Component Based Development Advanced part

1
Component Based DevelopmentAdvanced part

• Persistence using Hibernate
• By Robert Bialek
• 2005.05.18
• (based on Chistian Bauer's tutorial Hibernate
  Team)

2
Overview

• Persistence
• Object-Relational Mapping (ORM)
• Hibernate
• Hibernate - Example
• Supported Query Languages

3
Complexity of persistence

• Mapping object model(OM) to relational schema(RS)
• Relational model one-many, many-many relations
• Object model association, aggregation,
  composition, multiplicity and inheritence
• Keeping OM and RS in sync
• Keeping SQL up-to-date when the models change
• Querying data where to write the query code ?
• Performance SQL for every operation?

4
Goal Transparent Persistence

• Any class can be a persistent class
• No interfaces have to be implemented
• No persistent superclass has to be extended
• Persistent classes can be used outside of the
  persistence context (Unit Tests)
• Full portability without any dependency

5
Solution Object-Relational Mapping (ORM)

• Advantages
• Structural mapping more robust
• Less error-prone code
• Optimized performance all the time
• Vendor independence
• However
• Relational model is important
• Always ensure data integrity using database
• The data in the database will be around much
  longer than the application

6
Hibernate what is it?

• Popular Open Source ORM framework
• It is not abstract, high level persistence
  framework like JDO or EJB
• It doesn't try to abstract away complexity to a
  generic persistence framework
• Instead, it specializes only in transparent
  persistence of objects in relational database
• Works with many different databases
• Relies on SQL databases while staying in Java OO
  paradigm
• EJB 3.0 - Hibernate implements the persistence
  API and query language defined by EJB 3.0

7
Hibernate - features

• Persistence for POJOs (Plain Old Java Objects)
• Flexible and intuitive mapping
• Support for fine-grained object models
• Powerful, high performance queries
• Dual-Layer Caching Architecture (HDLCA)
• Retrieve objects by OID or using a query (HQL,
  QBE, SQL)

8
Hibernate requirements for a persistent class

• JavaBean specification (or POJOs)
• No-arg constructor
• Accessor methods for properties
• Collection property is an interface
• Identifier property

9
Hibernate - Example
10
Hibernate XML MappingAuctionItem.hbm.xml to
reside at the same location as the compiled class
(or .jar)

• lthibernate-mappinggt
• ltclass nameAuctionItem table"AUCTION_ITEM"gt
• ltid name"id" column"ITEM_ID"gt
• ltgenerator class"native"/gt
• lt/idgt
• ltproperty name"description" column"DESCR"/gt
• ltmany-to-one name"successfulBid"
• column"SUCCESSFUL_BID_ID"/gt
• ltset name"bids" cascade"all" lazy"true"gt
• ltkey column"ITEM_ID"/gt
• ltone-to-many class"Bid"/gt
• lt/setgt
• lt/classgt
• lthibernate-mappinggt

11
Hibernate Mapping Configuration

• ltclassgt - mappings of a class and its elements
• ltcomponentgt - dependent class (without primary
  key)
• ltpropertygt - mappings of class' properties
• ltidgt - mapping of the primary key

12
Hibernate Usage steps

• // 1. Build an item
• AuctionItem a new AuctionItem()
• a.setName(Robert)
• // 2. Fire up Hibernate
• Configuration cfg new Configuration()
• .addClass(AuctionItem.cla
  ss)
• SessionFactory sf cfg.buildSessionFactor
  y()
• // 3. Open Session
• Session sess sf.openSession()
• // 4. Save AuctionItem and close Session
• Transaction t sess.beginTransaction()
• sess.save(a)
• t.commit()
• sess.close()

13
Hibernate Use example 1

• Retrieve an AuctionItem and change the
  description
• Session session sessionFactory.openSession()
• Transaction tx session.beginTransaction()
• AuctionItem item
• (AuctionItem) session.get(ActionItem.class,
  itemId)
• item.setDescription(newDescription)
• tx.commit()
• session.close()

Get by primary key
14
Hibernate Use example 2

• Retrieve an AuctionItem and create a new
  persistent Bid
• Bid bid new Bid()
• bid.setAmount(bidAmount)
• Session session sessionFactory.openSession()
• Transaction tx session.beginTransaction()
• AuctionItem item
• (AuctionItem) session.get(ActionItem.class,
  itemId)
• bid.setItem(item)
• item.getBids().add(bid) // (No managed
  associations)
• tx.commit()
• session.close()

15
Supported Query Languages

• Hibernate Query Language (HQL)
• minimal object-oriented dialect of ANSI SQL
• Criteria Queries (QBC)
• extensible framework for query objects
• includes Query By Example (QBE)
• Native SQL queries
• direct pass through with automatic mapping
• named SQL queries in meta-data

16
HQL Example

• List allAuctions session.createQuery(
• select item
• from AuctionItem item
• join item.bids as bid
• where item.description like Hibernate
• and bid.amount gt 100
• ).list()
• i.e. get all the AuctionItems with a Bid worth
  more
• than 100 and an item description that begings
  with Hibernate

17
QBC Example
List auctionItems session.createCriteria(Auctio
nItem.class) .setFetchMode("bids",
FetchMode.EAGER) .add( Expression.like("descr
iption", Hibernate) ) .createCriteria("succ
essfulBid") .add( Expression.gt("amount",
100) ) .list() i.e. get all the
AuctionItems with a Bid worth more than 100 and
an item description that begings with Hibernate
18
Conclusion

• Hibernate
• simple to use and powerful ORM framework
• no dependencies between the application and the
  database
• Supports EJB and POJOs
• Retrieving persistent objects based on
• OID (primary key)
• Various queries

19
Resources

• ttp//www.hibernate.org/
• ttp//www.systemmobile.com/articles/IntroductionTo
  Hibernate.html Introduction to hibernate
• ttp//www.hibernate.org/159.html Presentations