Data Persistence and Object-Relational Mapping

Data Persistence and Object-Relational Mapping

Data Persistence and Object-Relational Mapping Slides by James Brucker, used with his permission 1 Goal Applications need to save data to persistent storage. Persistent storage can be database, directory service, or other. For O-O programming, we'd like to save and retrieve objects to/from storage. Java Application

object Persistent Storage 2 The Problem with Databases Databases store data in rows in tables, which are not like objects. We can simulate object associations and collections using relations between rows in tables. Preserving uniqueness of objects and some object properties using persistence is difficult.

Some conceptual differences exist, referred to as the Object-Relational Paradigm Mismatch 3 Object-Relational Mapping Purpose: save object as a row in a database table retrieve data from tables and create objects save and recreate associations between objects

Design Goals: separate object-relational mapping services from the rest of our program minimize the impact of changing database vendor or database schema 4 An Example An Event Manager application with these classes: 5 Object-Relational Mapping Map between an object and a row in a database table. Location id: int

name: String address: String Data Mapper PK LOCATIONS id INTEGER name VARCHAR(80) address VARCHAR(160) Class should have an identifier attribute Data Mapper convert object to table row data, convert data types,

instantiates objects Database Table identifier is usually the primary key of table 6 Mapping an Object ku : Location object diagram id = 101 name = "Kasetsart University" address = "90 Pahonyotin ..." save( ) id 101 102 LOCATIONS

name address Kasetsart University 90 Pahonyotin ... Seacon Square 120 Srinakarin ... 7 O-R Mapping Code for Location (1) Location ku = new Location( "Kasetsart University" ); ku.setAddress( "90 Pahonyotin Road; Bangkok" ); // save the location dataMapper.save( location ); Issues: data mapper should choose a unique ID for persisted objects what happens if same data is already in the table? 8 O-R Mapping Code for Location (2) // retrieve the location

Location ku1 = dataMapper.find( "Kasetsart University" ); Location ku2 = dataMapper.find( "Kasetsart University" ); how to we tell find what field to search for? id? name? our code does same find twice, does mapper return the same object? ( ku1 == ku2 ) => true or false? // update the address ku1.setAddress( "Kampaengsaen Road; Kampaengsaen" ); is the address updated automatically in the database? 9 Transparent Persistence With transparent persistence, any changes to a persistent object are automatically propagated to the database. Location ku = new Location( "Kasetsart University" ); ku.setAddress( "90 Pahonyotin Road; Bangkok" ); // save the location dataMapper.save( ku ); // change the address ku.setAddress( "Kampaengsaen, Nakorn Pathom" );

id 101 102 LOCATIONS name address Kasetsart University Kampaengsaen ... Seacon Square 120 Srinakarin ... 10 O-R Mapping of n-to-1 Associations Event id: int name: String startDate: Date location: Location

* Location id: int 1 name: String address: String 11 O-R Mapping of n-to-1 Associations Event id: int name: String startDate: Date location: Location * Location id: int

1 name: String address: String The Data Mapper converts a n-to-1 association to a foreign key relation (persist) or foreign key to object (retrieve). EVENTS PK id INTEGER name VARCHAR start_date TIMESTAMP FK location_id INTEGER LOCATIONS PK id INTEGER name VARCHAR

address VARCHAR 12 O-R Mapping Code for Event Event event = new Event( "Java Days" ); Location ku = new Location( "Kasetsart University" ); ku.setAddress( "90 Pahonyotin Road; Bangkok" ); event.setLocation( ku ); event.setStartDate( new Date(108,Calendar.JULY, 1) ); // save the event dataMapper.save( event ); when we save the event, does dataMapper save the location, too? 13 O-R Mapping Code for Event // retrieve the event Event evt = dataMapper.find( "Java Days" ); Location location = evt.getLocation( ); // null?

when we get the event, does the dataMapper create the location, too? // delete the event Event evt = dataMapper.find( "Java Days" ); dataMapper.delete( evt ); does the dataMapper delete the location, too? what if other events (still in database) also refer to this location? 14 O-R Mapping of 1-to-n Associations Event id: int name: String startDate: Date Speaker speakers id: int name: String *

telephone: String 15 O-R Mapping of 1-to-n Associations Event id: int name: String startDate: Date Speaker speakers id: int name: String * telephone: String Event has a collection of Speakers. The Data Mapper must save collection as Speaker entries with FK reference to Event. EVENTS

