comp9321 web application engineeringcs9321/17s1/lectures/lec08/lec-08.pdf · cohesion is defined as...

COMP9321 Web Application EngineeringSemester 1, 2017

Dr. Amin BeheshtiService Oriented Computing Group, CSE, UNSW Australia

Week 8

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http://webapps.cse.unsw.edu.au/webcms2/course/index.php?cid= 2457

Assignment 1

Marks on GradeBook

Assignment 2

is due end of Week 9: Sunday, 7 May 2017, 23:59:59

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J2EE Design Patterns

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Last Week, Design Pattern Part I:

Model View Controller: • MVC is the J2EE BluePrints recommended architectural design pattern for

interactive applications.

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.

First..

What is Cohesion & Coupling?

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Cohesion

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Cohesion is defined as the ‘degree of interaction within a module’.

Seven levels of Cohesion:

7. Functional 6. Informational5. Communicational4. Procedural3. Temporal2. Logical1. Coincidental

Cohesion

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Coincidental: module performs multiple, completely unrelated actions

(degrades maintainability & modules are not reusable)

Cohesion

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Logical: module performs series of related actions, one of which is selected by calling module.(interface difficult to understand & module difficult to reuse)

Cohesion

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Temporal : module performs series of actions related in time.

(code spread out -> not maintainable or reusable)

Cohesion

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Procedural : module performs series of actions related by procedure to be followed by product.

(not reusable)

Cohesion

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Communicational : A communicationally cohesive module is one which performs several functions on the same input or output data. For example, obtain author, title, or price of book from bibliographic record, based on a passed flag (not reusable)

Cohesion

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Informational : module performs a number of actions, each with its own entry point, with independent code for each action, all performed on the same data structure

(This is an ADT!)

Cohesion

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7. Functional6. Informational5. Communicational4. Procedural3. Temporal2. Logical1. Coincidental

Functional : module performs exactly one action

(more reusable, corrective maintenance easier, easier to extend product )

Coupling

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Coupling is defined as the ‘degree of interaction between two modules’.

Seven levels of Coupling:

5. Data4. Stamp3. Control2. Common1. Content Worst

Coupling

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Content : one module directly references contents of the other module. e.g., accessing local data of another module.

(almost any change to M1 requires changes to M2 )

Coupling

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Common : two modules have write access to the same global data.

(difficult to reuse + module exposed to more data than necessary )

Coupling

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Control : one module passes an element of control to the other.

(modules are not independent)

Coupling

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Stamp : data structure is passed as parameter, but called module operates on only some of individual components.

(affects understanding + unlikely to be reusable + passes more data than necessary)

Coupling

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Data : every argument is either a simple argument or a data structure in which all elements are used by the called module.

Each datum is an elementary piece, and these are the only data shared; e.g., passing an integer to a function that computes a square root.

(maintenance is easier) <good design has high cohesion & weak coupling>

Service Locator Pattern

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ContextService lookup and creation involves complex interfaces and network operations.

Problem• The service locator pattern is a design pattern used in software development to

encapsulate the processes involved in obtaining a service with a strongabstraction layer.

• When J2EE clients interact with the server side components (EJB: EnterpriseJava Beans) or DataSources, clients must locate the service component, whichreferred to as a lookup operation in JNDI: Java Naming and Directory Interface.

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Solution

• Using a central registry known as the "service locator", which on request returnsthe information necessary to perform a certain task.

• Service Locator object will abstract all JNDI usage to hide the complexities ofinitial 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

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Service Locator:The Service Locator abstracts the API lookup services, vendor dependencies,lookup complexities, and business object creation, and provides a simple interfaceto clients.

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InitialContext:The InitialContext object is the start point in the lookup and creation process.

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ServiceFactory:The ServiceFactory object represents an object that provides life cyclemanagement for the BusinessService objects. eg., The ServiceFactory object forenterprise beans is an EJBHome object.

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BusinessService: is a role that is fulled by the service that the client is seeking toaccess. 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.

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To build a service locator pattern, we need:

https://www.tutorialspoint.com/design_pattern/service_locator_pattern.htm

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 designpattern that implements inversion of control forresolving 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) atdeployment time.

• This is a top-down approach, in contrast to a bottom-up one whereinthe 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 persistentstorage, such as to a database, varies greatly depending on the type of storage(relational databases, object-oriented databases, flat files, and so forth) and thevendor implementation.

Problem

• For many applications, persistent storage is implemented with differentmechanisms, and there are marked differences in the APIs used to access thesedifferent persistent storage mechanisms. Other applications may need to access datathat resides on separate systems.

• An example is where data is provided by services through external systems such asbusiness-to-business (B2B) integration systems, credit card bureau service, and soforth.

http://www.oracle.com/technetwork/java/dataaccessobject-138824.html

Data Access Object

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Solution

• Use a Data Access Object (DAO) to abstract and encapsulate all access to the data source. The DAO manages the connection with the data source to obtain and store data.

Data Access Object: Sequence Diagram

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Business Delegate

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Business Delegate

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Context

• A multi-tiered, distributed system requires remote method invocations to send andreceive data across tiers. Clients are exposed to the complexity of dealing withdistributed components.

Problem

• Presentation-tier components interact directly with business services. This directinteraction exposes the underlying implementation details of the business serviceapplication program interface (API) to the presentation tier.

• As a result, the presentation-tier components are vulnerable to changes in theimplementation of the business services: When the implementation of the businessservices change, the exposed implementation code in the presentation tier mustchange 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 andbusiness services.

• The Business Delegate hides the underlying implementation details of the businessservice, such as lookup and access details of the EJB architecture.

• Another benefit is that the delegate may cache results and references to remotebusiness services. Caching can significantly improve performance, because it limitsunnecessary and potentially costly round trips over the network.

Business Delegate

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• Client: requests the BusinessDelegate to provide access to theunderlying business service.

• BusinessDelegate: uses a LookupService to locate the requiredBusinessService component.

Business Delegate Sequence Diagrams

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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.

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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.

Microservice

e.g. Microservices in IBM Bluemix

https://developer.ibm.com/bluemix/2015/01/19/microservices-bluemix/

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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 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 catalog 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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Synchronizer Token

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Synchronizer Token

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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• From Modules to Objects, Professor James Landay, Software Engineering Lecture

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