ObjectOriented Persistence Techniques Using OSS

1
Object-Oriented Persistence Techniques Using
OSS

• By Josh Schendel

2
Overview

• Understanding the Problem
• Database Refresher
• Object-Oriented Persistence
• In .NET
• In Java (Hibernate)

3
Understanding the Problem

• Software exists to solve a problem!
• Every SE professor at this school

Image from plg.uwaterloo.ca/ migod/fun/
4
The Good Old Days

• In the early days of programming
• Hardware presented limitations
• Fewer clock cycles to solve the problem
• Make best use of RAM and other resources
• Efficiency was critical!
• Programming teams were smaller
• Many programs written by a single person
• Maintenance generally consisted of bug-fixes

Image from www.upss.com/menus.htm
5
Software Evolved

• All these GOTOs are hard to follow!
• Structural programming presents a more
  understandable flow of logic
• This main() is too long!
• Functional programming allows code to be split up
  into smaller parts
• I want to reuse parts of this program!
• Object-oriented programming abstracts data and
  methods into highly reusable data structures

6
Transitioning to OOWhy did it happen?

• Maintenance is a growing concern
• Project groups are getting larger
• Code not always maintained by original author
• Integrated, robust software in demand
• Efficiency is good, but
• Current hardware alleviates many efficiency
  concerns
• Tricks made in the interest of increased
  efficiency are often unintelligible and hinder
  maintainability

7
The Point

• The evolution of software has stressed
  maintainability over efficiency by employing
  objects
• Not all attributes simple, can contain other
  objects
• Polymorphism
• The fastest programs are NOT object-oriented!

8
The Point

• The evolution of relational database software has
  stressed parallelization and scalability over
  object-oriented support
• Transactional-based processing to allow
  multiprocessing without corruption of data
• Many database packages charge based on
  multiprocessor utilizations
• Can only store SIMPLE data types!!
• Strings
• Dates
• Integers

9
The Problem

• Object-oriented programming solutions
• Efficiency-based database solutions

• How can we persist our complex, polymorphic
  objects in these relational database systems in a
  maintainable, object-oriented fashion?

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Overview

• Understanding the Problem
• Database Refresher
• Object-Oriented Persistence
• In .NET
• In Java (Hibernate)

11
Umm Whats Persistence?

• 3 types of data in an application
• Transient
• Persistent
• Detached

Information from http//www.hibernate.org/hib_docs
/v3/reference/en/html/objectstate.html
12
Transient Data

• Does not need to be saved or retained after
  program exits
• Does not contribute significant amount of state
  to the object
• Often temporary or derived information, and often
  local to a function

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Persistent Data

• Needs to be saved or retained after program
  exits
• Contributes a significant amount of state to the
  object
• Often all private class variable for a data
  object contain persistent data
• Persistent data used to reconstruct the saved
  object after it has been saved

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Detached Data

• Data that was loaded from a database but has
  since been changed or copied
• Out of synch with the database
• Can create undesirable states

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K, Whats a Relational Database?

• A relational database is a collection of data
  items organized as a set of formally-described
  tables from which data can be accessed or
  reassembled in many different ways without having
  to reorganize the database tables.http//searc
  horacle.techtarget.com/sDefinition/0,,sid41_gci212
  885,00.html

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A Database Looks Like
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Relational Database Structure

• Server A running instance of the database
  software. Each server has multiple
• Databases Contain a set of isolated stored
  procedures, functions, and multiple
• Tables (next page)

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Relational Database Structure

• Table An entity that stores a set of data.
• Columns Each stores a single attribute, or
  field
• Rows Each row is a record, or an instance of
  data in its corresponding table

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Tables
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Normalization

• Not all information about a system can be
  persisted in a single table
• Not all information about a system is unrelated
• Information is normalized across several tables
• Related information is linked via keys

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Relationships
Image from www.koffice.org/kexi/
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Database Terms

