Lecture 15Data Source Layer

Reading Object-relational impedance mismatch Data Source Patterns

– Data Transfer Object– Row Data Gateway– Table Data Gateway– Active Record– Data Mapper– Record set

Agenda Design Objectives Object-Relational Impedance Mismatch Problems with database programming Data Source Patterns Spring JDBC RU Data Framework Content Example

Design Objectives

The Three Layers Presentation

– User’s interface to the system– User can be another system– Accepts input, displays views

Domain– The Application of the system– The “Business logic”– Tends to creep into presentation and data source

Data Source– Connection to the database– Also Persistence

Relational Databases We are dealing with relational databases

– Wide-spread and well understood– SQL based– These are important in enterprise software

Alternatives– NoSQL databases– O/R Mappers

Connecting to Data Sources Programs need to interface the Data

Source– Usually this means relational databases

Database vendors usually supply drivers for database

Rational databases use SQL language– Fairly standard

ProgramDatabaseclasses

Driver Database

Objectives Hide SQL from the Domain Layer Access to database needs to ensure

– Speed and data integrity– Concurrent access of many clients

Database independence– It can be an objective to keep the system

independent of particular database technology Data Source Layer needs to be

maintainable– Database will change

Impedance Mismatch

Mapping Objects to Tables Object Relational Impedance Mismatch Objects are different than tables

– Encapsulation, Accessibility, Interfaces, classes, inheritance

Data types are different– Booleans, dates, string etc.

Structural differences– Classes contain other classes

The Legacy Problem In most cases the data model exists

– The schema already exists– We cannot assume that we create the schema

Data tends to stick where it lends– Cannot assume that our application controls

the schema– The schema will likely outlive the application

The Usability Problem The Database API determines the usability

of the data access– Should be easy to use

The programming model is important– Does matter how efficient and good a

persistence framework is, if it is complex and cumbersome to use

Tools may help, but should not be used to conceal excessive complexity– If tools are required to generate data access

the programming model is likely to be complex

Using Databases Programmers tend to want to solve all

problems in their domain– Should we solve all problems in our object

domain?– Should we write everything in Java or C#?

Databases are good at what they do– But it’s necessary to let them do it in a natural

way

Database Code Database programming can be very

repetitive– Opportunities for reusability– JDBC is too low-level

Code is Bad!– Don’t write code unless you have to– Try to write code for the business layer

Persistence Frameworks are difficult to build– Use the frameworks that exist

Which of these statements is false

A) Data tends to stick where it lendsB) Database programming tends to be low-levelC) Objects tend to map nicely to the databaseD) Database programming tends to be repetitive

QUIZ

Which of these statements is false

A) Data tends to stick where it lendsB) Database programming tends to be low-levelC) Objects tend to map nicely to the databaseD) Database programming tends to be repetitive

QUIZ

✔

Data Source Patterns

Domain Layer Patterns Recap Transaction Script

– Organizes business logic by procedures where each procedure handles a single request from the presentation

Domain Model– An object model of the domain that

incorporates both behaviour and data Table Module

– A single instance that handles the business logic for all rows in a database table or view

Good Design Separate database code from other code

– Provide database classes to access the database

– All SQL code in the same place– Factories for each database– Use of Connection Pools

Error handling– SQLException is isolated in the Data Source

Layer– Wrap in domain specific exceptions – use of

runtime exceptions

Design Example Domain Model uses gateways

Useful Patterns Data Transfer Object

– An object that carries data between processes in order to deduce the number of method calls

Record Set– An in-memory representation of tabular data

Data Transfer ObjectAn object that carries data between processes in

order to deduce the number of method calls Object that is used to transfer data

between layers– Data Source returns data objects to web layer

Data Transfer Object How it Works

– Similar to Value Object but is constructed to carry data between layers

– Data source layer creates DTO for transfer– DTOs holds data – get/set method– Can be mutable or immutable– Could have methods to transform data – for

example serialize the data or convert to XML– Simple Domain Objects can be used as DTO

• Creates dependencies

Data Transfer Object Assembling DTO from domain objects

– Assembler reduces dependencies

When To Use It– Whenever you need to transfer multiple items of

data between two processes in a single method call

Record Set An in-memory representation of tabular data

How It Works– Contains the result of a database query– Common in ADO.NET and JDBC– One record is current, clients can traverse the set– Usually provided by the database code

When to Use It– When returning data from a query

Data Source Patterns Table Data Gateway

– Acts as a Gateway to a database table Row Data Gateway

– Acts as a Gateway to a single record in a data source Active Record

– Wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data

Data Mapper– A layer of Mappers that moves data between objects

and database while keeping them independent

Pattern Overview

Data Source Layer Domain layer has influence on the

patterns Transaction Script

– Table Data Gateway for single access to a table– Row Data Gateway for single access to a row of

a table Domain Model

– Active Record or Row Data Gateway– Data Mapper

Table Module– Table Data Gateway with Record Set

Data Source Patterns Table Data Gateway

– Acts as a Gateway to a database table Row Data Gateway

– Acts as a Gateway to a single record in a data source Active Record

– Wraps a row in a database table or view, encapsulates the database access, and adds domain logic on that data

Data Mapper– A layer of Mappers that moves data between objects

and database while keeping them independent

Table Data GatewayAn object that acts as a Gateway to a

database table. One instance handles all the rows in the table.

