ObjectRelational Mapping with Hibernate

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Object-Relational Mapping with Hibernate
Brian Sam-Bodden Principal Partner Integrallis
Software, LLC.
August 1 - 5, 2005
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Contents

• The Problem
• Object-Relational Mapping
• Hibernate Core Concepts
• Hibernate Architecture
• Hibernate Mappings
• Configuring Hibernate
• Working with Persistent Objects
• Persistence Lifecycle

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Contents

• Working with Collections
• More on Association Mappings
• HQL The Hibernate Query Language
• Query by Criteria
• Query by Example
• Caching
• Other Hibernate Features
• Conclusions

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The ProblemTwo Different Worlds

• Object-Oriented Systems
• Represent a problem in terms of objects
• Semantically richer, encapsulate data and behavior

• Relational Databases
• Efficient data storage, retrieval and integrity
  of the data
• Provide a normalized set of data

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The ProblemNatural Divergence

• Early on in a systems life, both models tend to
  look alike
• As the system matures two models diverge
• They grow to do what they do best
• Object model gets richer
• Data model changes to accommodate operational
  factors

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The ProblemObject-Relational Mismatch

• Granularity
• Object Oriented vs. Database Types
• User-Defined Column Types (UDT) are not portable
• Inheritance Polymorphism
• Not supported in the relational model
• No polymorphic queries
• Columns are strictly typed

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The Problem Object-Relational Mismatch

• Identity Mismatch
• Object Identity vs. Object Equality
• Different objects can map to the same column or
  columns
• Database Identity Strategies
• Natural Keys versus Surrogate Keys

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The Problem Object-Relational Mismatch

• Associations
• Object References versus Foreign Keys
• Directionality of Associations
• Tables associations are always one-to-many or
  many-to-one
• Object associations can be many-to-many

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The Problem Object-Relational Mismatch

• Object Graph Navigation
• Object-oriented applications walk the object
  graph
• Object-oriented systems access what they need
• With database we need a plan of what to
  retrieve
• N1 Problem is a common symptom

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Object-Relational Mapping

• Tools/techniques to store and retrieve objects
  from a database
• From the code perspective it behaves like a
  virtual object database
• A complete ORM solution should provide
• Basic CRUD functionality
• An Object-Oriented Query Facility
• Mapping Metadata support
• Transactional Capability

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HibernateRelational Persistence For Idiomatic
Java

• Provides
• Transparent persistence
• Object-querying capabilities
• Caching
• Works with fine-grained POJO-based models
• Supports most Databases
• Created by Gavin King, now part of the JBoss
  Group

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Hibernate Relational Persistence For Idiomatic
Java

• Declarative programming
• Uses Runtime Reflection
• Query Language is SQL-like
• Leverages programmers knowledge
• Mappings
• Typically defined as XML documents
• But you never have to write XML if you dont want
  to (XDoclet, JSE 1.5 Annots)

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Hibernate Relational Persistence For Idiomatic
Java
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HibernateCore Concepts

• Session Factory
• is a cache of compiled mappings for a single
  database.
• used to retrieve Hibernate Sessions
• Session
• Short lived
• Temporary bridge between the app and the data
  storage
• Provides CRUD operations on Objects
• Wraps a JDBC Connection / J2EE Data Source
• Serves as a first level object cache

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Architecture
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ArchitectureChoices

• How much you use depends on your needs
• Lightweight
• Basic persistence
• Fully Integrated
• Transactions
• Caching
• Connection Pooling

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O/R Mappings

• Hibernate Mapping (.hbm.xml) Files define
• Column to Field Mappings
• Primary Key mapping generation Scheme
• Associations
• Collections
• Caching Settings
• Custom SQL
• And many more settings

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HBM XML File

• lthibernate-mapping
• package"com.integrallis.techconf.domain"gt
• ltclass name"Address"gt
• ltid name"Id" column"PK_ID"
  type"integer"gt
• ltgenerator class"identity" /gt
• lt/idgt
• ltproperty name"StreetAddress /gt
• ltproperty name"AptNumber" /gt
• lt/classgt
• lt/hibernate-mappinggt

