Operation Manual TCD 2012 L04/06 V2 TCD 2013 L04/06 V2 312 ...
L04 base patterns
-
Upload
olafur-andri-ragnarsson -
Category
Technology
-
view
427 -
download
2
description
Transcript of L04 base patterns
Lecture 04Base Patterns
Agenda Base Patterns
– Gateway, Mapper, Layerd Supertype, Separated Interface, Registry, Value Object, Plugin, Service Stub, Record Set
From Problem to Patterns– Using design patterns
Reading Fowler 18
Gateway (466)An object that encapsulates access to an
external system or resource Wrap external APIs into an interface
– API is usually for accessing some external resource• Examples: JDBC, JDom, financial software
Gateway (466) How It Works
– Create a simple API and use it access the external API through a Gateway
– All access is easily defined– Change in the resource does not require
changes in the client software– Gateways should be simple – complex logic
should not be in the clients of the Gateway– Gateways can be generated
Gateway (466) When to Use It
– Gateway is useful when accessing external service
– Can be applied with Service Stub (504)– Clear benefit is that is makes it easy to swap
out one kind of resource for another
Mapper (473)An object that sets up communiction
between two independent objects Create communication between two
systems but you still need to make them independent
Mapper (473) How it Works
– A Mapper is an insulating layer between subsystems– It controls the details of communication between
them without either subsystem being aware of it– Mappers are fairly easy as they are well-defined– The tricky part is what system invokes them – third
party system or make the Mapper an Observer When to Use it
– When you want to decouple different parts of a system
Layer Supertype (475)A type that acts as the supertype
for all types in its layer Super class that contains common
functionality in a layer How it works
– Use this pattern when you have common features from all objects in a layer
Layer Supertype (475) When to use it
– When you have common features from all objects in a layer.
Example– Domain objects can
have a common superclass for ID handling
class DomainObject...
private Long ID; public Long getID() { return ID; } public void setID(Long ID) { this.ID = ID; } public DomainObject(Long ID) { this.ID = ID; }
Example: Drawing system Shape class revisited
– All objects in the drawing layer must have an origin (x and y) and implement Drawable
public abstract class Shape implements Drawable{ protected int x,y;}
Separated Interface (476)Defines an interface in a separate package from its implementation
Decouples parts of a system– Controls the dependencies between packages– Implementation can easily be changed
How it works– Interface and implementation is placed in
separate packages– Client uses the interface– Implementation can be determined at
configuration time
Separated Interface Layered System
– Domain layer depends on Data Source layer– Data Source layer cannot access Domain layer
Data Source Layer
Domain Layer
JDBCCode
InterfaceRowCallBackHandler
processRow(ResultSet rs)
Concreate classRowCallBackHandler
processRow(ResultSet rs)
implements
Code reading SQL
Execution calls
Separated interface
Separated Interface (476) Implementation is placed in a separate
packageDevelopers of the client package are responsible for the interface
Separated Interface (476) Placing the Separated
Interfaced in a third package
When to use it– When you need to break a dependency
between two parts of a system
Separated Interface (476)
Separated Interface (476) Instantiating the implementation
– User of the interface should not know the implementation
Solutions– Use a Factory and Plugin method– Use Dependency Injection
Separated Interface (476)public interface FeedHandler{ public void processObject (FeedEntry entry);}
public class ReaderClient implements FeedHandler{ ... public ReaderClient() { FeedReader reader = ReaderFactory.getFeedReader(); reader.setFeedHandler(this); reader.read("http://rss.news.yahoo.com/rss/tech"); }
public void processObject(FeedEntry entry) { System.out.println(entry); }}
Callback
Registry (480)A well-known object that other objects can use to find common objects and services
A registry is a global object How It Works
– Object that can easily be accessed at any time– Only one object available at any time– Provides services or information– Can have different scopes– Usually not mutable data– Example: System Settings, Loggers
Singleton Registry (480) Only one instance running
When to Use It– As a last resort
public class Registry{ private static Registry soleInstance = new Registry();
public static Registry getInstance() { return soleInstance; }
private Registry() { } ...}
Registry registry = Registry.getInstance();//registry = new Registry (); Does not work
Value Object (486)A small simple object, like money or date
range, whose equality isn’t based on identity
Small and easily created objects that hold and represent some data
How it works– Not based on identity– Equality is based on comparing values of the object– Can be immutable (example is the Date class)
When to use it– When you’re basing equality on something other than
identify
Value Object (486) Examples
– Date, Money
class Money...
