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Aspect-Oriented ProgrammingAspect-Oriented Programming

Dimple KaulACCRE

Vanderbilt University Nashville, Tennessee

http://www.dre.vanderbilt.edu/~dkaul/

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Talk Outline

Problem Scenario Aspect Oriented Programming Aspect & Object Oriented Programming Aspect Terminology Examples Installation of AJDT First Steps Conclusion Questions

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Problem Scenario (1/2)

class Foo { public void foo () { bar.doSomething (); }}

class Bar { static void doSomething () { baz.doSomething (); }}

class Baz { static void doSomething () { for (int i = 0; i < 100; i++) { doSomething(i); } }}

Regular OO Java Adding new logging functionality

class Foo { public void foo () { logger.log ("Start -- Foo.foo()"); bar.doSomething (); logger.log ("End -- Foo.foo()"); }}

class Bar { static void doSomething () { logger.log ("Start -- Bar.doSomething()"); baz.doSomething (); logger.log ("End -- Bar.doSomething()"); }}

class Baz { static void doSomething () { logger.log ("Start -- Baz.doSomething()"); for (int i = 0; i < 100; i++) { doSomething(i); } logger.log ("End -- Baz.doSomething()"); }}.

Logging code is inserted at many places in the code

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Problem Scenario (2/2)

Rewriting former logging example in Aspect Oriented Programming way. Original code will not change….

aspect Logging {

pointcut log () : execution (void *.*()) || execution (static void Baz.doSomething()) ; before(): log() { Logger.log("Start --" + thisJoinPoint.getSignature ()); }

after(): log() { Logger.log( “End --" + thisJoinPoint.getSignature ()); }}

This procedure is executed when methods written as "execution(X)" are executed.

"thisJoinPoint" object has an information about the method called.

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Problem

Software systems consists of several concerns

For example: In a credit card processing

Primary Concern (System core)

processing of payment

Secondary Concern (System level)

authentication, security & logging etc

Object Oriented Programming provide good modularity for primary concerns i.e., the main business logic

Adding secondary concerns (crosscutting concerns) with primary concern result into a system which is difficult to understand and evolve and result into code tangling & scattering

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Aspect Oriented Programming (1/2)

AOP shows great promise in specialization : Functionality can often be changed without re-factoring code

No need for knowing “ahead of time” functionality If a system does not have logging or exception handling

functionality

These concerns can be added anytime without even touching original code

Components can be integrated incrementally

Implementations are easier to design, understand, & maintain

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Aspect Oriented Programming(2/2)

Supports the reuse of existing code by applying new features in a controlled and localized way

AOP promises higher productivity, improved quality, & better ability to implement newer features

AOP has been implemented in different languages (for example, C++, Smalltalk, C#, C, and Java)

AOP builds on top of other programming paradigms: object-oriented, procedural or functional

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What is AOP?

Aspect-oriented programming (AOP) methodology facilitates modularization of crosscutting concerns

Separation of concerns

breaking down of a program into distinct parts that overlap in functionality as little as possible

“Untangle your code into cross-cutting, loosely coupled aspect”

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Aspect

Requirement

AspectAspect

Requirement Requirement

(b) Aspect Oriented Model

Class Class Class Class

Requirement Requirement Requirement

(a) Object Oriented Model

Requirement is dependent on multiple classes

“[...] In programs P, whenever condition C arises, perform action A”

Each requirement can have separate aspect

AOP & OOP

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AOP & OOP Terminology

Object Oriented Aspect OrientedClass – code unit that encapsulates methods & attributes.

Aspect – code unit that encapsulates pointcuts, advice, & attributes.

Method signatures – define the entry points for the execution of method bodies.

Pointcut – define the set of entry points (triggers) in which advice is executed.

Method bodies – implementations of the primary concerns.

Advice – implementations of the cross cutting concerns.

Compiler – converts source code into object code.

Weaver – instruments code (source or object) with advice.

