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1 Aspect-Oriented Programming Dimple Kaul ACCRE Vanderbilt University Nashville, Tennessee

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1 1 Aspect-Oriented Programming Dimple Kaul ACCRE Vanderbilt University Nashville, Tennessee

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

3 3 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 --"); bar.doSomething (); logger.log ("End --"); } 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

4 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 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 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 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 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 9 Aspect Requirement Aspect Requirement (b) Aspect Oriented Model Class 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

10 10 AOP & OOP Terminology Object OrientedAspect Oriented Class – 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

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

12 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 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 14 AOP Terminology JoinPoints & 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 15 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

16 16 AOP Terminology Advice 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 17 AOP Terminology Advice 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 18 AOP Terminology Advice 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 19 AOP Terminology Others… 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( 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 20 Example showing Advice, Pointcut and Aspect 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 ()); } Pointcut Piece of Advice Aspect

21 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 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 23 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);; } 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);; } // less interesting items removed. }

24 24 Example (Adding pre-condition checking) package org.thewhittakers.banking; public aspect AccountConstraintsAspect { pointcut preventNegativeAmounts(Account account, double amount) : (execution(* || 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"); }

25 25 Example Exception Handling 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); } 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; }

26 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 27 Uses Cases for AOP Not only for Logging & Exception Handling Thread Safety Multi-Object Protocol Performance optimization  Middleware Specialization using AOP ( Timing / Monitoring /Tracing Various kinds of invasive/non-invasive instrumentation Authentication & Authorization Transactional management /Locking Session Handling Synchronization Caching

28 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 29 Installing AspectJ Download AJDT plugin for eclipse from: 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 30 First Steps Converting 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 31 Step 1: Select Project from the package explorer & Right click & choose “Convert to AspectJ Project” from the context menu

32 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 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.

34 34

35 35

36 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 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 38 Skeletal Aspect

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

40 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 41 History of AOP AOP has been following other technologies like OOP Worked in academia Popping up more & more in real world

42 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 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 44 Questions ??

45 45 References (2004). AspectJ Sample Code. Retrieved May 11, 2004, from the AspectJ documentation: home/sample-code.html home/sample-code.html (2004). You Arent Gonna Need It. Retrieved May 11, 2004, from the Extreme Programming Wiki: 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: aspect_p.html aspect_p.html Laddad, R. (2002). I want my AOP! Part 2. Retrieved May 11, 2004, from JavaWorld: aspect2_p.html aspect2_p.html Laddad, R. (2003). AspectJ in Action: Practical Aspect-Oriented Programming. Greenwich, CT: Manning Publications.

46 46 Defining Pointcuts Calling methods & constructors Advice is inserted after argument evaluation, but before calling. Access to caller’s context PointcutDescription 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 ( Call to any MyClass ' constructor with any arguments call ( 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

47 47 Defining Pointcuts Control flow based pointcuts PointcutDescription 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

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

49 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 PointcutDescription 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

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

51 51 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.

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