private long amount; private Currency currency;
public Money(double amount, Currency currency) { this.currency = currency; this.amount = Math.round(amount * centFactor()); } ...
Value Object Example: DateGregorianCalendar cal = new GregorianCalendar();
cal.set(1865, Calendar.APRIL, 14);Date d1 = cal.getTime();cal.set(1963, Calendar.NOVEMBER, 22);Date d2 = cal.getTime();
System.out.println(d1.equals(d2));
cal.set(1756, Calendar.JANUARY, 27);Date d3 = cal.getTime();Date d4 = cal.getTime();
System.out.println(d3.equals(d4));falsetrue
Plugin (499)Links classes during configuration
rather than compilation Use plugin to provide specific implantation
– Plugins implement specific interface use by the client application code
– Decision at configuration time or run time– Use factory to load in the plugin– For example: on plugin for test, another for
production
Plugin (499)
caller a plugin factory a plugin configuration
getPlugin
lookupPluginByType
newa plugin
A caller obtains a Plugin implementation of a separated interface
When to Use It– Use plugin when you have behavior that
requires different implementations based on runtime environment
Plugin (499) ReaderClient uses ReaderFactory to get an
interface to FeedReader
reader.properties define the name of the actual implementation class
ReaderClient ReaderFactory reader.properties
getFeedReader
props.getProperty("reader")
newFeedReader
Plugin (499)public ReaderClient(){ FeedReader reader = ReaderFactory.getFeedReader(); ...} public class ReaderFactory
{ public static FeedReader getFeedReader() { ... try { props.load(new FileInputStream(new File("reader.properties"))); instanceClass = Class.forName(props.getProperty("reader")); reader = (FeedReader)instanceClass.newInstance(); } ... return reader; }} reader=RssFeedReader
Service Stub (504)Removes dependence upon problematic
services during testing Enterprise systems often need to access
external system– Can be out of developers control
Service Stub (504) Service stub provides implementation for
development and testing purposes– Runs locally and in-memory– Implements the same interface of the gateway
used to access the real service When to Use It
– Service stub is useful when dependence on a particular service is hindering development or testing
– Called “Mock Object” in the extreme programming world
Service Stub Examplespublic class ReaderStub extends AbstractFeedReader{ public void read(String url) { feedHandler.processEntry(new FeedEntry("title1", "Bla bla bla")); feedHandler.processEntry(new FeedEntry("title2", "Bla bla bla")); feedHandler.processEntry(new FeedEntry("title3", "Bla bla bla")); }}
title1Bla bla blatitle2Bla bla blatitle3Bla bla bla
reader=ReaderStub
reader.properties
Record Set (508)An in-memory representation of tabular data
Allows you to access database data from other objects– Scroll through a list of data
Record Set (508) How it Works
– Record set are usually provide by database classes (JDBC or ADO.NET)
– Look exactly like the results of a database query
– Provides abstraction from the database code When to Use It
– When you need a common way to manipulate data from a relational database
Summary Base Patterns
– Gateway, Mapper, Layerd Supertype, Separated Interface, Registry, Value Object, Plugin, Service Stub, Record Set
Next: From Problem to Patterns– Using design patterns
QUIZ
Question #1 You use this patterns when you need to
break a dependency between two parts of the system
A) RegistryB) GatewayC) Separated InterfaceD) Plugin
Question #2 Intent of a pattern is this: An object
that sets up communication between two objects
A) GatewayB) MapperC) RegistryD) Value Object
Question #3 Sketch of a pattern is his
A) PluginB) MapperC) RegistryD) Service Stub
Question #4 Use this pattern when you find that
dependence on a particular service is hindering your development and testing
A) MapperB) Record SetC) Service StubD) Gateway
Using Design Patterns
Using Design Patterns Normally we don’t start with patterns