Aspect Oriented Programming languages include AspectJ, AspectC++, & JBOSS AOP

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Aspect-Oriented Development

Softwarerequirements

AOP

OOP

Aspects

ClassesInterfaces Executable

software

Concerns

Concern identifier

WEAVER

Existing OOP Project

AOP

OOP

Aspects

ClassesInterfaces

Executablesoftware

ConcernsWEAVER

New functionality

Concern identifier

Concerns

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Aspect Oriented Programming

Implications of tangling & scattering on software design Maintainability: Nightmare for huge systems

Poor traceability: simultaneous coding of many concerns in a module breaks linkage between the requirement & its implementation

Lower productivity: developer is paying too much attention to peripheral issues rather than the business logic

Less code reuse: cut-&-paste code between modules is the lowest form of reuse and is more error prone

Harder re-factoring: changing requirements means touching many modules for a single concern

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Dynamic VS Static crosscutting

Dynamic crosscutting define additional behavior to run at certain

well-defined points in the execution of the program

Static crosscutting modify the static structure of a program (e.g.,

adding new methods, implementing new interfaces, modifying the class hierarchy)

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AOP TerminologyJoinPoints & Pointcut

JoinPoints:Well-defined points in the execution of a program: Method call / execution Constructor call / execution Object initialization

Pointcuts: A set of join point, plus, optionally, some of the values in

the execution context of those join points. Can be composed using Boolean operators || , && Matched at runtime

For more detail see AspectJ Quick Reference Manual

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AOP Terminology JoinPoints & Pointcut contd… Example:

pointcut set() : execution( * *.set*(..) ) && this(Point) Captures all executions of any method that begins with set in

an object of type Point

pointcut abc() : call ( public void MyClass.myMethod(..) ) Captures call of myMethod method of MyClass class with any

number of arguments

within ( org.package.* ) Captures all join point where the associated code is defined in

the package “org.package.*”

withincode ( void Figure.move() ) Capture all join points where the associated code is defined in

the method void Figure.move()

“*” is wild card“..” is multi-part wild card

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AOP TerminologyAdvice

It executes when a pointcut matches before(): runs just prior to the join point

before(): log() { Logger.log("Start --" + thisJoinPoint.getSignature ()); }

after(): runs just after the join pointafter(): log() { Logger.log(("End --" + thisJoinPoint.getSignature ()); }

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AOP TerminologyAdvice contd…

after() returning – runs after the method returns normallyafter() returning(int x) : call(int getX()) {

System.out.println("Returning int value " + x + " for p = " + p);

}

after() throwing – runs after the method throws an exception abruptlyafter() throwing (SQLException ex) : inDataLayer() {

logException(ex);

}

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AOP TerminologyAdvice contd…

around(): runs before &/or after, with the operation taking place via a call to proceed(). Note, if proceed is not called, then the advised code is skipped.

pointcut log () : execution (void *.*()) || execution (static void Baz.doSomething()) ;

around(): log() { Logger.log("Start --" + thisJoinPoint.getSignature ()); proceed(); Logger.log( “ End --" + thisJoinPoint.getSignature ());}

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AOP TerminologyOthers… Inter-type declaration: Allows to add method, fields or interfaces to existing

classes from within aspectsaspect VisitAspect {

Point.acceptVisitor(Visitor v) { v.visit(this);

} }aspect DeclareErrorWarning { declare error : get(java.io.PrintStream System.out) &&

within(figures..*) : "illegal access to System.out";}

Aspect: container holding point cuts & advice

Weaver: the tool that instruments the primary concerns with the advice based on matched pointcuts.

Method added

Declared error

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Example showing Advice, Pointcut and Aspectaspect Logging {

pointcut log () : execution (void *.*()) || execution (static void Baz.doSomething()) ; before(): log() { Logger.log("Start --" + thisJoinPoint.getSignature ()); }

after(): log() { Logger.log(“End --" + thisJoinPoint.getSignature ()); }

Pointcut

Piece of AdviceAspect

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Weaving Types

Advice is inserted at Compile-time: source code is instrumented

before compilation. (AspectC++) Link-time: object code (byte code) is

instrumented after compilation (AspectJ) Load-time: specialized class loaders

instrument code (AspectWerkz) Run-time: virtual machine instruments or

application framework intercepts loaded code (JBossAOP, XWork).