• Connection String A string of characters that is
  used in establishing a connection to a database
• SQL Structured Query Language, a protocol used
  to specify information desired from a database
• Join A means of combining the contents of
  records from different tables based on certain
  criteria
• Key A field or set of fields on a table for
  which the table is constrained in such a way that
  no two records can coexist in the table with the
  same values

Terms from Hibernate in Action
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Database Terms

• Record One row of a table or resultset.
• Query An instance of a SQL statement that has
  been sent to a database to be performed
• Resultset The results of a query. All desired
  fields of all records that fit the criteria in
  the query are returned
• Serialization The conversion of an object into a
  meaningfully formatted string for which the
  object could use to reload its state at the time
  of conversion

Terms from Hibernate in Action
24
Overview

• Understanding the Problem
• Database Refresher
• Object-Oriented Persistence
• In .NET
• In Java (Hibernate)

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3-Tiered Development

• Presentation Layer
• GUI
• Business Logic Layer
• Policy layer
• Monkeys!
• Data/Services Layer
• Database interactions
• Focus is on making Data Layer as
• object-oriented as the rest of the system

Picture and terms from http//www.officewizard.com
/books/clientserver/ClientServerComputing.htm
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Challenges of an OO Data Layer

• Normalization
• Tables are split up to reduce redundant data
• Challenges developers capacity to treat each
  table as an object
• Composition
• Objects contain other objects
• Contained objects must be stored in separate
  table
• Inheritance
• Store inherited objects in
• Separate table?
• Same table, separate table for additional
  attributes?
• Same table, make table include attributes for all
  descendents?

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Pre-.NET Database Interfacing

• ADO
• ActiveX Data Objects
• Supplied with VB6 for classic ASP applications
• Set of objects used in interfacing with a
  database
• Link to database remained open while iterating
  through the returned data
• Implemented using DOM

Terms from http//www.vbwm.com/articles/2002/tcole
/adonet1/
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.NET Database Interfacing

• ADO.NET heavily revamps traditional ADO
• Disconnected
• On a transaction a connection is opened, data is
  retrieved, and connection is automatically closed
• Saves server and database resources!
• Implemented using XML
• VERY easy for ADO.NET objects to read XML or to
  serialize contents into XML
• Doesnt assume one size fits all
• DataReader
• DataSet

Terms from http//www.vbwm.com/articles/2002/tcole
/adonet1/
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.NET Database Interfacing

• DataReader
• Lightweight and quick
• Should be used for quick and easy database
  queries
• Able to quickly and efficiently iterate over a
  resultset
• Read-only
• Often used to bind datagrids

Terms from http//www.vbwm.com/articles/2002/tcole
/adonet1/
30
.NET Database Interfacing

• DataSet
• Heavyweight but powerful
• Each query places the resultset in a table in
  the DataSet
• Can join tables within the DataSet
• Can query the DataSet directly to filter
  records
• Quick and easy conversion to and from XML
• Great for XML parsing and persistence!

Terms from http//www.vbwm.com/articles/2002/tcole
/adonet1/
31
.NET Database Interfacing

• DataAdapter
• Mediates connection between DataReader/DataSet
  and the database
• Stores important database information
• Connection string
• SQL query to be sent
• Name of stored procedure (to be discussed)
• Parameters

Terms from http//www.vbwm.com/articles/2002/tcole
/adonet1/
32
Code!
Sub LoadCustomers() Dim myConnection As New
OleDb.OleDbConnection( _
"ProviderMicrosoft.Jet.OLEDB.4.0 Data
SourceC\Program _ Files\Microsoft
Visual) Dim myDataAdapter As New
OleDb.OleDbDataAdapter( _ "Select from
Customers", myConnection) Dim myDataSet As New
DataSet() ' Fill DataSet with table, call it
'Customers' myDataAdapter.Fill(myDataSet,
"Customers")   ' Display first CompanyName
field of first row in Customers table
MessageBox.Show(myDataSet.Tables("Customers").Rows
(0)("CompanyName")) End Sub
Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
33
But Josh Doesnt Like It
Sub LoadCustomers() Dim myConnection As New
OleDb.OleDbConnection( _
"ProviderMicrosoft.Jet.OLEDB.4.0 Data
SourceC\Program _ Files\Microsoft
Visual)