Also called Data Access Objects – DAO How It Works

– Simple interface to a table with several find methods and methods for maintaining data

– CRUD methods (Create, Read, Update, Delete)– Acts as a gateway to a table– One gateway for each table– Finders return Collection of DTOs or Record Set

Table Data Gateway

When to Use It– Works for Table Module since it is based on

Record Set – Useful for web application where Domain

Model is used

Row Data GatewayAn object that acts as a Gateway to a single

record in a data source. There is only one instance per row.

How It Works– Object that is exactly one

single record– Each table column is a

field in the object– Do not have logic– Finder object– Can be generated

Row Data Gateway When to Use It

– Works well for simple domain layer for example Transaction Script

Active RecordAn object that wraps a row in a database table or view, encapsulates the database access, and

adds domain logic on that data How It Works

– Each object can read andstore itself

– Contain domain logic When to Use It

– When Domain Logic is not too complex

– When using Transaction Script

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Person

Data MapperA layer of Mappers that moves data between

objects and a database while keeping them independent of each other and the

mapper itself Sparates the in-memory objects from the

databse

Data Mapper How It works

– Simple Data Mappers map in-memory object to table on a field-to-field basis

– Others need to map more complicated object hierarchies to multiple tables

– Mapper uses Identity Map to see if object is already loaded

For insert and updates– The mapper must know what objects have changed,

which are new, and which must be destroyed– Unit of Work pattern

Data Mapper When to Use It

– Database and object model must be independent

– Data Mappers are useful with Domain Model – For simple Domain Model an Active Record

could be used, but as it becomes more complicated some mapping is needed

O/R mapping solutions can provide the mappers– For example Hibernate

Data Mapper Simple example

– Loading from the database

Data Mapper Simple example

– Updating data– Client asks the mapper to save a domain

object– The mapper pulls the data out of the domain

object and saves to the database

Data Source class maps nicely to the rows in a table and contains some useful methods

A) Row Data GatewayB) Table Data GatewayC) Active RecordD) Data Mapper

QUIZ

Data Source class maps nicely to the rows in a table and contains some useful methods

A) Row Data GatewayB) Table Data GatewayC) Active RecordD) Data Mapper

QUIZ

✔

Spring JDBC

Spring JDBC Spring JDBC packages

– org.springframework.jdbc datasource

– Classes for connecting to the database core

– Base classes for accessing the database – JdbcTemplate

object– Classes that support updating, inserting and deleting

data from the database support

– Utility classes

Loading the DataSource Code

Configuration – data.xml

Resource res = new FileSystemResource("data.xml");XmlBeanFactory factory = new XmlBeanFactory(res);DataSource ds = (DataSource)factory.getBean("dataSource");

<beans> <bean id="dataSource“ class= "org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="driverClassName"> <value>net.sourceforge.jtds.jdbc.Driver</value> </property> <property name="url”> <value>jdbc:jtds:sqlserver://honn.ru.is:1433</value> </property> <property name="username"><value>andri</value></property> <property name="password"><value>abc123</value></property> </bean></beans>

JdbcTemplate Main class of core package

– Simplifies queries Template Method pattern

– JdbcTemplate handles the processing and calls our code

– Dependency Injection

JdbcTemplate Example ParameterizedRowMapper<String> rm = new ParameterizedRowMapper<String>() { public String mapRow(ResultSet rs, int rowNum) throws SQLException { return rs.getString("title"); } };

JdbcTemplate tpl = new JdbcTemplate(getDataSource()); Collection<String> col = tpl.query("select * from contents", rm); for(String s : col) { System.out.println(s); }

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Collecting Data Spring Interface RowMapper

– An interface used by JdbcTemplate for mapping returned result sets

– Class that implements this interface can be used to collect data

public interface ParameterizedRowMapper<T> extends RowMapper<T> { Object mapRow(ResultSet rs, int rowNum) throws SQLException; }

ContentRowMapperpublic class ContentRowMapper implements ParameterizedRowMapper<Content>{ public Content mapRow(ResultSet rs, int rowNum) throws SQLException { Content content = new Content (rs.getInt (1), // id rs.getString (2), // title rs.getString (3), // link rs.getString (4), // description rs.getDate (5), // pubdate rs.getString(6)); // author return content; }}

Using ContentRowMapper JdbcTemplate method query takes

RowMapper interface as parameter ContentRowMapper crm = new ContentRowMapper();

JdbcTemplate tpl = new JdbcTemplate(ds); List l = tpl.query("select * from contents", crm);

Iterator i = l.iterator(); Content cont; while (i.hasNext()) { cont = (Content) i.next(); System.out.println(cont); }

