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Mats Strandberg

ORM With Hibernate

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Mats Strandberg

Presentation

Impedance mismatchWhat is ORMWhen/why use ORMUnderstand Hibernate basicsSee Java using Hibernate APISee SQLUnderstand Hibernate featuresAsk questions

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Mats Strandberg

Crisp Utbildningsdag (Crisp RD)

Crisp RDEvery second Friday, usually by employeeSome presentations at KTH

Mats StrandbergWorked with several OODBs since 1990Used RDBs in several OO projectsUsed Hibernate in one commercial projectWorked with Java since 1996

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Mats Strandberg

Simple Domain Model

Event

Participant

Venue

Address

0..n

0..n

1

1

0..n

0..n

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Mats Strandberg

Simple Domain Model

Event

Participant

Venue

Address

0..n

0..n

1

1

name

street

cityname

name

0..n

0..ndate

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Mats Strandberg

Object Diagram

hibernateRd: Event

mats : Participant

theVenue : Venue

theAddress : Address

name=”rum 1537”

street=”Osquars Backe 2”

city=”Stockholm”name=”Mats Strandberg”

name=”Hibernate RD”

date=”20060929 9.00”

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Mats Strandberg

Example Code and Mapping File

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Mats Strandberg

Relational Schema

Events

Participants

VenuesAddresses

* VenueId* AddressId

street

* ParticipantId

* EventId

name

EventParticipations* ParticipantId

* EventIdname

date

EventVenuename

VenueAddresscity

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Mats Strandberg

Example execution

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Mats Strandberg

On Root Objects

In an object graph usually there’s roots, where navigation starts.

Root

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Navigation

event.getVenue().getAddress().getStreet();

Event

Participant

Venue

Address

name

street

cityname

name

date

SELECT street FROM Addresses WHERE AddressId=(SELECT VenueAddress FROM Venues WHERE VenueId=(SELECT EventVenue FROM Events WHERE EventId=1));

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Mats Strandberg

Query

List list = // get events

for (Iterator iter = list.iterator(); iter.hasNext(); ) {

Event event = (Event) iter.next();

Address address = event.getVenue().getAddress();

if ("Stockholm".equals(address.getCity())) {

System.out.println("'" + address.getStreet() + "' is in Stockholm");

}

}

Event

Participant

Venue

Address

name

street

cityname

name

date

Get streets in Stockholm

SELECT street FROM Addresses WHERE city="Stockholm";

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Mats Strandberg

Object Queries

Roots may not be enough for searchWe need at least class extension orOQL

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Mats Strandberg

HQL

List list =

session.createQuery(

"select street from Address where city='Stockholm'").list();

for (Iterator iter = list.iterator(); iter.hasNext(); ) {

System.out.println("'" + iter.next() + "' is in Stockholm");

}

Event

Participant

Venue

Address

name

street

cityname

name

date

Get street in Stockholm

SELECT street FROM Addresses WHERE city="Stockholm";

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Mats Strandberg

Example: Hibernate + Domain model

Take a look at: JavaSQL

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Mats Strandberg

Why a Database?

Need for persitent stateSupport for transactionsLarge data setsMultiple concurrent applications share dataData distributionUsually disk based (persistence with single node)

Only add complexity to solve a real problem

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Mats Strandberg

Reasons for Object Persistence vs RDBReasons to use OODB or ORM:

Use of OO in design and programming(avoid impedance mismatch)Domain Model Intense solutionHierarchic dataNavigational access

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Mats Strandberg

Reasons For ORM vs OODB

Legacy RDB (Relational Database)(RDB) vendor independenceVendor stability(?)Schema migrationTools

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Mats Strandberg

Alternatives to ORM

Hand coded persistance layerSerializationEJB/CMPOODB

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Hibernate

Open SourceLGPL Licencehttp://hibernate.org

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Hibernate is ”non-intrusive”

This means:Persistence is orthogonal to classPersisting a instance is a run-time decision

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Requirements for a Persistent Class

Hibernate is said to ”non-intrusive”, however:Classes must have a no-arg constructorClasses should have a private Long id;Classes may have private database attributes

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Mats Strandberg

Understand ORM to use it

”The effective use of ORM technology in all but the simplest of enterprise environments requires understanding and configuring how the mediation between relational data and

objects is performed”

Linda DeMichiel, Lead Architect EJB, Sun

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Impedance Mismatch

Identity GranularityObject navigationSubtypesPolymorphic associations

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Inheritance

Table per concrete class

Aa1

Bb1

Cc1

B

a1

C* id

a1

* id

A* id

a1

b1

b1 c1

c1

A* id

B* id

C* id

a1

b1 c1

Table per class

hierarchy

Table per class

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Mats Strandberg

Application Transactions

A.k.a. long running transactionsAn object graph can be detachedUpdates can be done while detachedThe object graph can later be attached to a session

session.close() detaches the objectssession.update(object) attaches the object

NOTE: Other updates may be clobbered!

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Mats Strandberg

Detached objects + automatic versioningHandle concurrent updates by versioning

A version attribute must be added to classes involvedDetach object by session.close() Updates can be done while detachedAttach with session.update(object) Exception thrown by Hibernate at flush (commit) if version mismatch

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Lazy vs Eager fetch

Event Venue Address

name street

city

name

date

Venue$

name

isA

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Consider Performance

Iterating over a Class (Event) that has a n to m association

List list =

session.createQuery("from Event").list();

for (Iterator i = list.iterator(); i.hasNext(); ) {

Event event = (Event) i.next();

out.println("EVENT name:" + event.getName());

}

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Mats Strandberg

Lazy initialization

A Proxy is usedGetters are overridden, e.g. Event.getName();

The Proxy is a subclass of your persistent class, e.g extends EventRequires build-time bytecode instrumentationBeware of explicit comparison of runtime class, passing of class objects etc.

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CachingFirst-level Cache

Session

Second-level Cache

Query CacheCache Concurrency Strategy

Cache Provider

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Testing without database?

Testing business logic:Hibernate is non-intrusive -> Use POJOs in a transient wayThink of transaction demarcationBeware of embedding HQL as it requires a DB.

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Hibernate Product Suite

Hibernate CoreHibernate AnnotationsHibernate EntityManager Hibernate ToolsNHibernateJBoss Seam

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Roadmap

Production:Hibernate 3.1NHibernate 1.0 for :NET (Hibernate 2.1)

Development:Hibernate 3.2 (EJB 3.0)NHibernate 1.2 (.NET Framework 2.0)NHibernate 3.x? (Hibernate 3)

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Entity Manager

Entity Manager implements a complete EJB3 persistence provider (together with Hibernate Annotations)EJB-QL based on HQLAutomatic VersioningDetached EntitiesNon-managed set-up is rather differentEJB3 has a large number of persistence contexts

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Features

Two Level CacheLocking strategies (e.g. Optimistic Locking)Application TransactionsComposition InheritancePolymorphismPersistence by reachabilityFetching strategies: Lazy vs EagerLazy initialization

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Books on Hibernate

Etc.

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Things to Mention

LockingJPA – Java Persistence APIIndexHibernate vs Manual ORM: When using Hibernate there is a ”standard” for how the mapping has been done. This is good for maintenanceBidirectional relations are handled at code level