• Connection String
• Hard coded
• Connection Strings are volatile
• Change when switch from development to production
  databases
• Make private constant for the class?
• Not good enough Connection strings are
  generally constant throughout a project
• Will end up redefining the same constant in every
  class
• Solution Project-wide constant

Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
34
But Josh Doesnt Like It
Sub LoadCustomers() Dim myConnection As New
OleDb.OleDbConnection(CONSTANTS.CONN_STRING)
Dim myDataAdapter As New OleDb.OleDbDataAdapter(
_ "Select from Customers",
myConnection) Dim myDataSet As New DataSet()
' Fill DataSet with table, call it
'Customers' myDataAdapter.Fill(myDataSet,
"Customers")   ' Display first CompanyName
field of first row in Customers table
MessageBox.Show(myDataSet.Tables("Customers").Rows
(0)("CompanyName")) End Sub

• Replaced hard-coded connection string

Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
35
But Josh Doesnt Like It
Sub LoadCustomers() Dim myConnection As New
OleDb.OleDbConnection(CONSTANTS.CONN_STRING)
Dim myDataAdapter As New OleDb.OleDbDataAdapter(
_ "Select from Customers", myConnection)

• Query mixed in with the code!
• Modern databases offer stored procedures to aid
  in partitioning the Data Layer
• Stored procedures offer many advantages over
  formulating a query in-code

Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
36
Stored Procedures

• Set of pre-written SQL statements that reside in
  the database
• Code references the stored procedure instead of
  building the SQL statement
• Advantages
• Compilation
• Maintainability

Terms from http//www.vbwm.com/articles/2002/tcole
/adonet1/
37
Stored Procedures

• Compilation
• Compilation of stored procedures does NOT
  produce byte code!
• Syntax checking done when saving the stored
  procedure
• You will not be allowed to save a stored
  procedure that does not compile
• Catch query errors at compile time rather than
  run time!
• Bonus! Compilation allows speeds up queries

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Stored Procedures

• Maintainability
• Stored procedures can be reused throughout an
  application
• If the database changes, only need to update each
  affected query once in the stored procedure
• Readability enhanced
• Syntax highlighting
• Dont need to escape as many characters since the
  procedure does not need to be encased in a string

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Stored Procedures
40
But Josh Doesnt Like It
Sub LoadCustomers() Dim myConnection As New
OleDb.OleDbConnection(CONSTANTS.CONN_STRING)
Dim myDataAdapter As New OleDb.OleDbDataAdapter(
_ GET_CUSTOMERS myConnection)
myDataAdapter.SelectCommand.CommandType
CommandType.StoredProcedure Dim myDataSet As
New DataSet() ' Fill DataSet with table,
call it 'Customers' myDataAdapter.Fill(myDataS
et, "Customers")   ' Display first
CompanyName field of first row in Customers table
MessageBox.Show(myDataSet.Tables("Customers").
Rows(0)("CompanyName")) End Sub

• Replaced SQL with stored procedure

Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
41
But Josh Doesnt Like It
Dim myConnection As New OleDb.OleDbConnection(C
ONSTANTS.CONN_STRING) Dim myDataAdapter As New
OleDb.OleDbDataAdapter( _ GET_CUSTOMERS
myConnection) myDataAdapter.SelectCommand.Comma
ndType CommandType.StoredProcedure Dim
myDataSet As New DataSet() ' Fill DataSet
with table, call it 'Customers'
myDataAdapter.Fill(myDataSet, "Customers")  