– We start with problems to solve– From Problem to Pattern
Must have clear objectives for the design– The patterns will come as they are needed
Establish Design Principles– This applies to your application
Remember the separation of concern
From Problem to Pattern How do I reuse common functionality of
my objects?–Layered Supertype
How do I access an external service without becoming too dependant on it?–Gateway
How do I avoid creating unwanted dependencies?–Separated Interface
From Problem to Pattern How do I test my client code using a
service that I don’t have access to?–Service Stub
How do I link to my implementation class using configuration–Plugin
How can I keep common object available within the application–Registry
Refactoring Design, redesign, refactor
– Make the design as complete as possible– But be prepared to change design as you code– Unit tests become very important
Code Smell– Think of your code as a baby:
“If it smells, change it!”
Refactoring Refactoring is the process of improving
design in little steps at a time– Minimizes risks – calculated – Changes are controlled– Code can improve– Less likely to smell
The Danger Code Dept
“I’ll fix it later”
The four most dangerous and expensive words in programming
MailService
Mail Service We are building an web application
– One important service is sending messages in email
– We need to access the e-mail service
Mail Service We decide to use JavaMail API
– Problem is that this API is pretty low-level and complicated
– Lots of “noise” – not good to have the domain developers worry about that
What Design Pattern can we use here?
Mail Service Gateway We build a simple gateway to handle mail
– Domain developers don’t worry about the service
– We can easily change to a different mail API
Gateway (466)An object that encapsulates access to an
external system or resource Wrap external APIs into an interface
– API is usually for accessing some external resource• Examples: JDBC, JDom, financial software
MailSender (1/2) Class that sends the mail
– Method send that takes care of sending the mail
public class MailSender{ public void send(String from, String to, String subject, String body) { String smtpServer = "mail.ru.is"; try {
MailSender (2/2) Properties props = System.getProperties(); props.put("mail.smtp.host", smtpServer); Session session = Session.getDefaultInstance(props, null); Message msg = new MimeMessage(session); msg.setFrom(new InternetAddress(from)); msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to, false)); msg.setSubject(subject); msg.setText(body); msg.setSentDate(new Date()); Transport.send(msg); } catch (Exception ex) { ex.printStackTrace(); } }}
MailSender Name the problem with his class
public class MailSender { public void send(String from, String to, String subject, String body) { String smtpServer = "mail.ru.is"; try { Properties props = System.getProperties(); props.put("mail.smtp.host", smtpServer); Session session = Session.getDefaultInstance(props, null); Message msg = new MimeMessage(session); msg.setFrom(new InternetAddress(from)); msg.setRecipients(Message.RecipientType.TO,InternetAddress.parse(to, false)); msg.setSubject(subject); msg.setText(body); msg.setSentDate(new Date()); Transport.send(msg); } catch (Exception ex) { ex.printStackTrace(); } }}
MailSender Problem
– Many parameters instead of an object– Mail server is hard-coded– Exception handling is bad
public void send(String from, String to, String subject, String body) { String smtpServer = ”smtp.ru.is"; try { ... } catch (Exception ex) { ex.printStackTrace(); } }}
MailService Interface for the domain developers
– Program-to-interfaces Principle
– So let MailSender implement this interface
public interface MailService{ public void send(String from, String to, String subject, String body);}
public class MailSender implements MailService{ public void send(String from, String to, String subject, String body) { ...
