Post on 16-Apr-2017
Agenda
Design ObjectivesObject-Relational Impedance MismatchProblems with database programmingData Source PatternsSpring JDBCRU Data Framework Content Example
Reading
Object-relational impedance mismatchData Source Pattern• Data Transfer Object• Row Data Gateway• Table Data Gateway• Active Record• Data Mapper• Record set
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”– Has the tendency to creep into presentation and data source▪ Data Source– Connection to the database
We are dealing with relational databases• Wide-spread and well understood• SQL based, transactionally safe• These are important in enterprise softwareDrawbacks• Bottleneck in a modern Internet system• Impedance MismatchAlternatives• NoSQL databases• Object-Relational Mappers (ORM)
Relational Databases
Programs need to interface the Data Source• Usually this means relational databasesDatabase vendors usually supply drivers for databaseRational databases use SQL language• Fairly standard
Connecting to Data Sources
Databaseclasses Driver DatabsaseProgram
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
Object Oriented world
classes, interfaces,
encapsulation, inheritance,
polymorphism
Relational Database world
Tables, view, primary keys, foreign keys,
stored procedures
Object-Relational 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 pretty low-level▪ Too much similar 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– Examples: Spring JBDC, Hibernate, JPA, NHibernate, iBatis
https://en.wikipedia.org/wiki/List_of_object-relational_mapping_software
Whichofthesestatementsisfalse
A) Datatendstostickwhereitlends B) Databaseprogrammingtendstobelow-level C) Objectstendtomapnicelytothedatabase D) Databaseprogrammingtendstoberepetitive
QUIZ
Domain Layer Patterns Recap▪ Transaction Script– Organises 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
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
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
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 and
store itself– Contain domain logic▪ When to Use It– When Domain Logic is not
too complex– When using Transaction Script
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-lastname-firstname-NumerOfDependents
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
▪ Separates the in-memory objects from the database
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– 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
DataSourceclassmapsnicelytotherowsinatableandcontainssomeusefulmethods
A) RowDataGateway B) TableDataGateway C) ActiveRecord D) DataMapper
QUIZ
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; }
ContentRowMapper
public 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(); }
Content Example▪ Table contents– Contains content information
CREATE 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 class
factory = 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 AbstractDataSource which implements DataSource
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>
RU Database▪ hrnem.ru.is
▪ Each student has their own database▪ Username and passwords sent out▪ Check the access