• This code is reusable and non-OO
• Developers care about objects, not records in a
  DataSet!
• Instead of repeating the code every time we want
  the customers in the database, lets make a
  function that returns the resultset as objects of
  the expected type

Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
42
But Josh Doesnt Like It
Function getCustomer(ByVal id As Integer) As
Customer Dim myConnection As New
OleDb.OleDbConnection(CONSTANTS.CONN_STRING)
Dim myDataAdapter As New OleDb.OleDbDataAdapter(
_ GET_CUSTOMER myConnection)
myDataAdapter.SelectCommand.CommandType
CommandType.StoredProcedure
myDataAdapter.Parameters.Add(_at_id, id) Dim
myDataSet As New DataSet() ' Fill DataSet
with table, call it 'Customers'
myDataAdapter.Fill(myDataSet, "Customers")  
String CompanyName myDataSet.Tables(Customers
) _
.Rows(0).Item(CompanyName) Customer
customer new Customer (CompanyName, )
Return customer End Function

• This code is reusable and non-OO
• Developers care about objects, not records in a
  DataSet!
• Instead of repeating the code every time we want
  the customers in the database, lets make a
  function that returns the resultset as objects of
  the expected type

Code from from http//www.developer.com/net/vb/art
icle.php/10926_1540311_7
43
Object-Oriented Persistence in .NET

• .NET offers powerful ADO.NET objects
• By defining constants, employing stored
  procedures, and isolating Data Layer operations
  into object-oriented functions, we can emulate an
  object-oriented database from a relational one

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Overview

• Understanding the Problem
• Database Refresher
• Object-Oriented Persistence
• In .NET
• In Java (Hibernate)

45
Java Persistence

• Wouldnt it be great if all you had to do to save
  an object to the database was say

session.save(customer)
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Introducing Hibernate!

• Hibernate
• Open-source framework
• Sits between Policy and Data layer
• Transforms the frontend of a relational database
  into an object-oriented database

Information from Professional Hibernate
47
How Does it Work?

• Installation
• JAR file with classpath
• Supporting files
• hibernate.properties
• hibernate.cfg.xml

Information from Professional Hibernate
48
How Does it Work?

• hibernate.properties
• Non-XML format
• Discrepancies between this and hibernate.cfg.xml
  lose out in favor of hibernate.cfg.xml
• Will likely be depracated in the near future

Information from Professional Hibernate
49
How Does it Work?

• hibernate.cfg.xml
• XML format
• Contains all the information needed to run
  Hibernate
• This includes
• Connection String / JDBC driver information
• Need only be in ONE place in the project!
• Database dialect and other settings
• Class-to-table mappings or locations to
  class-to-table mapping files

Information from Professional Hibernate
50
How Does it Work?

• Database dialect
• Dialect A form of SQL that is constrained or
  contains proprietary keywords or formats that are
  specific to certain databases
• Since Hibernate takes care of much of the SQL, it
  must know correct dialect for the database

Information from Professional Hibernate
51
How Does it Work?

• Class-to-table mappings
• Tells Hibernate what classes to persist, what
  types each field is, and where all the
  information should be persisted.
• Tells Hibernate how to generate IDs
• Provide FREE DOCUMENTATION on how the
  relationship between object-oriented data in the
  application and the corresponding data in the
  database!