Testing Testing MailService and MainSender is
easypublic class TestMail{ public static void main(String[] args) { MailService mail = new MailSender(); mail.send("[email protected]", // from "[email protected]", // to "Hallo", // subject "So does this stuff work"); // body }}
TestingTestMail: sending mail.Done.
NOTE: mail.ru.is is not good for testing!
Testing What is the problem with clients like this?
public class TestMail{ public static void main(String[] args) { MailService mail = new MailSender(); mail.send("[email protected]", // from "[email protected]", // to "Hallo", // subject "So does this stuff work"); // body }}
Improvements Problem
– MailSender implementation class is exposed to the domain layer
Solution– Use the Plugin Pattern– Create a factory that will read a
configuration file and load the mail implementation class
– Client will use the MailService interface
Plugin (499)Links classes during configuration
rather than compilation Use plugin to provide specific implantation
– Plugins implement specific interface use by the client application code
– Decision at configuration time or run time– Use factory to load in the plugin
• For example: one plugin for test, another for production
Factory with a Plugin Create a MailFactory class
– Loads mail.properties file– Creates the class specified in the properties file
and returns interface MailService– Clients use MailService and are not exposed to
particular implementation– It’s easy to change the properties file
Improvements Problem
– Can we make the loading of properties and class more generic?
– Other factories might need this functionality also Solution:
– Create a Layered Supertype– MailFactory extends Factory
Layer Supertype (475)A type that acts as the supertype
for all types in its layer Super class that contains common
functionality in a layer How it works
– Use this pattern when you have common features from all objects in a layer
Layer Supertype (475)A type that acts as the supertype
for all types in its layer Super class that contains common
functionality in a layer How it works
– Use this pattern when you have common features from all objects in a layer
Plugin Pattern
Factory Has two methods
– loadProperties– loadClass
Exception handling– Create a new exception class that we will use– FactoyException– Log the error
FactoryException Extends Exception
– Checked exception– Callers must catch this exception or explicitly
throw itpublic class FactoryException extends Exception{ public FactoryException(String message) { super(message); }
public FactoryException(String message, Throwable cause) { super(message, cause); }}
Factorypublic class Factory { Logger logger = Logger.getLogger(LogInfo.LOG_NAME);
protected Properties loadProperties(String filename) throws FactoryException { Properties props = new Properties(); try { props.load(new FileInputStream(new File(filename))); } catch (FileNotFoundException fnfex) { String msg = "Factoy: File '" + filename + "' not found."; logger.severe(msg); throw new FactoryException(msg, fnfex); } ... return props; }
Testing Fails Exception is thrown and message is
logged2.9.2007 16:49:34 is.ru.honn.rubook.factory.Factory loadClassSEVERE: Factoy: Class 'is.ru.honn.rubook.mail.MailServiceStubx' not found.
Testing Fails Problem
– MailService implementation classes have to handle FactoryException or pass it on
– Do we want clients to worry about some factory?
Solution– One solution is to catch FactoryException
and throw unchecked MailService exception
MailFactorypublic class MailFactory extends Factory{ public MailService getMailService() { MailService service; try { service = (MailService)loadClass( loadProperties("mail.properties"). getProperty("mail.service.class")); } catch(FactoryException fex) { throw new MailServiceException ("Unable to send e-mail", fex); } return service; }}
MailServiceException Extends RuntimeException
– Unchecked exception– Callers decide if they want to catch it
public class MailServiceException extends RuntimeException{ public MailServiceException(String message) { super(message); }
public MailServiceException(String message, Throwable cause) { super(message, cause); }}
Testing Using the MailFactory class
– We can catch the MailServiceException or ignore it
– Notice we have not only abstracted the Mail API but also the exception handling
public class TestMail{ public static void main(String[] args) { MailFactory mf = new MailFactory(); MailService mail = mf.getMailService(); mail.send("[email protected]", "[email protected]", "Hello", "Hello"); }}
Improvements Problem
– Exception handling in our original MailSender is bad
Solution– Use the MailServiceException public void send(MailMessage message)
{ try { ... } catch (Exception ex) { String msg = "Sending mail failed: " + ex.getMessage(); logger.severe(msg); throw new MailServiceException(msg, ex); }
SEVERE: Sending mail failed: Unknown SMTP host: mail.ru.is
Improvements Problem
– What if we don’t have access to the SMTP server at this time?