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AspectJ & AOP

AspectJ is an aspect-oriented extension to the Java programming language

It was originally developed and co-founded by Gregor Kiczales and his team at Xerox PARC

Later Xerox group’s work was integrated with Eclipse Java IDE

Freely available implementation Compiler, tools and plugins are Open source

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Example (Simple bank account class)

package org.thewhittakers.banking;public class Account implements Loggable { private double balance; private String owner;

public Account(String owner, double initialBalance) { this.setOwner(owner); this.credit(initialBalance); } public void credit(double amount) { this.balance += amount; } public void debit(double amount) { this.balance -= amount; } public void transferTo(Account other, double amount) { this.debit(amount); other.credit(amount); } // less interesting items removed.}

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Example (Adding pre-condition checking)

package org.thewhittakers.banking;

public aspect AccountConstraintsAspect { pointcut preventNegativeAmounts(Account account, double amount) : (execution(* Account.credit(double)) || execution(* Account.debit(double))) && this(account) && args(amount); pointcut preventOverdraft(Account account, double amount) : execution(* Account.debit(double)) && this(account) && args(amount); before(Account account, double amount): preventNegativeAmounts(account, amount) { if (amount < 0) throw new RuntimeException("Negative amounts not permitted"); } before(Account account, double amount): preventOverdraft(account, amount) { if (account.getBalance() < amount) throw new RuntimeException("Insufficient funds"); }}

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Example Exception HandlingCatch any exception in all the public methods: aspect CatchException{ // Catch any exception in all the public methods void around(): execution(public * *(..)){ try{ proceed(); } catch(Exception e){ System.out.println("Printing exception"); //Handle exception } }}

Catch SQL exceptions that need to be logged:after() throwing (SQLException ex) : inDataLayer() { logException(ex);}

Throwing of exception:aspect ThrowException { private boolean Point.inGroup = false; before(Point p): execution(void Group.add(FigureElement)) &&

args(p) { if (p.inGroup) { throw new IllegalStateException(); } else { p.inGroup = true; } }}

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Example of Persistence

aspect DatabaseAspect {

pointcut transactionalMethods (): execution (/* pattern for transactional

methods */) ; before(): transactionalMethods () { initialiseDatabase() ; } after() returning: transactionalMethods() { commitTransaction() ; } after() throwing: transactionalMethods() { rollbackTransaction() ; }}

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Uses Cases for AOP

Not only for Logging & Exception Handling Thread Safety Multi-Object Protocol Performance optimization

Middleware Specialization using AOP (http://www.dre.vanderbilt.edu/~dkaul/pdf/acm_aspect.pdf)

Timing / Monitoring /Tracing Various kinds of invasive/non-invasive instrumentation Authentication & Authorization Transactional management /Locking Session Handling Synchronization Caching

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Why bother with AOP?

Capture the crosscutting concern explicitly Both the behavior of the concern The specification of its applicability

Change is easier Change the aspect – no grepping

Aspects can be plugged in or out easily Many people suggest use of patterns, template

and careful programming as alternative to AOP Research has proved that all these proposed ideas

always fail to localize the crosscutting concerns. Then tend to have some code that remains in base structure

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Installing AspectJ

Download AJDT plugin for eclipse from: http://download.eclipse.org/technology/ajdt/30/update

AspectJ Development Tools (AJDT) project provides Eclipse platform based tool support for AOSD with AspectJ

Runtime library required for AspectJ is a very small library of about 35K

It creates normal java class files & can be execute on any JVM

Aspect files can have .Java or .aj extension Weaves into class files Produces standard Java bytecode

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First StepsConverting existing Java project

It enables us to use AspectJ language to implement the applications , & AspectJ compiler to build it

Converting regular existing Java project to AspectJ will be like this…

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Step 1:

Select Project from the package explorer & Right click & choose “Convert to AspectJ Project” from the context menu

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Step 2:

Some changes in the Package Explorer are seen

First, the project icon has changed from the Java project icon J to AspectJ project icon AJ

Second, a new jar file has been added to the project's build path, using the Eclipse path variable ASPECTJRT_LIB

Creates build configuration file which stores information about the build of project

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First Steps (contd..)Creating new AspectJ projects

To do this, you use the New AspectJ Project Wizard. You can reach the wizard via the Eclipse workbench menus by selecting File -> New -> Project, & then AspectJ Project.

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First Steps (contd..)Configuring Workbench

First time you convert old java project to Aspectj or create new Aspectj project we need to configure workbench

Preference window will come up to set some settings

You can also change these preferences at any stage by going through eclipse workbench menus by selecting Window -> Preference -> AspectJ

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Create New Aspect

To creating new aspect for a package

Select package in the package explorer & right click to go to context menu & do New ->Aspect

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Skeletal Aspect

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Moving back to regular Java project

Easy to revert back to your regular java project

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Select Project from the package explorer & Right click & choose “Remove AspectJ Nature” from the context menu