Information from Professional Hibernate
52
Example
lthibernate-configurationgt lt!-- a
SessionFactory instance listed as /jndi/name --gt
ltsession-factory name"javahibernate/S
essionFactory"gt lt!-- properties --gt
ltproperty name"connection.datasource"gtjava/co
mp/env/jdbc/MyDBlt/propertygt ltproperty
name"dialect"gtorg.hibernate.dialect.MySQLDialectlt
/propertygt ltproperty name"show_sql"gtfalse
lt/propertygt ltproperty name"transaction.fa
ctory_class"gt org.hibernate.transactio
n.JTATransactionFactory lt/propertygt
ltproperty name"jta.UserTransaction"gtjavacomp/U
serTransactionlt/propertygt lt!-- mapping
files --gt ltmapping resource"org/hibernate
/auction/Item.hbm.xml"/gt ltmapping
resource"org/hibernate/auction/Bid.hbm.xml"/gt
lt!-- cache settings --gt
ltclass-cache class"org.hibernate.auction.Item"
usage"read-write"/gt ltclass-cache
class"org.hibernate.auction.Bid"
usage"read-only"/gt ltcollection-cache
collection"org.hibernate.auction.Item.bids"
usage"read-write"/gt lt/session-factorygt lt/hi
bernate-configurationgt
53
Hibernate and SQL

• HQL (Hybernate Query Language)
• Hibernate offers a simplified query language
• Very similar to traditional SQL
• Uses dialects to convert the HQL into appropriate
  SQL
• SQL
• Hibernate also allows straight SQL statements to
  be executed
• Good for optimizing queries that Hibernate
  doesnt compile efficiently

Information from Professional Hibernate
54
Sessions

• Each time a database interaction is required, a
  session must be used.
• Sessions created from the Singleton
  SessionFactory class
• Types of Database Interactions
• Select
• Load
• Insert
• Update
• Delete

Information from Professional Hibernate
55
Select

• What is it?
• Query that creates a resultset that contains the
  specified columns for all records that meet a
  criteria
• How does Hibernate make it easy?

try Session session
sessionFactory.openSession() List users
session.find(from Users) // HQL
session.close() catch (Exception e)
Information from Professional Hibernate
56
Load

• What is it?
• Query that retrieves a unique record from a table
  that has the same ID
• How does Hibernate make it easy?

// ID initialized elsewhere try
User myUser new User() // blank Session
session sessionFactory.openSession()
List users session.load(myUser, ID) // HQL
session.close() catch (Exception e)
Information from Professional Hibernate
57
Insert

• What is it?
• Query that adds a new record to a table with
  specified properties
• How does Hibernate make it easy?

User myUser new User(Josh, Schendel,
schendej) try Session session
sessionFactory.openSession()
session.save(myUser) // HQL
session.close() catch (Exception e)
Information from Professional Hibernate
58
Update

• What is it?
• Query that changes the record in the database
  that has same ID as the object
• How does Hibernate make it easy?

//User loaded from database
myUser.setName(Mark) try Session
session sessionFactory.openSession()
session.update(myUser) // HQL
session.close() catch (Exception e)
Information from Professional Hibernate
59
Delete

• What is it?
• Query that removes a record that has the same ID
  from the database
• How does Hibernate make it easy?

//User loaded from database try
Session session sessionFactory.openSession()
session.delete(myUser) // HQL
session.close() catch (Exception e)
Information from Professional Hibernate
60
Final Notes About Hibernate

• Simplifies object persistence dramatically
• Supports more complicated queries
• Mapping documents
• Written once
• Used everywhere to simplify persistence
• Fully support all joins and most SQL transactions
  via dialects
• Object-oriented interface to the database without
  the cost of an object-oriented database

Information from Professional Hibernate
61
Bibliography
Bauer, Christian, and King, Gavin. Hibernate in
Action. Manning Publishing. 2005
Iverson, Will. Hibernate, a J2EE Developers
Guide. Pearson. 2005
Minter, Dave, and Linwood, Jeff. Pro Hibernate
3. Wiley Publishing. 2005
Pugh, Eric, and Gradecki, Joseph. Professional
Hibernate. Wiley Publishing. 2004
HIBERNATE - Relational Persistence for Idiomatic
Java. 3.11. 2/23/06. lthttp//www.hibernate.org/
hib_docs/v3/reference/en/html/objectstate.htmlgt
Visual Basic Web Magazine. Cole, Tom. 2/23/06.
lthttp//www.vbwm.com/articles/2002/tcole/adonet1/gt