JDO Presentation given at Java User Group Switzerland (JUGS) on 31.03.05

Java Data Objects (JDO) Overview and Future Michael Vorburger, Vertical*i Java User Group Switzerland Zurich, 31.03.05

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

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

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?

Object Persistence Concepts
Graph of data objects (“Domain Model”)
API: persist(), Persistence by Reachability
Transactions
Query!
Internal: Memory Management, Caching, etc.

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!

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)

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

JDO API: PMF & PM
PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory(Properties p);
javax.jdo.PersistenceManager pm = pmf.getPersistenceManager();
pm.makePersistent(customer);
customer = pm.getObjectById(oid,true);

JDO Metadata & Enhancement
JDO XML metadata describes which classes can be persisted (<class name=…>), with relevant options (<collection element-type=…>, <extension …>)
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.

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. JNDI}; ut = ejbContext.getUserTransaction()
ut.begin(); /* Do Stuff */ ut.commit();
Declarative EJB container managed transactions
Core JDO Book Chapter 11

JDO API: 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 many more query facilities!

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.

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

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.

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 (?)
…

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!

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?!

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)

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.

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.

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.

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.

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.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)

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".

Q & A
Thanks for listening!
To play with “running code” after this: http://www.jpox.org – great tutorials!
Questions?
Drinks.

JDO Presentation given at Java User Group Switzerland (JUGS) on 31.03.05

by Michael Vorburger

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JDO Presentation given at Java User Group Switzerland (JUGS) on 31.03.05 Presentation Transcript