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O/R Mappings

• Hibernate uses smart defaults
• Mapping files can be generated
• Manually
• Tedious, boring and error prone
• Total control of the mapping
• From POJOs
• XDoclet, Annotations
• From Database Schema
• Middlegen, Synchronizer, Hibernate Tools

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Configuring Hibernate

• Hibernate Session Factory configured via
  hibernate.cfg.xml
• CFG file determines which mappings to load
  (.hbm.xml files)
• In your application SessionFactory is a singleton
• You request a Session as needed to work with
  persistent objects

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The Big Picture
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Working with Persistent ObjectsSaving an Object

• Address address new Address()
• ...
• Session session null
• Transaction tx null
• try
• session factory.openSession()
• tx session.beginTransaction()
• session.save(address)
• tx.commit()
• finally
• session.close()

• POJO is created
• Start a Session
• Transaction is started
• The object is saved
• The transaction is committed
• The session is closed

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Working with Persistent ObjectsLoading an Object

• Address address null
• Session session null
• session factory.openSession()
• try
• address (Address)
• session.get(Address.class, id)
• finally
• session.close()

• Start a Session
• POJO is loaded
• PK and Object Class are provided
• The session is closed

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Working with Persistent ObjectsDeleting an Object

• Session session null
• Transaction tx null
• try
• session factory.openSession()
• tx session.beginTransaction()
• session.delete(address)
• tx.commit()
• finally
• session.close()

• Start a Session
• Transaction is started
• The object is deleted
• The transaction is committed
• The session is closed

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Persistence LifecyclePossible States

• Transient
• Not Associated with a database table
• Non-transactional
• Persisted
• Object with Database Identity
• Associates object with a Session
• Transactional
• Detached
• no longer guaranteed to be in synch with the
  database

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Persistence Lifecycle
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Working with Collections

• Can represent a parent-child/one-many
  relationship using most of the available Java
  Collections
• Persistence by reach-ability
• No added semantics to your collections
• Available Collection mappings are
• ltsetgt, ltlistgt, ltmapgt, ltbaggt, ltarraygt and
  ltprimitive-arraygt
• In the database mappings are defined by a foreign
  key to the owning/parent entity

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Working with CollectionsA simple example
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Working with Collections A simple example

• public class Conference implements Serializable
• ...
• // Tracks belonging to this Conference
• private Set tracks
• public Set getTracks () return tracks
• public void setTracks (Set tracks)
  this.tracks tracks
• // maintain bi-directional association
• public void addTrack(Track track)
• if (null tracks) tracks new
  HashSet()
• track.setConference(this)
• tracks.add(track)

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Working with Collections A simple example

• lthibernate-mapping package"com.integrallis.tcms.d
  omain"gt
• ltclass name"Conference"gt
• ltid column"PK_ID" name"Id
• type"integer"gt
• ltgenerator class"identity" /gt
• lt/idgt
• ...
• ltset name"Tracks" inverse"true"
• cascade"all" lazy"false"gt
• ltone-to-many class"Track" /gt
• ltkey column"FK_CONFERENCE_ID" /gt

• Class/Table
• How to generate PK
• Other field mappings
• Java Set for the Tracks
• Class on the many side
• Foreign Key on the many side

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Association MappingsSupported Associations

• One-to-one
• Maintained with Foreign Keys in Database
• One-to-many / Many-to-one
• Object on the one side
• Collection on the many side
• Many-to-Many
• Use a mapping table in the database

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Association MappingsSome Details

• Inverse attribute used for bi-directional
  associations
• Lazy Loading
• Configured per relationship
• Uses dynamic proxies at Runtime
• Cascading Styles
• none no operations are cascaded
• all all operations are cascaded
• Every operation in Hibernate has a corresponding
  cascade style

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Hibernate Query LanguageHQL

• An objectified version of SQL
• Polymorphic Queries
• Object parameters in Queries
• Less verbose than SQL
• Doesnt hide the power of SQL
• SQL joins, Cartesian products
• Projections
• Aggregation (max, avg) and grouping
• Ordering
• Sub-queries
• and more