PK id INTEGER name VARCHAR start_date TIMESTAMP FK location_id INT SPEAKERS PK id INTEGER name VARCHAR telephone VARCHAR FK event_id INTEGER 16 O-R Mapping Code for Collections (1) Event event = new Event( "Java Days" );

event.setLocation( ku ); // add event speakers Speaker gosling = new Speaker( "James Gosling" ); Speaker yuen = new Speaker( "Prof. Yuen" ); event.getSpeakers().add( gosling ); event.getSpeakers().add( yuen ); // save the event dataMapper.save( event ); Issues: same issues as many-to-1 association 17 O-R Mapping Code for Collections (2) // retrieve the event Event evt = dataMapper.find( "Java Days" ); Set speakers = evt.getSpeakers( ); out.println( "Speakers for " + evt.getName( ) ); for( Speaker spkr : speakers ) out.print( spkr.getName() );

what kind of collection does dataMapper return? can we use any collection we want in the Event class? public class Event { private Set speakers = new ______; // ? what kind of collection ? public setSpeakers( Set speakers ) { this.speakers = speakers; } 18 O-R Mapping of Ordered Collections Event id: int name: String startDate: Date Session sessions id: int {ordered}* name: String speaker: Speaker 19

O-R Mapping of Ordered Collections Event id: int name: String startDate: Date Session sessions id: int {ordered}* name: String Event has a list or array of Sessions. The Data Mapper must store a foreign key and a list index in the Session table. EVENTS PK id INTEGER name VARCHAR start_date TIMESTAMP

FK location_id INT SESSIONS PK id INTEGER name VARCHAR FK event_id INTEGER session_idx INT FK speaker_id INT 20 O-R Mapping Code for a List // add sessions to the event we already saved Event event = dataMapper.find( "Java Days" ); Speaker gosling = dataMapper.find( "James Gosling" ); Session opening = new Session( "Opening Ceremony" ) ); opening.setSpeaker( gosling ); // make opening be the 1st session (sessions is a List)

event.getSessions().add( 0, opening ); ... add more sessions ... // update the event dataMapper.update( event ); does dataMapper use the existing data for gosling in the new Session? what if our update changes the indices of other objects in the list? 21 O-R Mapping of m-to-n Associations Event id: int name: String startDate: Date events * Attendee id: int

attendees * name: String telephone: String 22 O-R Mapping of m-to-n Associations Event id: int name: String startDate: Date events * Attendee id: int attendees * name: String telephone: String

EVENT_ATTENDEE PK id INTEGER FK event_id INTEGER FK attendee_id INTEGER EVENTS PK id name start_date FK location_id INTEGER VARCHAR TIMESTAMP INT ATTENDEES PK id INTEGER

name VARCHAR telephone VARCHAR 23 Design of a Data Mapper Problem: What behavior do we need for a data mapper? What operations should it perform? 24 Object-Relational Operations: CRUD Common O-R operations are: Create - save (persist) a new object in the database Retrieve an object from the database Update data for an object already saved in database Delete object's data from the database 25

Design Model for Data Mapper Data Mapper T load( id ) : T find( query : String ) : T[*] findAll( ) : T[*] A UML Type Parameter save( object : T ) update( object : T ) delete( object : T ) The method to "load" an Object by its identifier is sometimes named: load( id ) the Hibernate method name and Spring name

find( id ), findById( id ) get( id ) similar to load( id ) but no exception if id is not found 26 A Data Mapping for Event Class Data Mapper is also called "Data Access Object" (DAO). Hibernate uses the term data access object. We use DAO in data mapper names, e.g. EventDao. EventDao findById( id: int ) : Event find( query: String ) : Event[*] save( evt: Event ) update( evt: Event )

delete( evt: Event ) 27 Layered Design User Interface data xfer object ui event Application Logic CRUD request Domain Objects DAO ORM API Other Services domain object O-R Mapping Framework JDBC API

Foundation Classes domain object ResultSet, etc. JDBC 28 What's Next? The Choices: 1. Write the OR Mapping yourself using Java and JDBC 2. Use an existing O-R Framework

SQL Fundamentals JDBC Fundamentals Configure Database Compare O-R frameworks Learn to use one framework Configure Database

29 Transactions These frameworks generally go beyond CRUD They also allow something called a transaction begin_xtn(); operations on the database commit_xtn() or abort_xtn();

Has all or nothing guarantees. And they support locking for concurrency control. 30 Persistence Frameworks Hibernate - widely used open-source persistence framework for Java. Persistence of POJOs, uses mapping files and object-query language to decouple Java from database. NHibernate for .Net languages. iBatis - simple, uses SQL maps. Database schema not transparent to Java code.

Entity Enterprise JavaBeans - uses EJB container services to perform persistence. Resource hog. Cayenne - Apache project, has GUI modeler that eliminates need to write xml files. Can reverse engineer database or generate database schema & Java code. TopLink (Oracle), Torque (Apache DB), Castor, ... 31 Standards and APIs

Java Data Objects (JDO) - transparent persistence of POJOs; defines query language (JDOQL) and standard for XML descriptors. implementations: Kodo, JPOX Java Persistence API (JPA) - part of the EJB 3.0, defines OR standard, query language (JPQL), and standalone POJO or EJB server-based persistence. implementations: TopLink Essentials (Glassfish project), OpenJPA. Hibernate is JPA compliant. Article: Adopting a Java Persistence Framework, http://today.java.net/pub/a/today/2007/12/18/adopting-javapersistence-framework.html 32

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