Introduction to business component technologies

Component definitions Szyperski:

A software component is a unit of composition with contractually specified interfaces and explicit context dependencies only. A software component can be deployed independently and is subject to composition by third parties

D'Souza and Wills: A software component is a coherent package of software artefacts

that can be independently developed and delivered as a unit and that can be composed, unchanged, with other components to build something larger

UML v2.4: A component represents a modular part of a system that

encapsulates its contents and whose manifestation is replaceablewithin its environment

More definitions: http://en.wikipedia.org/wiki/Software_componentry

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Context dependencies Component may require security, transaction, logging and

other middleware services provided by component container (aka component framework)

Component may require other components to be made available (required interface)

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Problem with these definitions Definitions above are correct, but they do not stress

differences between a component and a class:

public class Component implements Interface { ...

}

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Essential characteristics of a componentComponent Oriented Development in OSGi with Declarative Services, Spring Dynamic Modules andApache iPOJO, Neil Bartlett, Heiko Seeberger, 2009

May provide services to other components and use services from other components

Have a life cycle

Active participants in the system

They need middleware services, such as transactions, security, logging, etc.

Characteristic of future component technologies:

Aware of and adapt to their environment

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Old analogyComponent Oriented Development in OSGi with Declarative Services, Spring Dynamic Modules andApache iPOJO, Neil Bartlett, Heiko Seeberger, 2009

inactive

do not provide/use services

no life-cycle

do not adopt to environment

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New analogyComponent Oriented Development in OSGi with Declarative Services, Spring Dynamic Modules andApache iPOJO, Neil Bartlett, Heiko Seeberger, 2009

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How to distinguish a component from a class? Components cannot live without special environment

called container container instantiates components and controls their life-

cycle

container (not programmer!) binds components together (required interfaces with provided interfaces)

container provides services to components

Instances of a component are created by container, never by programmer itself (operator new is prohibited) Conversely, instances of classes are created by programmer

using operator new.

Later we will look at components in more details (slides about modules and components)

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Concepts: business logic and middleware Business logic

models real life business objects (such as accounts, loan, and inventories)

prescribes how business objects interact with one another enforces the routes and the methods by which business

objects are accessed and updated is part of functional requirements creates essential added value that client is willing to pay for.

Middleware - software that provides services to applications beyond those available from the operating system For example: transactions, logging, security, messaging, etc. if is mentioned in requirements document then only as part

of non-functional requirements; often is assumed as matter-of-course and not documented at

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Explicit middleware Money transfer between two accounts:

void transfer(Account account1, Account account2, long amount) {

1: Perform a security check2: Create audit log record3: Start a transaction4: Load data from the database5: account1 -= amount; account2 += amount;6: Store updated data to the database7: Commit the transaction

} Business logic is intertwined with the logic to call middleware

services Lowers developer productivity, difficult to write and maintain.

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Implicit middleware Programmer writes only business logic:

void transfer(Account account1, Account account2, long amount){

account1 -= amount; account2 += amount;}

If middleware services are needed, we declare that, for example, in: XML file, annotations.

Middleware services are called by component container with the help of request interceptor.

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The Essence of Component-oriented Technologies

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Client - other component or some program

Request Interceptor

Component

Transaction Service

Security Service

My own middleware implementation

My APIMy API

Security API

Security API

Transaction API

Transaction API

Main Conclusions Component-based technologies enable us to separate

business logic from middleware services

Provide means to apply Separation-of-Concernsprinciple in practice

Client never communicates with component instance directly – there always will be an intermediary (request interceptor, proxy, etc.)

Sometimes even more than one intermediary

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Modern business component technologies CDI (Contexts and Dependency Injection)

http://cdi-spec.org/

SpringFramework IoC container http://docs.spring.io/spring/docs/current/spring-framework-

reference/htmlsingle/#beans

EJB (Enterprise JavaBeans) https://java.net/projects/ejb-spec/

OSGi https://www.osgi.org/developer/downloads/

Microsoft Application Platform Book: Microsoft Application Architecture Guide, 2nd Edition

Component Guidelines

Designing Business Components

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