Insert SimpleJdbcInsert

– Class for inserts public int add(Content content) { SimpleJdbcInsert insertContent = new SimpleJdbcInsert(getDataSource()) .withTableName("contents") .usingGeneratedKeyColumns("id");

Map<String, Object> parameters = new HashMap<String, Object>(5); parameters.put("title", content.getTitle()); parameters.put("link", content.getLink()); parameters.put("description", content.getDescription()); parameters.put("pubdate", content.getPubDate()); parameters.put("author", content.getAuthor()); return insertContent.executeAndReturnKey(parameters).intValue(); }

RU Data FrameworkContent Example

Content Example Table contents

– Contains content informationCREATE TABLE contents( id int Identity (1, 1) primary key NOT NULL, title varchar(128), link varchar(512) unique, description text, pubDate datetime, author varchar(128),)

Content Example Gateway class for the contents table

– ContentDataGateway interface contains the CRUD operations

– Class ContentData implements the gateway and provides the JDBC code

– Content is simple JavaBean – acts as Data Transfer Object

RU Data Framework Classes and interfaces for accessing the

database– Implementation of the Data Gateway

For Table Data Gateway– Each table has an Gateway interface– Implementation in Data classes– Factory pattern returns the implementation for

each Date Gateway

RuDataAccessFactory Factory for creating Gateway interfaces

– Reads information about the DataSource– Spring Bean Definition file: data.xml– Uses Spring Bean Factory – RuDataAccessFactory reads information on

each gateway interface and which classes to use as implementation

– Code using the gateway interface calls getDataAccess in the factory classfactory = RuDataAccessFactory.getInstance("data.xml");contentDataGateway = (ContentDataGateway) factory.getDataAccess("contentDataAccess");

Data Definition File data.xml

<beans> <!-- Data Source --> <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" > <property name="driv erClassName"> <value>net.sourceforge.jtds.jdbc.Driver</value></property> <property name="url"> <value>jdbc:jtds:sqlserver://honn.ru.is:1433</value></property> <property name="username"><value>andri</value></property> <property name="password"><value>abc123</value></property> </bean> <!– Content Gateway Interface --> <bean id="contentGateway" class="is.ru.honn.tube.data.content.ContentData"/> </bean></beans>

DataSource DataSource is a connection to a database

– Driver Class Name (net.sourceforge.jtds.jdbc.Driver)– URL (jdbc:jtds:sqlserver://honn.ru.is:1433)– username (andri)– password (abc123)

RU framework uses Spring – Load the DataSource information– DriverManagerDataSource

extends AbstractDataSourcewhich implementsDataSource

DataSource in Ru Framework RU Framework uses Spring to get

DataSource

Using RU Framework

RuDataAccess RuDataAccess is base interface for Data

gateway interfaces– All gateway interfaces extend RuDataAccess– Has methods to set and get DataSource

RuData RuData is a class implementing

RuDataAccess– Handles DataSource– Data classes extend this class

ContentDataGateway Contains all the method that are needed

to manage contents– Gateway to the contents table– Pattern Table Data Gateway

public interface ContentDataGateway extends RuDataAccess{ public int add(Content content); public List<Content> getContents();}

ContentDatapublic class ContentData extends RuData implements ContentDataGateway{ public int add(Content content) { SimpleJdbcInsert insertContent = new SimpleJdbcInsert(getDataSource()) .withTableName("contents") .usingGeneratedKeyColumns("id");

Map<String, Object> parameters = new HashMap<String, Object>(5); parameters.put("title", content.getTitle()); parameters.put("link", content.getLink()); parameters.put("description", content.getDescription()); parameters.put("pubdate", content.getPubDate()); parameters.put("author", content.getAuthor()); return insertContent.executeAndReturnKey(parameters).intValue(); }

ContentData public List getContents() { JdbcTemplate queryContent = new JdbcTemplate(getDataSource()); List<Content> contents = queryContent.query ("select * from contents", new ContentRowMapper()); return contents; }

Usagepublic class ContentServiceData implements ContentService { private ContentDataGateway contentDataGateway = null; public ContentServiceData() { RuDataAccessFactory factory = null; try { factory = RuDataAccessFactory.getInstance("data.xml"); } catch (RuException e) { ... } contentDataGateway = (ContentDataGateway) factory.getDataAccess("contentDataGateway"); } public void addContent(Content content) { contentDataGateway.add(content); } public List<Content> getContents() { return contentDataGateway.getContents(); }}

Contents Use ContentServiceData instead of a

Service Stub– Change one line in the app.xml file

<bean id="contentService" class="is.ru.honn.tube.service.ContentServiceData"> </bean>

Contents Use ContentServiceData instead of a

Service Stub– Change one line in the app.xml file

Summary Design Objectives

– Object-relational impedance mismatch– The Usability Problem– The Legacy Problem

Patterns– Table Data Gateway– Row Data Gateway– Active Record– Data Mapper– Data Transfer Object and Record set