COMP9321 Web Application Engineering

Transcription

1 COMP9321 Web Application Engineering Design Patterns II Dr. Basem Suleiman Service Oriented Computing Group, CSE, UNSW Australia Semester 1, 2016, Week 7 1

2 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 COMP9321, 16s1, Week 3 2

3 Week 6, Design Pattern Part I: Design guidelines: Servlets, JSP, JavaBeans J2EE Design Patterns 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. 3

4 This Week, Design Pattern Part II: J2EE Design Patterns 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 4

5 Service Locator Pattern 5

6 Service Locator Pattern 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. 6

7 Service Locator Pattern 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 7

8 Service Locator Pattern Relationships To build a service locator pattern, we need: Service Locator InitialContext ServiceFactory BusinessService 8

9 Service Locator Pattern 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 9

10 Service Locator Pattern 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: 10

11 Service Locator Pattern 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: 11

12 Service Locator Pattern 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. 12

13 Service Locator Pattern Participants and Responsibilities 13

14 Identifying Service Locator Pattern in the phonebook lab 14

15 Identifying Service Locator Pattern in the phonebook lab 15

16 Identifying Service Locator Pattern in the phonebook lab 16

17 Dependency Injection 17

18 Dependency 18

19 SAX Books Parser Example 19

20 What is "dependency injection"? 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. 20

21 Benefits of "dependency injection" 21

22 Data Access Object 22

23 Data Access Object 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. 23

24 Data Access Object (DAO) Pattern 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. 24

25 Data Access Object Pattern Relationships 25

26 Data Access Object: Participants and Responsibilities 26

27 Business Delegate 27

28 Business Delegate 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. 28

29 Business Delegate 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. 29

30 Business Delegate Pattern Relationships Client: requests the BusinessDelegate to provide access to the underlying business service. BusinessDelegate: uses a LookupService to locate the required BusinessService component. 30

31 Business Delegate Responsibilities 31

32 Business Delegate Responsibilities 32

33 Business Delegate, API, and API Engineering 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. 33

34 Business Delegate, API, and API Engineering 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. 34

35 Business Delegate, API, and API Engineering 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. 35

36 More J2EE Patterns Core J2EE Patterns Catalog: 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. 36

37 A few more things to consider 37

38 Guarding a View 38

39 Guarding a View 39

40 Guarding a View 40

41 Guarding a View 41

42 Guarding a View 42

43 Duplicate Form Submissions 43

44 Duplicate Form Submissions 44

45 Duplicate Form Submissions Synchronizer Token: a better pattern for handling duplicate form submission 45

46 Synchronizer Token 46

47 Synchronizer Token 47

48 Background Tasks 48

49 Background Tasks 49

50 Background Tasks 50

51 References Core J2EE patterns, Deepak Alur, John Crupi and Dan Marlks, Prentice Hall Patterns of Enterprise Application Architecture, Martin Fowler, Addison- Wesley html 51

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