Java程序辅导

COMP9321 Web Application Engineering 
 
Design Patterns II 
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http://webapps.cse.unsw.edu.au/webcms2/course/index.php?cid=2442 
Dr. Basem Suleiman 
Service Oriented Computing Group, CSE, UNSW Australia 
 
Semester 1, 2016, Week 7 
Acknowledgement/Contributions  
Service Oriented Computing Group, CSE, UNSW Australia  
• Dr. Helen Paik 
• Prof. Boualem Bentallah  
• Dr. Srikumar Venugopal  
• Dr. Moshe Chai Barukh  
• Dr. Amin Beheshti 
• Dr. Basem Suleiman   
• Many others from service oriented computing group 
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J2EE Design Patterns 
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Week 6, Design Pattern Part I: 
 
Design guidelines:  
• Servlets, JSP, JavaBeans  
 
Model View Controller:  
• MVC is the J2EE BluePrints recommended architectural design pattern for 
interactive applications 
• Two MVC architectures, centralized and multiple controllers  
 
Front Controller (Command):  
• For providing a central dispatch point to handle all incoming requests. 
J2EE Design Patterns 
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This Week, Design Pattern Part II: 
 
Service Locator:  
• Typically used in business layer for locating resources (such as database 
connection) 
 
Data Access Object:  
• A typical pattern for data access layer (linking the data storage layer with 
the application) 
 
Business Delegate:  
• A pattern to reduce coupling between presentation-tier clients and 
business services 
Service Locator Pattern 
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Service Locator Pattern 
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The service locator pattern is a design pattern used in software 
development to encapsulate the processes involved in obtaining a 
service with a strong abstraction layer. 
 
Context 
 
• Service lookup and creation involves complex interfaces and network operations. 
 
Problem 
 
• When J2EE clients interact with the server side components (EJB: Enterprise 
Java Beans) or DataSources, clients must locate the service component, which  
referred to as a lookup operation in JNDI:  Java Naming and Directory Interface 
 
• Locating a JNDI-managed service object is common to all clients that need to 
access that service object 
 
• It is easy to see that many types of clients repeatedly use the JNDI service, and 
the JNDI code appears multiple times across these clients. This results in an 
unnecessary duplication of code in the clients that need to look up services. 
Service Locator Pattern 
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Solution 
 
• Using a central registry known as the "service locator", which on request returns 
the information necessary to perform a certain task. 
 
• Service Locator object will abstract all JNDI usage to hide the complexities of 
initial context creation and lookup operations 
 
• Multiple clients can reuse the Service Locator object to reduce code complexity, 
provide a single point of control 
msdn.microsoft.com 
Service Locator Pattern – Relationships  
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To build a service locator pattern, we need: 
 
Service Locator 
 
InitialContext 
 
ServiceFactory 
 
BusinessService  
Service Locator Pattern 
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To build a service locator pattern, we need: 
 
Service Locator:  
The Service Locator abstracts the API lookup services, vendor dependencies, 
lookup complexities, and business object creation, and provides a simple 
interface to clients 
 
InitialContext 
 
ServiceFactory 
 
BusinessService  
 
 
Service Locator Pattern 
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To build a service locator pattern, we need: 
 
Service Locator:  
The Service Locator abstracts the API lookup services, vendor dependencies, lookup 
complexities, and business object creation, and provides a simple interface to clients. 
 
InitialContext:  
The InitialContext object is the start point in the lookup and creation process. 
 
ServiceFactory: 
 
BusinessService: 
Service Locator Pattern 
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To build a service locator pattern, we need: 
 
Service Locator:  
The Service Locator abstracts the API lookup services, vendor dependencies, lookup 
complexities, and business object creation, and provides a simple interface to clients. 
 
InitialContext:  
The InitialContext object is the start point in the lookup and creation process. 
 
ServiceFactory:  
The ServiceFactory object represents an object that provides life cycle management for 
the BusinessService objects. eg., The ServiceFactory object for enterprise beans is an 
EJBHome object. 
 
