Post on 06-May-2015
Aspect-oriented Programming
Prof. Dr. Ralf LämmelUniversität Koblenz-LandauSoftware Languages Team
https://github.com/101companies/101repo/tree/master/languages/AspectJ/aspectJSamples
Non-101samples available here:
http://101companies.org/wiki/Contribution:aspectJ
See special copyright message
at the end of the slide deck.
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Elevator pitchSuppose you need to implement some tracing (aka logging) functionality in your app. Chances are that code needs to be scattered allover the app; see below. Can we do better than this. Yes, use AOP!
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
class Point { void set(int x, int y) { TraceSupport.traceEntry("Point.set"); this.x = x; this.y = y; TraceSupport.traceExit("Point.set"); }}
class TraceSupport { static int TRACELEVEL = 0; static protected PrintStream stream = null; static protected int callDepth = -1; static void init(PrintStream _s) {stream=_s;} static void traceEntry(String str) { if (TRACELEVEL == 0) return; callDepth++; printEntering(str); } static void traceExit(String str) { if (TRACELEVEL == 0) return; callDepth--; printExiting(str); }}
TraceSupport
Many classes (objects) interact with the trace facility.
Tracing without AOP
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Logging is a crosscutting concern.These concerns don’t fit into traditional modules.
Other examples:• Error handling• Synchronization• Security• Power management• Memory management• Performance optimizations
Costs of tangled code:• Difficult to understand• Difficult to change• Increases with size of system• Increases maintenance costs• Very difficult to get rid of, if at all
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Aspect-Oriented Programming
• Crosscutting concerns– … are inherent to complex systems.– … serve important purposes.– … have a natural structure.– … can be captured in new kinds of modules.– … require designated language and tool support.
An aspect is a well-modularized crosscutting concern.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Tracing with AOP
414
aspect PointTracing {
pointcut trace(): within(com.bigboxco.boxes.*) && execution(* *(..));
before(): trace() { TraceSupport.traceEntry(tjp); } after(): trace() { TraceSupport.traceExit(tjp); }}
class Point { void set(int x, int y) { this.x = x; this.y = y; }}
The classes (objects) do not to contain code that anticipates the interaction with the trace facility.
An aspect in the AspectJ language – which is a Java
extension
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Aspects – benefits in design and coding
• Objects no longer responsible for using the trace facility.• Trace aspect encapsulates that tracing responsibility.• Changing the tracing concern affects one module.• Removing tracing from the design is trivial.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
AspectJ – an implementation of AOP
• Well-integrated extension to Java• A general-purpose AOP language• Compiles to JVM-compatible .class files• Semantics relies on load-time weaving• IDE support for Eclipse et al.: AJDT• Freely available implementation• Invented in the 90-ties at XEROX Parc• Now maintained and used by IBM• Other major AOP implementations for Java:
– JBoss– AspectWerkz
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Logistics
Recommended working environment for AspectJRecent EclipseAJDT (includes AspectJ compiler)
Use Eclipse UPDATE to install AJDT.Eclipse/AspectJ tips:
Make sure weaving is enabled.Make use of IDE hints to study weaving.
Simple examples(demos)
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Scenario• Report when “main” method is called.• Report when execution is completed.
“Hello World” of AspectJ
DEMO
See package helloworld
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Scenario• Refuse negative withdrawal.• Refuse negative deposits.• Refuse negative balance.
A safer account class
420
DEMO
See package accounts
AOP language concepts based on a
bigger example
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
operations that move elements
factory methodsDisplay
*
2Point
getX()getY()setX(int)setY(int)moveBy(int, int)
Line
getP1()getP2()setP1(Point)setP2(Point)moveBy(int, int)
Figure
makePoint(..)makeLine(..)
FigureElement
moveBy(int, int)
A simple figure editor
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
A simple figure editor
class Line implements FigureElement{ private Point p1, p2; Point getP1() { return p1; } Point getP2() { return p2; } void setP1(Point p1) { this.p1 = p1; } void setP2(Point p2) { this.p2 = p2; } void moveBy(int dx, int dy) { ... }}
class Point implements FigureElement { private int x = 0, y = 0; int getX() { return x; } int getY() { return y; } void setX(int x) { this.x = x; } void setY(int y) { this.y = y; } void moveBy(int dx, int dy) { ... }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
A crosscutting concern: figure updating
• Figures are collections of figure elements.• The latter move frequently.• Displays show figures.• Displays need to be updated when moves happen.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Join points along the execution of method call(Move a line, and in turns its points)
a Line
a Point
returning or throwingdispatch
dispatch
a method call
returning or throwinga method execution
returning or throwinga method execution
myObject.moveBy(2, 2)
myObject.p1.moveBy(2, 2)
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Join-point terminology
• Several kinds of join points– method & constructor call– method & constructor execution– field get & set– exception handler execution– static & dynamic initialization
a Line
dispatch
method call join points
method execution join points
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
The language construct “pointcut”
• Pointcuts provide a means to identify join points.• A pointcut is a kind of predicate on join points that:
– can match or not match any given join point, and– optionally, pulls out some of the values at that join point.