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Hibernate Query LanguageHQL

• Simplest HQL Query
• Return all Addresses
• session sessionFactory.openSession()
• // query string Address refers to a Class not
  a Table
• String queryString "from Address"
• // create, configure and execute the query
• List addresses session.createQuery(queryString).
  list()

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Hibernate Query LanguageHQL

• A more elaborate HQL Query
• Return all Tracks in a Conference
• Conference conference
• session sessionFactory.openSession()
• // build a query string
• String queryString from Tracks as t where
  t.Conference conf
• // create, configure and execute the query
• List addresses session.createQuery(queryString)
• 
  .setObject(conf, conference)
• 
  .list()

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Hibernate Query LanguageHQL Parameters

• Named parameters removed positional problems
• May occur multiple times in the same query
• Self-documenting
• List tracks new ArrayList()
• tracks.add(JSE")
• tracks.add(JEE")
• Query q session
• .createQuery("from Sessions s where
  s.Track in (trackList)")
• q.setParameterList(trackList", tracks)
• List sessions q.list()

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Hibernate Query LanguageHQL Parameters

• Specify bounds upon your result set
• The maximum number of rows
• The first row you want to retrieve
• Query query session.createQuery("from Users")
• query.setFirstResult(50)
• query.setMaxResults(100)
• List someUsers query.list()

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Query by Criteria

• Same query using the Criteria API
• Return all Tracks for a given Conference
• Conference conference
• session sessionFactory.openSession()
• // create and expression to match the given
  conference
• Expression exp Expression.eq(Conference",
  conference)
• List addresses session
• 
  .createCriteria(Tracks.class)
• .add(exp)
• .list()

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Query by Example

• Return all Address for a given Street Name
• Address address new Address()
• Address.setStreetAddress(Main Street)
• // create example object
• Example example Example.create(address)
• 
  .ignoreCase()
• 
  .enableLike(MatchMode.ANYWHERE)
• // create, configure and execute the query
• List matches session.createCriteria(Address.clas
  s)
• 
  .add(example)
• .list()

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Caching in Hibernate

• 1st Level
• Transaction scoped cache provided by the Session.
  
• Always available
• 2nd Level
• Process scoped cache
• Shared by all Sessions
• Optional
• Pluggable implementation, can be clustered
• Query
• Use with care, profile first

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Caching in Hibernate
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Caching in Hibernate

• Caching setting in HBM for Read Only
• ltclass nameReferenceData
• tableREFDATAgt
• ltcache usageread-only /gt
• ..
• lt/classgt

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Caching in Hibernate

• Caching setting in HBM for Read/Write
• Profile the application before applying
• ltclass nameReadMostly
• tableREADMOSTLYgt
• ltcache usagenonstrict-read-write /gt
• ..
• lt/classgt

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Caching in Hibernate

• Query Caching, most difficult to get right
• Analyze/Profile First. Let usage determine which
  queries to cache
• String queryString from ...
• List hits session
• .createQuery(queryString)
• .setObject(user, user)
• .setCacheable(true)
• .list()

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Other Hibernate Features

• Multiple Strategies for supporting Inheritance
• Scalar Query Results
• Projections
• Custom SQL for Loading, Updating and Deleting
• J2EE/JTA Friendly
• Spring Integration

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ConclusionsWhy should you use Hibernate?

• Its a well designed product. Look at the source!
• It covers 80 of your persistence needs and does
  a fine job on the other 20
• You get to work with POJOs, not coarse-grained
  EJBs
• Performance is good! Boils down to the generated
  SQL
• If you dont like it, you can always use custom
  SQL

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ConclusionsWhy should you use Hibernate?

• Good separation of integration tier and the rest
  of the application
• Avoid reinventing the well
• Good community support, large pool of talent

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Lets Look at some CodeQuick Live Demos

• Some CRUD Demos
• Inheritance Demo
• using Table-Per-Class-Hierarchy
• Hibernate Components
• Hibernate code in a J2EE app

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• Questions?

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• Thank you!

You can contact me with questions at
bsbodden_at_integrallis.com Our website http//www.i
ntegrallis.com Updated Slides Online
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• Shameless Plug -)

get the book
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• Shameless Plug -)

so that I can feed him
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