It is a good idea not use Aspectj related artifacts in actual project otherwise we may get build errors Good practice to keep

aspect file extension as .aj

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History of AOP

AOP has been following other technologies like OOP

Worked in academia Popping up more & more in real world

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Conclusion

AOP is an evolutionary step Not a replacement of OOP, but is used to enhance it Decomposes the system into primary & crosscutting

concerns which map more directly into requirements. Easy to understand system by reducing tangling &

scattering. Joinpoints, pointcuts, & advice are used to instrument

primary concerns with crosscutting concerns If you’ve got an orthogonal concern that is about exactly

one place in the original code, & you’re sure that that orthogonal concern will not propagate to other loci as the system evolves, it is probably a bad idea to use AOP

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Related Work

Traditional techniques: Code re-factoring Ahead-of-time design

Modern techniques: Feature-Oriented Programming

Incremental stepwise refinement Feature modularity features are basic design

components

Hybrid Programming (FeatureC++) Mix of AOP & FOP Still research is going on

But have shortcomings:

• Manual & Error prone• High Memory consumption• Performance overhead

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Questions??

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References

AspectJ.org. (2004). AspectJ Sample Code. Retrieved May 11, 2004, from the AspectJ documentation: http://dev.eclipse.org/viewcvs/indextech.cgi/~checkout~/aspectj-home/sample-code.html

C2.com. (2004). You Arent Gonna Need It. Retrieved May 11, 2004, from the Extreme Programming Wiki: http://xp.c2.com/YouArentGonnaNeedIt.html.

Gradecki, J., & Lesiecki, N. (2003). Mastering AspectJ: Aspect-Oriented Programming in Java. Indianapolis, IN: Wiley Publishing.

Laddad, R. (2002). I want my AOP! Part 1. Retrieved May 11, 2004, from JavaWorld: http://www.javaworld.com/javaworld/jw-01-2002/jw-0118-aspect_p.html

Laddad, R. (2002). I want my AOP! Part 2. Retrieved May 11, 2004, from JavaWorld: http://www.javaworld.com/javaworld/jw-03-2002/jw-0301-aspect2_p.html

Laddad, R. (2003). AspectJ in Action: Practical Aspect-Oriented Programming. Greenwich, CT: Manning Publications.

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Defining Pointcuts

Calling methods & constructors Advice is inserted after argument evaluation, but before

calling. Access to caller’s context

Pointcut Description

call (public void MyClass.myMethod(String))

Call to myMethod() in MyClass taking a String argument, returning void, & public access

call (* MyClass.myMethod*(..))Call to any method with name starting in "myMethod" in MyClass

call (MyClass.new(..))Call to any MyClass' constructor with any arguments

call (MyClass+.new(..))Call to any MyClass or its subclass's constructor. (Subclass indicated by use of '+' wildcard)

call (public * com.mycompany..*.*(..))All public methods in all classes in any package with com.mycompany the root package

“*” is wild card“..” is multi-part wild card

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Defining Pointcuts

Control flow based pointcuts

Pointcut Description

cflow (call (* MyClass.myMethod(..))

All the join points in control flow of call to any myMethod() in MyClass including call to the specified method itself

cflowbelow (call (* MyClass.myMethod(..))

All the join points in control flow of call to any myMethod() in MyClass excluding call to the specified method itself

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Defining Pointcuts

Context capturing pointcuts Can attach names to the types to capture the variables for use

inside the associated advice.

Pointcut Description

this (JComponent+)All the join points where this is instanceof JComponent

target (MyClass)All the join points where the object on which the method is called is of type MyClass

args (String,..,int)All the join points where the first argument is of String type & the last argument is of int type

args (RemoteException)All the join points where the type of argument or exception handler type is RemoteException

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Defining Pointcuts

Execution of methods & constructors Advice is inserted in the method or constructor body itself.

Access to callee’s context. Replace call with execution. Field access – read or write

Pointcut Description

get (int MyClass.x)Execution of read-access to field x of type int in MyClass

set (int MyClass.x)Execution of write-access to field x of type int in MyClass

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Defining Pointcuts

Pointcuts & logical operators Can be combined with &&, ||, & !

before(): execution(public * *(..)) && within(figures.*) { Log.write(thisJoinPoint);}

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What is AOP?

The AOP-style solution Three phase implementation (Laddad, 2003)

Aspectual decomposition: based on requirements, extract concerns, identifying them as primary & crosscutting.

Concern implementation: code each concern separately – primary (OO), crosscutting (AO).

Aspectual re-composition: tools weave the separately implemented code together into a final instrumented software system.