Solution– Use a Service Stub– Create the class MailServiceStub that will simply
log out the mail sent– Could also write in file
Service Stub (504)Removes dependence upon problematic
services during testing Enterprise systems often need to access
external system– Can be out of developers control
MailServiceStubpublic class MailServiceStub implements MailService{ Logger logger = Logger.getLogger(LogInfo.LOG_NAME);
public void send(String from, String to, String subject, String body) { logger.info("Sending mail from '" + from + "' to '" + to + "' Subject: '" + subject); }}
2.9.2007 16:36:08 is.ru.honn.rubook.mail.MailServiceStub sendINFO: Sending mail from '[email protected]' to '[email protected]' Subject: 'Hello
mail.service.class=is.ru.honn.rubook.mail.MailServiceStub
mail.properties
Improvements Problem
– What if we need to add new parameter?
Solution– Use an object to group parameters– Easy to change without changing the interface
public interface MailService { public void send(String from, String to, String subject, String body);}
public interface MailService{ public void send(MailMessage message);}
MailMessage Typical Data Transfer Objectpublic class MailMessage { private String from; private String to; private String subject; private String body;
public MailMessage(String from, String to, String subject, String body) { this.from = from; this.to = to; this.subject = subject; this.body = body; } public String getFrom() { return from; } public void setFrom(String from) { this.from = from; }...
Improvements Problem
– The mail server in MailSender is still hardcoded Solution
– Place in the configuration file– Let the factory inject the name into the
Mail Servicepublic interface MailService{ public void setMailServer(String mailServer); public void send(MailMessage message);}
Injecting the Mail Server Name
New MailFactoy getMailService injects the name into the
servicepublic class MailFactory extends Factory{ public MailService getMailService() { ... loadProperties("mail.properties"); service = (MailService)loadClass(getProperties(). getProperty("mail.service.class")); service.setMailServer(getProperties(). getProperty("mail.server")); // injection return service; }}
mail.service.class=is.ru.honn.rubook.mail.MailSendermail.server=mail.ru.is
Improvements Problem
– loadProperties loads the file each time used Solution
– Load once then usepublic class Factory{ private Properties properties = new Properties();
protected Properties loadProperties(String filename) throws FactoryException { ... return properties; } public Properties getProperties() { return properties; } ...
Improvements Problem
– All mail server implementations must store server name and set function
– Common functionality in multiple classes Solution
– Create a Layered Supertype– Take care of the common functionality– Make the send method abstract
AbstractMailService Implements MailService
– Provides handling of the mail server propertypublic abstract class AbstractMailService implements MailService{ protected String mailServer;
// this is used by the factory to inject public void setMailServer(String mailServer) { this.mailServer = mailServer; }
public String getMailServer() { return mailServer; }}
MailSender Extends AbstractMailService
– Does not have to implement the MailServer interface
– Can use the getMailServer methodpublic class MailSender extends AbstractMailService{ public void send(MailMessage message) { try { Properties props = System.getProperties(); props.put("mail.smtp.host", getMailServer()); ...
Summary Base Patterns
• Gateway, Mapper, Layerd Supertype, Separated Interface, Registry, Value Object, Plugin, Service Stub, Record Set
We start with problems to solve– Then we find the patterns to use– Must have clear objectives for the design
Beware of code smell Refactoring is the process of improving design
in little steps at a time Example case study
– Mail service with a configurable factory