Aspect Oriented Programmingwith AspectJ

Ted LeungSauria Associates, LLC

twl@sauria.com

Overviewn Why do we need AOP?n What is AOPn AspectJ

Why do we need AOP?n Modular designs are not cut and driedn Responsibilities can be assigned to one or

more classesn Examples:

n Every servlet for the administrative part of the site must check for a logged in administrator user

n Site navigation : changing country in the UI (via multiple means) must update a bunch of data structures

n Maintaining both ends of a two way relationshipn Logging and tracing

What is AOP?n Introduces the notion of crosscutting

concernsn Concerns that you want to modularizen Concerns whose implementation is all over

n Introduces language mechanisms for identifying and capturing crosscutting concerns

Why bother with AOP?n Capture the crosscutting concern

explicitlyn Both the behavior of the concernn The specification of its applicability

n Change is easiern Change the aspect – no grepping

n Aspects can be plugged in or out

AspectJ and AOPn AspectJ is an aspect-oriented extension to

Javan One Concept : Join Pointsn Four constructs

n Pointcut Designators (Pointcuts)n Advicen Introductionn Aspects

n Aspects are composed of advice and introductions, and attached to pointcuts

Hello World AOP Style

public aspect HelloAspect {pointcut entry() : execution(public static void main(String[]));

after() : entry() {System.out.println("Hello World");

}}

public class HelloWorld {public static void main(String args[]) {}

}

Join Pointsn A well defined point in the program flown Created each time:

n A method is calledn A method executesn A field is get/setn An Exception handler executesn A dynamic initializer (constructor) executesn A static initializer executes

Pointcuts : dynamic crosscutsn A declarative specification of a set of

join pointsn Easy to change versus copying calls or

code to where they belongn Examples:

n Calls to method X from within the control flow of method Y

n Calls to public methods

Pointcut designatorsn field get and set

n set(int MyClass.field)

n Method calln call(int add(int, int))

n Method executionn execution(int add(int,int)

n Exception Handlingn handler(IOException)

n Constructor Executionn initialization(class)

n Lexical control flown within(class), withincode(method)

n Dynamic control flown cflow(pointcut), cflowbelow(pointcut)

Composing pointcutsn Pointcut designators can be composedn &&

n target(MyClass) && call(void draw())

n ||n call(void draw) && target(MyClass1) || target(package.*))

n !n call(void draw()) && !target(MyClass)

Signatures

n Basic signature:n float compute(float, float)

n On a specific classn float Account.compute(float, float)

n Any 0-ary method on a specific classn Account.*()n Account.*(int)

n Any public method returning an intn public int *.*(..)

n Any method throwing IOExceptionn public *.*(..) throws IOException

Type Patternsn A type name

n vector

n Wildcardsn java.util.*Listn org.apache.xerces.xni.*n org.w3c..*

n Subtypesn java.util.AbstractList+

n Arraysn java.util.String[]

n Compositionn java.io.* || java.nio.*

Advicen Code that runs at each join point

selected by a pointcutn Kinds of advice

n beforen after

n after returningn after exception

n around

Example Object Model

Examplesn Fieldsn Wildcardsn ExceptionFlow

Accessing pointcut contextn We want to be able to access program

values at a join pointn Pointcuts can take parameters

n pointcut name(Type1 arg1, Type2 arg2) :args(arg1, arg2) && pointcut

n Advice can use those parametersn Around(Type1 arg1, Type2 arg2) : name(arg1, arg2) {

… use arg1 & arg2 in advice code}

Examplen Fields1n Servlet Based

Advice precedencen What happens when lots of advice

matches?n Dominates keyword

n aspect A dominates TypePattern {}

n Subaspect advice takes precedencen Otherwise undetermined

Introduction: static crosscutsn Aspect can introduce:

n introduce fieldsn Modifiers Type TypePattern.Id { = Expr };

n introduce methodsn Modifiers TypePattern.new(Formals){ Body }n Modifiers TypePattern.Id(Formals) { Body }

n Implement an interfacen declare parents : TypePattern implementsTypeList;

n Extend a classn declare parents : TypePattern extendsTypeList;

n Can introduce on many classes at once

Examplen SerialNumber

Aspect Extensionn Aspects can extend classesn Aspects can implement interfacesn Aspects can extend abstract aspects

n The sub aspect inherits pointcuts

Examplen Abstract tracing Aspect

Associated aspectsn How many aspects are instantiated?n singleton

n By defaultn aspect Id perthis(Pointcut)

n 1 per currently executing objectn aspect Id pertarget(Pointcut)

n 1 per target objectn aspect Id percflow(Pointcut)

n 1 per control flow entrancen aspect Id percflowbelow(Pointcut)

n 1 per cflowbelow entrance

Privileged Aspectsn Aspects normally obey Java access

control rulesn Aspects that can break encapsulation

n privileged aspect Id {}

Examplen UpdateAspect

Tool Supportn Antn IDE Support

n Emacsn Forten JBuilder

n AJDocn AJBrowsern Debugger

n Uses .lst files to do aspect weaving

AspectJ Statusn 1.0

n Released 11/30/2001

n 1.1n Faster increment compilation

n 2.0n Dynamic crosscutsn Work on bytecode files

Musingsn AspectJSPn Eclipsen No need for .lst’s

Development usesn Loggingn Tracingn Timingn Exception handling / loggingn Various kinds of invasive/non-invasive

instrumentation

n Flexibility of pointcut designators

Application usesn Consistency maintenance / checking

n Keeping both sides of a bi-directional relationship up to date

n Policiesn Securityn Session Handlingn Failure / Retryn Synchronization

n Context Passingn Avoids huge argument lists or carrier objects

n Multiple Views

To Learn Moren www.aspectj.orgn www.aosd.netn CACM 10/2001n These slides at:

n www.sauria.com