• Example: call(void Line.setP1(Point))
• Meaning: Matches if the join point is a method call with this signature.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pointcuts compose like predicates, using &&, || and !.
Example:
Meaning:
Composition of pointcuts
Matches whenever a Line receives a “void setP1(Point)” or “void setP2(Point)” method call.
a “void Line.setP2(Point)” call
or
a “void Line.setP1(Point)” call
call(void Line.setP1(Point)) || call(void Line.setP2(Point));
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Anonymous and named pointcuts
• Pointcuts can be named – for reuse.
• Example: pointcut move(): call(void Line.setP1(Point)) || call(void Line.setP2(Point));
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
After advice: actions to be taken after computation at join point
pointcut move(): call(void Line.setP1(Point)) || call(void Line.setP2(Point));
after() returning: move() { <code here runs after each move> }
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
A simple aspect for display updating
aspect DisplayUpdating {
pointcut move(): call(void Line.setP1(Point)) || call(void Line.setP2(Point));
after() returning: move() { Display.update(); }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Tangled code without an aspect
• Update calls are tangled through the code.• Discipline of updating must be checked by full inspection.• BTW, “weaving” effectively results in this code.
class Line { private Point p1, p2;
Point getP1() { return p1; } Point getP2() { return p2; }
void setP1(Point p1) { this.p1 = p1; Display.update(); } void setP2(Point p2) { this.p2 = p2; Display.update(); }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pointcuts – some elaborations
• Crosscutting multiple classes
• Crosscutting based on interface signatures
pointcut move(): call(void Line.setP1(Point)) || call(void Line.setP2(Point)) || call(void Point.setX(int)) || call(void Point.setY(int));
pointcut move(): call(void FigureElement.moveBy(int, int)) || call(void Line.setP1(Point)) || call(void Line.setP2(Point)) || call(void Point.setX(int)) || call(void Point.setY(int));
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pick up values at join pointExample: the target of a join point
Variable fe is bound to type by pointcut declaration.Variable fe is bound to value at join point.Advice can access value.
pointcut move(FigureElement fe): target(fe) && (call(void FigureElement.moveBy(int, int)) || call(void Line.setP1(Point)) || call(void Line.setP2(Point)) || call(void Point.setX(int)) || call(void Point.setY(int)));
after(FigureElement fe) returning: move(fe) { <fe is bound to the figure element> }
parameter mechanism
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pick up values at join pointExample: the arguments of an intercepted method
Variables x and y are bound to type by pointcut declaration.Variables x and y are bound to values at join point.Advice can access arguments.
pointcut move(int x, int y): args(x, y) && call(void FigureElement.moveBy(int, int)
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Recall: the earlier aspect for display updating
aspect DisplayUpdating {
pointcut move(): call(void Line.setP1(Point)) || call(void Line.setP2(Point));
after() returning: move() { Display.update(); }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
A revision of the aspect for display updating
aspect DisplayUpdating {
pointcut move(FigureElement fe): target(fe) && (call(void FigureElement.moveBy(int, int)) || call(void Line.setP1(Point)) || call(void Line.setP2(Point)) || call(void Point.setX(int)) || call(void Point.setY(int)));
after(FigureElement fe) returning: move(fe) { Display.update(fe); }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Intermediate summary• A design concern is crosscutting (CC) if:
– involves several objects or operations, and– implemented w/o AOP leads to distant code locations
• doing the same thing • doing a coordinated single thing
• Expected benefits of aspectual modularization of CC: – Good modularity, even in the presence of crosscutting concerns
• less tangled code, more natural code, smaller code• easier to maintain and to evolve• easier to reason about, debug, change • more reusable• more possibilities for generalization, plug and play
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pointcuts cont’d
call(void Point.setX(int))method/constructor call join points (actual method call)
execution(void Point.setX(int))method/constructor execution join points (actual running method)
initialization(Point)object initialization join points
staticinitialization(Point)class initialization join points (as the class is loaded)
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pointcuts cont’d• Field reference (“right value”)Example: get( int Point.x )Meaning: match if field x of type int of an object of class
Point is referenced in right-value manner.