BusinessService: 
Service Locator Pattern 
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To build a service locator pattern, we need: 
 
Service Locator:  
The Service Locator abstracts the API lookup services, vendor dependencies, lookup 
complexities, and business object creation, and provides a simple interface to clients. 
 
InitialContext:  
The InitialContext object is the start point in the lookup and creation process. 
 
ServiceFactory: The ServiceFactory object represents an object that provides life cycle 
management for the BusinessService objects. eg., The ServiceFactory object for 
enterprise beans is an EJBHome object. 
 
BusinessService: is a role that is fulled by the service that the client is seeking to 
access. The BusinessService object : 
• is created or looked up or removed by the ServiceFactory.  
• in the context of an EJB application is an enterprise bean.  
• the context of JDBC is a DataSource. 
 
Service Locator Pattern –  Participants and 
Responsibilities 
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Identifying Service Locator Pattern in the phonebook lab 
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Identifying Service Locator Pattern in the phonebook lab 
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Identifying Service Locator Pattern in the phonebook lab 
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Dependency Injection 
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Dependency 
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SAX Books Parser Example 
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What is "dependency injection" ? 
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• In software engineering, dependency injection is a software design 
pattern that implements inversion of control for 
resolving dependencies. 
 
• Dependency injection means giving an object its instance variables. 
 
• Dependency injection provides the ability to pass by reference (or 
"inject"), service objects into a client (a class or a delegate) at 
deployment time.  
 
• This is a top-down approach, in contrast to a bottom-up one wherein 
the clients discover or create service objects on their own.   
Benefits of "dependency injection" … 
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 Data Access Object 
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 Data Access Object 
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Context 
 
• Access to data varies depending on the source of the data. Access to persistent 
storage, such as to a database, varies greatly depending on the type of storage 
(relational databases, object-oriented databases, flat files, and so forth) and the 
vendor implementation. 
 
Problem 
 
• For many applications, persistent storage is implemented with different 
mechanisms, and there are marked differences in the APIs used to access these 
different persistent storage mechanisms. Other applications may need to access data 
that resides on separate systems.  
 
• An example is where data is provided by services through external systems such as 
business-to-business (B2B) integration systems, credit card, bureau service, and so 
forth. 
 http://www.oracle.com/technetwork/java/dataaccessobject-138824.html 
 Data Access Object (DAO) Pattern 
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Solution 
 
• Use a Data Access Object (DAO) pattern to abstract and encapsulate all access to the 
data source. The DAO manages the connection with the data source to obtain and store 
data. 
 
• The DAO implements the access mechanism required to work with the data source. The 
data source could be a persistent store like an RDBMS, an external service like a B2B 
exchange, or any others  
 
• The business component that relies on the DAO uses the simpler interface exposed by 
the DAO for its clients. The DAO completely hides the data source implementation 
details from its clients.  
 
• This pattern allows the DAO to adapt to different storage schemes without affecting its 
clients or business components, the DAO acts as an adapter between the component and 
the data source. 
 
http://www.oracle.com/technetwork/java/dataaccessobject-138824.html 
 Data Access Object Pattern – Relationships 
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http://www.oracle.com/technetwork/java/dataaccessobject-138824.html 
 Data Access Object: Participants and Responsibilities 
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http://www.oracle.com/technetwork/java/dataaccessobject-138824.html 
Business Delegate 
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Business Delegate 
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Context 
 
• A multi-tiered, distributed system requires remote method invocations to 
send and receive data across tiers. Clients are exposed to the complexity of 
dealing with distributed components. 
 
Problem 
 
• Presentation-tier components interact directly with business services. This 
direct interaction exposes the underlying implementation details of the 
business service application program interface (API) to the presentation tier.  
 
• As a result, the presentation-tier components are vulnerable to changes in 
the implementation of the business services: When the implementation of 
the business services change, the exposed implementation code in the 
presentation tier must change too. 
 http://www.oracle.com/technetwork/java/businessdelegate-137562.html 
Business Delegate 
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Solution 
 
• Use a Business Delegate to reduce coupling between presentation-tier 
clients and business services.  
 
