Aspect-oriented programming

Laura Ricci

A.A. 2001-2002

Index

Introduction to AOP

What are aspects?

The role of aspects in software design

AOP issues

AspectJ

What next?

Introduction Currently, the dominant programming

paradigm is object-oriented programming that:

Object orientation is a clever idea, but has

certain limitations

has been presented as a technology that can fundamentally aid software engineering

is reflected in the entire spectrum of current software development methodologies and tools

Introduction A new programming technique called aspect-

oriented programming (AOP):

makes it possible to clearly express those programs that OOP fail to support

enables the modularization of crosscutting concerns by supporting a new unit of sw modularity – aspects – that provide encapsulation for crosscutting concerns

What are aspects? The current working definition is (May 99,

Gregor Kiczales):

modular units that cross-cut the structure of other modular units

units that is defined in terms of partial information from other units

exist in both design and implementation

Concerns AOP is based on the idea that computer

systems are better programmed by separately specifying the various concerns of a system

Separation of concerns is an important software engineering principle guiding all stage of a sw development methodology

Concerns: are properties or areas of interest

can range from high-level notion ot low level-notion

can be functional or nonfunctional (systemic) back

Example of crosscutting concerns

Consider:

This requires every method that moves a figure element to do the notification

the UML for a simple figure editor in which there are two concrete classes of figure element, points and lines

the concern that the screen manager should be notified whenever a figure element moves

It is taken from C. of the ACM

Example of crosscutting concerns

Display

Figure FigureElement*

Point LinegetX()

getY()

getP1

setP1

DisplayUpdating

setX(int)

setY(int)

setP1(Point)

setP2(Point)

2

The red box is drawn around every method that must implement this concern

DisplayUpdating fits neither inside of nor around the other boxes in the figure

DisplayUpdating cuts across the other boxes

What are aspects? Aspects are similar to classes because:

have type

can extend classes and other aspects

can be abstract or concrete

can have fields, methods, and types as members

What are aspects? Aspect are different than classes because:

can additionally include as members pointcuts (picks out join points), advice (code that executes at each join point )

do not have constructor or finalizer and they cannot be created with the new operator

priviliged aspects can access private members of other types

What are aspect? Aspects may arise at any stage of the software

lifecycle

Common example of crosscutting aspects are design or architectural constraints, systemic properties or behaviours and features

Examples of how aspects cross-cut components

We wanted to implement a distributed digital library that stores documents in many forms and provides a wide range of operations on those documents

application components aspects

digital database, minimizing network

Library printers, traffic,

services synchronization

constraints,

failure handling

It is taken from AOP document

Examples of how aspects cross-cut components There are several aspects of concerns,

including: communication, by which we mean controlling the amount of network bandwidth the application uses by being careful about which objects get copied in remote method calls

coordination constraints, by which we mean the synchronization rules require to ensure that the component program behaves correctly

failure handling, by which we mean handling the many different forms of failure that can arise in a distributed system

The role of aspects in software design AOP aims at providing better means of

addressing the well-known problem of separation of concerns

Three basic approaches to addressing the process of separation of concerns:

language-based

framework-based

architecture-oriented

Language-based approach It is based on the definition of a set of

language constructs

Relevant concerns are identified at the problem domain and translated to aspectual construct

The final application is obtained by weaving the primary structure with the crosscutting aspects

Framework-based approach Provides more flexible constructs Concerns are materialized as aspectual classes

at the framework level Developers can customize these aspects using

the mechanism supported by the framework

These types of framework are known as AO

frameworks (explicitly engineers concerns)

Architecture-oriented approach Early identification of concerns using

architectural organizational models

Architectural view-point involves a higher level of abstraction than the previous approaches

It tipically comprises two stages

Architecture-oriented approach First, developers should determine the

problem architecture

Then, the approach enables several kinds of aspect materialization through different frameworks

Concerns are initially mapped to architectural construct

AOP technologies example

Empirical analysis based on a simulation case study of the temperature control system (TCS) of a building ()

This comprises a building with rooms requiring specific temperatures and a network consisting of radiators, pipes and a boiler

To simulate TCS, a simple mathematical model of temporal differential equations specifies the heat flow among the different components

It is taken from C. of the ACM

AOP technologies exampleRelevant aspects in TCS

Aspect

It refers to how the simulated entities should run

Relationships

Synchronization

Scheduling

It basically involves access to shared variables and race condition

Description

It works together with scheduling

It works together with synchronization

AOP technologies example Four different groups of programmers

implementing:

typical OO view

aspect language (AL)

reflective AO framework (R-AO)

event-based architectural framework (EBA)

