Aspect-oriented programming with AspectJ (as part of the the PTT lecture)

Aspect-oriented
Programming
Prof. Dr. Ralf Lämmel
Universität Koblenz-Landau
Software 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 pitch
Suppose 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!
See copyright notice
elsewhere in this deck.

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

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

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.

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

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.

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

Logistics
Recommended working environment for AspectJ
Recent Eclipse
AJDT (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.

Scenario
• Report when “main” method is called.
• Report when execution is completed.
“Hello World” of AspectJ
DEMO
See package helloworld

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

operations that move elements
factory methods
Display
*
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

A simple ﬁgure editor
class Line implements FigureElement{
private Point p1, p2;
Point getP1() { return p1; }
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) { ... }
}

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.

Join points along the execution of method call
(Move a line, and in turns its points)
a Line
a Point
returning or throwing
dispatch
dispatch
a method call
a method execution
a method execution
myObject.moveBy(2, 2)
myObject.p1.moveBy(2, 2)

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

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.

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));

Anonymous and named pointcuts
• Pointcuts can be named – for reuse.
• Example:
pointcut move():

After advice:
actions to be taken after computation at join point
pointcut move():
after() returning: move() {
<code here runs after each move>
}

A simple aspect for
display updating
aspect DisplayUpdating {
pointcut move():
Display.update();
}
}

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;
void setP1(Point p1) {
this.p1 = p1;
Display.update();
}
void setP2(Point p2) {
this.p2 = p2;
Display.update();
}
}

Pointcuts – some elaborations
• Crosscutting multiple classes
• Crosscutting based on interface signatures
pointcut move():
call(void Point.setX(int)) ||
call(void Point.setY(int));
pointcut move():
call(void FigureElement.moveBy(int, int)) ||
call(void Point.setY(int));

Pick up values at join point
Example: 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 Point.setY(int)));
after(FigureElement fe) returning: move(fe) {
<fe is bound to the figure element>
}
parameter
mechanism

Example: 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)

Recall: the earlier aspect
for display updating
pointcut move():
Display.update();
}
}

A revision of the aspect
for display updating
target(fe) &&
after(FigureElement fe) returning: move(fe) {
Display.update(fe);
}
}

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

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)

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.

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

AspectJ -- How does it work?
Two kinds of classes:
a) Classes compiled by AspectJ
b)Classes not compiled by AspectJ
Two weaving approaches:
a) Generated bytecode contains aspects.
b)Preexisting bytecode is transformed.

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

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

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;
}
}

Example: the arguments of an matched call
before(int newX):
assert newX >= MIN_X;
assert newX <= MAX_X;
}
Pointcut parameter,
as used previously,
see “target”.
Bind parameter to
arguments of
matched call

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;
}
}

Around advice: make calls valid
aspect BoundPointEnforcement {
void around(int 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));
}
}

Context of around advice
For each around advice with the signature
ReturnType around(T1 arg1, T2 arg2, …)
there is a special method with the signature
ReturnType proceed(T1, T2, …)
available only in around advice.
Meaning: “run what would have ran if this advice had not been defined”

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

A revision of the aspect for
display updating
target(fe) &&
pointcut topLevelMove(FigureElement fe):
move(fe) && !cflowbelow(move(FigureElement));
after(FigureElement fe) returning: topLevelMove(fe) {
Display.update(fe);
}
}

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.");
}
}
new is
rejected
We use
wildcards.

“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.

“compile-time advice” as an improvement
Compile error if factory methods are bypassed
class Figure {
...
}
(call(Point.new(..)) || call(Line.new(..)))
declare error: illegalNewFigureElement():
"Use factory method instead.";
}
}
Special kind
of advice

“compile-time advice” as an improvement
Compile error if factory methods are bypassed
class Figure {
...
}
call(FigureElement+.new(..))
declare error: illegalNewFigureElement():
"Use factory method instead.";
}
}
Use
subtyping
wildcard

Display updating with
compile-time factory check
DEMO
See package ﬁgures

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.

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 problem
with inter-type declarations
DEMO
See package expressions

http://101companies.org/wiki/
Contribution:aspectJ
DEMO

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.

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.

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.

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

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
m
ulti-partwild
card

Other AspectJ concepts
• Precedence among multiple aspects
• Pointcuts:
–Initialization
–Exceptions
• Reflection for join point
• Abstract pointcuts
• Aspect inheritance
• Subtyping constraints
• ...

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/

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.

Aspect-oriented programming with AspectJ (as part of the the PTT lecture)

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