customer);
customer = pm.getObjectById(oid,true);
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JDO Metadata & Enhancement JDO XML metadata describes which classes can be
persisted (), with relevant options(, )
JDO implementation run-time needs to interact withpersistence instances for lifecycle notification, load,store/flush. Generally, this is achieved by running abuild-time tool, a JDO [byte code] enhancer. (It addsthe PersistenceCapable interface from SPI API topersistent classes. This is no longer strictly for JDOcompliance,see BinaryCompatibility.)
Subject of hot debates strange, think AOP etc.
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JDOA
PI: Transactions Non-Managed
javax.jdo.Transaction jtx =pm.currentTransaction();
jtx.begin(); /* Do Stuff */ jtx.commit();
Managed (usually in J2EE container) JTAAPI
javax.transaction.UserTransaction ut;
ut = {lookup somewhere, e.g. JNDI};ut = ejbContext.getUserTransaction()
ut.begin(); /* Do Stuff */ ut.commit();
Declarative EJB container managed transactions
Core JDO Book Chapter 11
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JDOA
PI: Query and JDOQL Query q = pm.newQuery(Customer.class,
"customerName.startsWith('Tintin') &&lastOrder.price > 100");
Collection results = (Collection)q.execute();Iterator it = results.iterator();while (it.hasNext()) {
Customer cust = (Customer)it.next());
Parameters: q.declareParameters(String variable")and q.execute(variable) etc. etc. many manymore query facilities!
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JDO Queries: More of v1.0 JDOQL Filter can contain almost all Java
language operators, and some methods such asCollection.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.
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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() orrange :start to :end and java.util.List results
User-Defined Result Class, single results, Unique
Projections, Aggregates functions Standardized SQL pass-through
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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 (?)
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JDO & EJB2
.1 EJB 2.1 CMP Entity Beans mixed persistence
and remoting, with a complicated API and
development model (container) sorry, butconsider that dead, essentially.
Session and Message-Driven Beans however
certainly have their place. So JDO inside theimplementation logic of those makes a lot of
sense!
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JDO & EJB2
.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?!
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JDO & EJB: DTO (A
KA
VO) Either, traditionally write dedicated
SerializableDTO/VO in addition to PC,
maybe per service/use case. Nothing new;see Core J2EE Patterns etc.
Or, mark domain model PCs asSerializable
(although JDO will neverserialize) and use makeTransient (1.0) or
detachCopy (2.0)
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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 manualmakeTransient() approach from Core JDObook chapter 9.
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JDO in the Web Tier (directly) If your project does not need EJB-like remoting and
physical separation of tiers, directly using JDO inthe Web Tier (Struts, JSF, etc.) can be very nice,
and lead to a simple and efficient developmentmodel!
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; allowwrite changes in Controller only.
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EJB 3.0 EJB 3.0 EG decided to abandon EJB 2.1
Entity Beans, and create new and separatePersistence Doc and API:
This will be an ORM not a transparentPersistence API. Some other differences.However, will also be usable in J2SE,outside J2EE container.
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EJB 3.0
Also a POJO-style approach. Many realworld domain models will probably be usablewith both APIs!
Both a similarAPI, conceptually close forpractical purposes, e.g. life cycle aligned.EntityManager.persist(),EntityManager.createQuery(),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)
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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".
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Q & A
Thanks for listening!
To play with running code after this:
http://www.jpox.org great tutorials!
Questions?
Drinks.