AOP technologies example TCS evaluation according to the different implementation

Analyzed features: scheduling and synchronization

OO modeling: SS are scattered across the components requiring these facilities (inheritance anomalies)

EBA: SS are provided as built-in features in the framework. It may be difficult to customize some of these policies

R-AO – AL: There are specific aspects dealing with SS. They are kept separated, but some problems about the way they interact (aspect composition) may arise

AOP technologies example Several execution of the simulation programs

are maded

TCS performance with the different implementation:

results were very similar except in the case of R-AO

similarities found in the study seem to indicate that both EBA and AL run almost like standard code

AOP technologies example

To obtain a measure of the complexity of the implementations, it is gathered code statistics (NCSS) about:

number of methods per class

the NCSS per methods

cyclomatic complexity (CNN per methods)

In particular, the best results were obtained with EBA (this payoff comes mainly from the autonomy of component and decoupling prescribed by framework).

The role of aspects in software design

The central problem of aspect technologies, whatever the approach we consider, is not just about crosscutting or separation of concerns, but it involves deeper research about how to understand a number of software parts as separated artifacts and then integrate some of then into a coherent system

AOP issues AOP must address both what the programmer

can say and how the computer system will realize the program in a program system

AOP system:

provides a way of expressing crosscutting concerns

also ensures these mechanisms are conceptually straighforward and have efficient implementations

AOP issues How an AOP system specifies aspects:

What composition mechanisms the system provides:

join points

aspect parameterization

dominant decomposition

visibility

mechanism provided

AOP issues Implementation mechanisms:

Software process:

static/dinamic distinction

modular compilation

target representation

methodology or framework (the system provides for organizing the system-building activity)

reusability

domain-specificity

AspectJ Aspectj is:

Aspectj enables the modular implementation of a wide range of crosscutting concerns

a general-purpose Ao extension to Java

Java platform compatible

easy to learn and use

freely available under an Open Source license

AspectJ When written as an aspect the structure of a

crosscutting concern is explicit and easy to reason about

Aspects are modular AspectJ enables:

name-based crosscutting (tend to affect a small number of other classes)

property-based crosscutting (range from small to large scale)

AspectJ Adoption of it into an existing project can be a

straightforward and incremental task:

The goals of the AspectJ project are to make AOP technology available to a wide range of programmers, to build and support an AspectJ user community

to begin with development aspects

other paths are possible,depending on the needs of the projects

Example

What next? Software engineering researchers provide

some help to determine if it is beneficial for sw development organization to adopt AOP for building their sw products

A number of study have been conducted to asses the usefulness of AOP

Two basic techniques for assessing a programming technology:

experiments

case study

What next? Three areas emerge as important in

supporting the use of AOP:

exposing join points

managing aspect interface

structuring aspects

References Communications of the ACM (October 2001-Volume 44,

Number 10)AOP official web site: http://aosd.net/ AspectJ official web site: http://aspectj.org/Sites from which you can find documents about conferences

and several informations: http://www.ccs.neu.edu/home/lieber/connection-to-aop.html http://www.cs.man.ac.uk/cnc/mscprojects

/aspect/node1.html http://www.cs.ubc.ca/labs/se/papers/1999/icse99-aop.html http://www.eclipse.net/~tomo/pisa/oopsla2000/

p_netinant.html

Crosscutting

Crosscutting is how to characterize a concern than spans multiple units of OO modularity

Crosscutting concerns resist modularization using normal OO construct

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Pointcut Picks out join points

Can be defined in classes or in aspects and can be named or be anonymous

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Advice It is code that executes at each join point

picked out by pointcut

His power comes from the advice being able to access value in the execution context of a pointcut

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Join points Are well-defined points in the execution of a

program

Only those points that can be used in a disciplined and principled manner

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Framework It is based on decomposition of aspects in

system design that consists of components, aspects, and layers.

By separating the different aspects of each component, we can separate components, aspects, and layers from each other

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AspectJ example

An aspect is defined very much like a class and can have methods, fielsd, constructors, initializers, named pointcuts and advice

An example is tracing aspect that prints messages before certain display operation

The overall effect of this aspect is to print a descriptive message whenever the traced methods are called

It is taken from C. of the ACM

AspectJ exampleAspect SimpleTracing {

pointcut traced() :

call (void Display.update () ) ||

call (void Display.repaint (..) );

before() : traced () {

println(“Entering:” + thisJoinPoint);

}

void println (String str) {

<write to appropriate stream> } }

Traced identifies calls to several key methods on Display

Before advice on this pointcut uses a helper method of the aspect to print a message

Advice uses the thisJoinPoint special variable to an object that describes the current join point

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