Aspect-Oriented Programming - Advance Java Web Technology - Lecture Slides, Slides of Computer Science

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Aspect-Oriented Programming

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Programming paradigms

• Procedural programming
  • Executing a set of commands in a given sequence
  • Fortran, C, Cobol
• Functional programming
  • Evaluating a function defined in terms of other functions
  • Lisp, ML, OCaml
• Logic programming
  • Proving a theorem by finding values for the free variables
  • Prolog
• Object-oriented programming (OOP)
  • Organizing a set of objects, each with its own set of responsibilities
  • Smalltalk, Java, C++ (to some extent)
• Aspect-oriented programming (AOP)
  • Executing code whenever a program shows certain behaviors
  • AspectJ (a Java extension)
  • Does not replace O-O programming, but rather complements it

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Example

• class Fraction { int numerator; int denominator; ... public Fraction multiply(Fraction that) { traceEnter("multiply", new Object[] {that}); Fraction result = new Fraction( this.numerator * that.numerator, this.denominator * that.denominator); result = result.reduceToLowestTerms(); traceExit("multiply", result); return result; } ... }
• Now imagine similar code in every method you might want to trace Docsity.com

Consequences of crosscutting code

• Redundant code

• Same fragment of code in many places

• Difficult to reason about

• Non-explicit structure
• The big picture of the tangling isn’t clear

• Difficult to change

• Have to find all the code involved...
• ...and be sure to change it consistently
• ...and be sure not to break it by accident

• Inefficient when crosscuting code is not needed

Adapted from: www.parc.xerox.com/research/csl/projects/ aspectj/downloads/PARC-Workshop-2002.pptDocsity.com

Terminology

• A join point is a well-defined point in the

program flow

• A pointcut is a group of join points

• Advice is code that is executed at a pointcut

• Introduction modifies the members of a class

and the relationships between classes

• An aspect is a module for handling

crosscutting concerns

• Aspects are defined in terms of pointcuts, advice, and introduction Docsity.com

The Figure Element example

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Join points

• A join point is a well-defined point in the

program flow

• We want to execute some code (“advice”) each time a join point is reached
• We do not want to clutter up the code with explicit indicators saying “This is a join point”
• AspectJ provides a syntax for indicating these join points “from outside” the actual code

• A join point is a point in the program flow

“where something happens”

• Examples:
  • When a method is called Docsity.com

Pointcuts

• Pointcut definitions consist of a left-hand side

and a right-hand side, separated by a colon

• The left-hand side consists of the pointcut name and the pointcut parameters (i.e. the data available when the events happen)
• The right-hand side consists of the pointcut itself

• Example pointcut:

pointcut setter(): call(void setX(int));

• The name of this pointcut is setter
• The pointcut has no parameters
• The pointcut itself is call(void setX(int)) Docsity.com

Example pointcut designators II

• When the target object is of type SomeType

• target(SomeType)

• When the executing code belongs to class

MyClass

• within(MyClass)

• When the join point is in the control flow of a

call to a Test's no-argument main method

• cflow(call(void Test.main()))

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Pointcut designator wildcards

• It is possible to use wildcards to declare

pointcuts:

• execution(* *(..))
  • Chooses the execution of any method regardless of return or parameter types
• call(* set(..))
  • Chooses the call to any method named set regardless of return or parameter type
  • In case of overloading there may be more than one such set method; this pointcut picks out calls to all of them Docsity.com

Pointcut designator composition

• Pointcuts compose through the operations or (“||”), and

(“&&”) and not (“!”)

• Examples:
  • target(Point) && call(int *())
    • Chooses any call to an int method with no arguments on an instance of Point, regardless of its name
  • call(* *(..)) && (within(Line) || within(Point))
    • Chooses any call to any method where the call is made from the code in Point’s or Line’s type declaration
  • within() && execution(.new(int))
    • Chooses the execution of any constructor taking exactly one int argument, regardless of where the call is made from
  • !this(Point) && call(int *(..))
    • Chooses any method call to an int method when the executing object is any type except Point Docsity.com

Pointcut designators based on modifiers

• call(public * *(..))
  • Chooses any call to a public method
• execution(!static * *(..))
  • Chooses any execution of a non-static method
• execution(public !static * *(..))
  • Chooses any execution of a public, non-static method
• Pointcut designators can be based on Docsity.com

Kinds of advice

• AspectJ has several kinds of advice; here are

some of them:

• Before advice runs as a join point is reached, before the program proceeds with the join point
• After advice on a particular join point runs after the program proceeds with that join point - after returning advice is executed after a method returns normally - after throwing advice is executed after a method returns by throwing an exception - after advice is executed after a method returns, regardless of whether it returns normally or by throwing an exception Docsity.com

Example II, with parameters

• You can access the context of the join point:

• pointcut setXY(FigureElement fe, int x, int y): call(void FigureElement.setXY(int, int)) && target(fe) && args(x, y);
• after(FigureElement fe, int x, int y) returning: setXY(fe, x, y) { System.out.println(fe + " moved to (" + x + ", " + y + ")."); }

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