• Assignment (“left value”)Example: set( int Point.x )Meaning: match if field x of type int of an object of class
Point is referenced in left-value manner.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pointcuts cont’d
this( TypeName ) within( TypeName )withincode( MemberSignature )
any join point at which currently executing object is an instance of type name currently executing code is contained within type name currently executing code is specified methods or constructors
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
AspectJ -- How does it work?
Two kinds of classes:a) Classes compiled by AspectJb)Classes not compiled by AspectJTwo weaving approaches:a) Generated bytecode contains aspects. b)Preexisting bytecode is transformed.
beforeafter
around
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Controlling actions at join point
• before before proceeding at join point
• after returning a value at join point
• after throwing a throwable at join point
• after returning at join point either way
• around on arrival at join point gets explicit
control over when&if computation proceeds
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
An example to illustrate before/after/around“Contract checking”
• Pre-conditions–check whether parameter is valid
• Post-conditions–check whether values were set
• Condition enforcement–force parameters to be valid
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Before advice: assert preconditions
aspect BoundPointPreCondition { before(int newX): call(void Point.setX(int)) && args(newX) { assert newX >= MIN_X; assert newX <= MAX_X; } before(int newY): call(void Point.setY(int)) && args(newY) { assert newY >= MIN_Y; assert newY <= MAX_Y; }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Pick up values at join pointExample: the arguments of an matched call
before(int newX): call(void Point.setX(int)) && args(newX) { assert newX >= MIN_X; assert newX <= MAX_X; }
Pointcut parameter, as used previously,
see “target”.
Bind parameter to arguments of matched call
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
After advice: assert postconditions
aspect BoundPointPostCondition {
after(Point p, int newX) returning: call(void Point.setX(int)) &&
target(p) && args(newX) { assert p.getX() == newX; }
after(Point p, int newY) returning: call(void Point.setY(int)) && target(p) && args(newY) { assert p.getY() == newY; }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Around advice: make calls valid
aspect BoundPointEnforcement {
void around(int newX): call(void Point.setX(int)) && args(newX) { proceed( clip(newX, MIN_X, MAX_X) ); }
void around(int newY): call(void Point.setY(int)) && args(newY) { proceed( clip(newY, MIN_Y, MAX_Y) ); }
private int clip(int val, int min, int max) { return Math.max(min, Math.min(max, val)); }}
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Context of around advice
For each around advice with the signatureReturnType around(T1 arg1, T2 arg2, …)
there is a special method with the signatureReturnType proceed(T1, T2, …)
available only in around advice.
Meaning: “run what would have ran if this advice had not been defined”
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cflow
Control-flow-related pointcuts
cflow( Pointcut )all join points in the dynamic control flow of any join point picked out by Pointcut
cflowbelow( Pointcut )all join points in the dynamic control flow below any join point picked out by Pointcut
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
A revision of the aspect for display updating
aspect DisplayUpdating {
pointcut move(FigureElement fe): target(fe) && (call(void FigureElement.moveBy(int, int)) || call(void Line.setP1(Point)) || call(void Line.setP2(Point)) || call(void Point.setX(int)) || call(void Point.setY(int)));
pointcut topLevelMove(FigureElement fe): move(fe) && !cflowbelow(move(FigureElement));
after(FigureElement fe) returning: topLevelMove(fe) { Display.update(fe); }}
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compile-time advice
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Another example: Enforce factory methods makePoint and ...Line
class Figure { public Line makeLine(Line p1, Line p2) { new Line... } public Point makePoint(int x, int y) { new Point... } ...}
aspect FactoryEnforcement { pointcut illegalNewFigureElement(): (call(Point.new(..)) || call(Line.new(..))) && !withincode(* Figure.make*(..));
before(): illegalNewFigureElement() { throw new Error("Use factory method instead."); }}
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new is rejected
We use wildcards.
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
“Compile-time advice”
• Normally pointcuts define points in the actual execution of
the program to be affected by advice.