• The Business Delegate hides the underlying implementation details of the 
business service, such as lookup and access details of the EJB architecture. 
 
• Using a Business Delegate reduces the coupling between presentation-tier 
clients and the system's business services. 
 
• Another benefit is that the delegate may cache results and references to 
remote business services. Caching can significantly improve performance, 
because it limits unnecessary and potentially costly round trips over the 
network. 
http://www.oracle.com/technetwork/java/businessdelegate-137562.html 
Business Delegate Pattern – Relationships  
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http://www.oracle.com/technetwork/java/businessdelegate-137562.html 
• Client: requests the BusinessDelegate to provide access to the 
underlying business service.  
 
• BusinessDelegate: uses a LookupService to locate the required 
BusinessService component. 
Business Delegate  Responsibilities  
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http://www.oracle.com/technetwork/java/businessdelegate-137562.html 
Business Delegate  Responsibilities  
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http://www.oracle.com/technetwork/java/businessdelegate-137562.html 
Business Delegate, API, and API Engineering 
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What is API? 
 
• Application programming interface (API) is a set of routines, protocols, 
and tools for building software applications. 
 
• An API expresses a software component in terms of its operations, 
inputs, outputs, and underlying types. 
 
• An API defines functionalities that are independent of their respective 
implementations. 
 
• A good API makes it easier to develop a program by providing all the 
building blocks. A programmer then puts the blocks together. 
 
What is API Engineering? 
 
• API engineering is an application of engineering to the design, 
development, and maintenance of APIs. 
 
 
 
Business Delegate, API, and API Engineering 
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Web APIs? 
 
• Web APIs are the defined interfaces through which interactions 
happen between an enterprise and applications that use its assets. 
 
• When used in the context of web development, an API is typically 
defined as a set of HTTP request messages, along with a definition of the 
structure of response messages, which is usually in an XML or  JSON 
(JavaScript Object Notation) format.  
 
 
 
Business Delegate, API, and API Engineering 
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Web APIs? 
 
• While "web API" historically has been virtually synonymous for web 
service, the recent trend (so-called Web 2.0) has been moving away from 
Simple Object Access Protocol (SOAP) based web services and 
service-oriented architecture (SOA) towards more direct 
representational state transfer (REST) style web resources and 
resource-oriented architecture (ROA). 
 
• Part of this trend is related to the Semantic Web movement toward 
Resource Description Framework (RDF). 
 
• Web APIs allow the combination of multiple APIs into new applications 
known as mashups. 
 
 
http://www.programmableweb.com/ 
More J2EE Patterns 
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Core J2EE Patterns Catalog: 
 
http://www.oracle.com/technetwork/java/index-138725.html 
 
 
 
On this site, you will find the entire Java 2 Platform, Enterprise Edition (J2EE) Pattern 
catalogue from the book Core J2EE Patterns: Best Practices and Design Strategies  
authored by architects from the Sun Java Center. 
A few more things to consider 
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Guarding a View 
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Guarding a View 
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Guarding a View 
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Guarding a View 
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Guarding a View 
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Duplicate Form Submissions 
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Duplicate Form Submissions 
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Duplicate Form Submissions 
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Synchronizer Token: a better pattern for handling duplicate form 
submission  
Synchronizer Token 
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Synchronizer Token 
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Background Tasks 
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Background Tasks 
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Background Tasks 
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References 
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• Core J2EE patterns, Deepak Alur, John Crupi and Dan Marlks, Prentice 
Hall 
• http://www.oracle.com/technetwork/java/index-138725.html 
• Patterns of Enterprise Application Architecture, Martin Fowler, Addison-
Wesley 
• http://java.sun.com/blueprints/patterns/ 
• http://www.oracle.com/technetwork/articles/javase/index-
142890.html 
 
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