Aspect-Oriented Programming

1. Aspect-Oriented Programming Dimple Kaul ACCRE Vanderbilt University Nashville, Tennessee http://www.dre.vanderbilt.edu/~dkaul/

2. Talk Outline • Problem Scenario • Aspect Oriented Programming • Aspect & Object Oriented Programming • Aspect Terminology • Examples • Installation of AJDT • First Steps • Conclusion • Questions

3. Problem Scenario (1/2) Adding new logging functionality Regular OO Java 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()"); } } . 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); } } } Logging code is inserted at many places in the code

4. 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.

5. 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

6. 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

7. 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

8. 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”

9. Requirement Requirement Requirement Class Class Class Class (a) Object Oriented Model Requirement Requirement Requirement Aspect Aspect Aspect (b) Aspect Oriented Model AOP & OOP Requirement is dependent on multiple classes Each requirement can have separate aspect “[...]In programs P, whenever condition C arises, perform action A”

10. AOP & OOP Terminology Aspect Oriented Programming languages include AspectJ, AspectC++, & JBOSS AOP

11. Aspect-Oriented Development Concerns OOP Classes Interfaces Executable software Concern identifier Software requirements AOP Aspects WEAVER Existing OOP Project OOP Classes Interfaces Executable software Concerns Aspects New functionality Concern identifier AOP Concerns WEAVER

12. 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

13. 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)

14. 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

15. AOP TerminologyJoinPoints & 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

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

17. AOP TerminologyAdvice contd… • after() returning – runs after the method returns normally after() 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 abruptly after() throwing (SQLException ex) : inDataLayer() { logException(ex); }

18. 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 ()); }

19. AOP TerminologyOthers… • Inter-type declaration:Allows to add method, fields or interfaces to existing classes from within aspects aspect 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

20. Example showing Advice, Pointcut and Aspect Pointcut 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 ()); } Aspect Piece of Advice

21. 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).

22. 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

23. Example package org.thewhittakers.banking; publicclass Account implements Loggable { privatedouble balance; private String owner; public Account(String owner, double initialBalance) { this.setOwner(owner); this.credit(initialBalance); } publicvoid credit(double amount) { this.balance += amount; } publicvoid debit(double amount) { this.balance -= amount; } publicvoid transferTo(Account other, double amount) { this.debit(amount); other.credit(amount); } // less interesting items removed. } (Simple bank account class)

24. Example package org.thewhittakers.banking; publicaspect 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) thrownew RuntimeException("Negative amounts not permitted"); } before(Account account, double amount): preventOverdraft(account, amount) { if (account.getBalance() < amount) thrownew RuntimeException("Insufficient funds"); } } (Adding pre-condition checking)

25. Example Exception Handling 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; } } } Catch 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); }

26. Example of Persistence aspect DatabaseAspect { pointcut transactionalMethods (): execution (/* pattern for transactional methods */) ; before(): transactionalMethods () { initialiseDatabase() ; } after() returning: transactionalMethods() { commitTransaction() ; } after() throwing: transactionalMethods() { rollbackTransaction() ; } }

27. 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

28. 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

29. 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

30. 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…

31. Step 1: Select Project from the package explorer & Right click & choose “Convert to AspectJ Project” from the context menu

32. 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

33. 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.

36. 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

37. 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

38. Skeletal Aspect

39. Moving back to regular Java project • Easy to revert back to your regular java project

40. 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

41. History of AOP • AOP has been following other technologies like OOP • Worked in academia • Popping up more & more in real world

42. 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

43. 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

44. Questions??

45. 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.

46. Defining Pointcuts “*” is wild card “..” is multi-part wild card • Calling methods & constructors • Advice is inserted after argument evaluation, but before calling. Access to caller’s context

47. Defining Pointcuts • Control flow based pointcuts

48. Defining Pointcuts • Context capturing pointcuts • Can attach names to the types to capture the variables for use inside the associated advice.

49. 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

50. Defining Pointcuts • Pointcuts & logical operators • Can be combined with &&, ||, & ! before() : execution(public * *(..)) && within(figures.*) { Log.write(thisJoinPoint); }