• We can also define a pointcut just to produce a compile-
time action (typically an error) for any code location that
matches with the pointcut.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
“compile-time advice” as an improvementCompile error if factory methods are bypassed
class Figure { public Line makeLine(Line p1, Line p2) { new Line... } public Point makePoint(int x, int y) { new Point... } ...}
aspect FactoryEnforcement { pointcut illegalNewFigureElement(): (call(Point.new(..)) || call(Line.new(..))) && !withincode(* Figure.make*(..));
declare error: illegalNewFigureElement(): "Use factory method instead."; }}
Special kind of advice
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
“compile-time advice” as an improvementCompile error if factory methods are bypassed
class Figure { public Line makeLine(Line p1, Line p2) { new Line... } public Point makePoint(int x, int y) { new Point... } ...}
aspect FactoryEnforcement { pointcut illegalNewFigureElement(): call(FigureElement+.new(..)) && !withincode(* Figure.make*(..));
declare error: illegalNewFigureElement(): "Use factory method instead."; }}
Use subtyping wildcard
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Display updating withcompile-time factory check
DEMO
See package figures
inter-type declarations
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Inter-type declarations• An orthogonal concept compared to advice.• Purpose: add members to existing types:
– (static and instance) fields– (static and instance) methods, virtual ones included– …
• Syntax: prefix member name by target class: For instance:
public int Point.numbersOfPoints;
• New members are visible in aspect only.
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Scenario• An object model for an expression language.• The classes do not support any operations.• Add operations for evaluation and pretty-printing.
Solving the expression problemwith inter-type declarations
DEMO
See package expressions
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
http://101companies.org/wiki/Contribution:aspectJ
DEMO
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Summary• More seriously:
• AOP helps avoiding code tangling.• AOP helps with code modularization.• AOP supports modularization of crosscutting concerns.
• Less seriously:• AOP is an intriguing way to break encapsulation.• AOP is all about tracing and logging.• AOP has been obsoleted in Cobol.
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Byte-code transformation based on API for byte-code engineering
(A technique for AOP language implementation)
• Approach:– Load .class file– Use API to analyze and transform byte code.– Then, either:
• Load result into JVM, or• Save result into same or different .class file.
• BTW: • This is a form of meta-programming!• This is borderline reflection.
(We are using Java to transform byte code.)
Not covered in the lecture. This is for those interested.
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Scenario• Add timing code to a method in an existing .class file.• Update .class file itself; transformation is effective upon load.
This is demo-only subject. We don’t go any deeper into it.
DEMO
Byte-code engineering
See package bcel
Not covered in the lecture. This is for those interested.
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Language extension – summary
• Pointcuts– Pick out join points (“points in the execution of the program”) and
values at those points (arguments, results, exceptions) • Advice
– Additional action to take at join points in a pointcut• Before• After (returning or throwing)• Around
• Inter-type declarations (aka “open classes”)• Aspect
– A modular unit of crosscutting behavior– A generalization of the class form of modular unit– Comprised of declarations for:
• Advice• Inter-types• Pointcuts• Fields, constructors, and methods
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Wildcards in pointcuts• target(Point)• target(graphics.geom.Point)• target(graphics.geom.*) any type in graphics.geom• target(graphics..*) any type in any sub-package of graphics
• call(void Point.setX(int))• call(public * Point.*(..)) any public method on Point• call(public * *(..)) any public method on any type
• call(void Point.setX(int))• call(void Point.setY(*))• call(void Point.set*(*))• call(void set*(*)) any setter
• call(Point.new(int, int))• call(new(..)) any constructor
“*” is wild card
“..” is multi-part wild card
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Other AspectJ concepts
• Precedence among multiple aspects• Pointcuts:
– Initialization– Exceptions
• Reflection for join point• Abstract pointcuts• Aspect inheritance• Subtyping constraints• ...
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(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Further reading• AspectJ/AOP tutorials
[1] Intro at eclipse.org[2] The seminal “AspectJ overview”[3] “I want my AOP” (provides sources, too)[4] “The Paradoxical Success of AOP”
• AspectJ sample code:– Install AspectJ and go to doc/examples directory Examples are included in the suite for the present lecture.– Some additional pointers:
• http://www.aspectprogrammer.org/eclipseaspectj/• http://mail.eclipse.org/aspectj/sample-code.html• http://www.cs.wustl.edu/~mdeters/seminar/fall2002/notes/code/• http://stderr.org/doc/aspectj-doc/examples/
(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Copyright notice
This slide deck adopts a considerable amount of slides from the following deck: © 2004 “Aspect-Oriented Programming with AspectJ™” by Julie Waterhouse, Mik Kersten, eclipse.org/aspectj, IBM, UBC; http://kerstens.org/mik/publications/aspectj-tutorial-oopsla2004.ppt. That deck has served as an OOPSLA 2004 tutorial. Most of the slides showed up in many other decks by the key representatives of AspectJ. If you derive any work from the present slide deck, you must keep the copyright notice in tact. All remaining copyright, if any, is with Ralf Lämmel.