an empirical assessment of the crosscutting concern problem
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An Empirical Assessment of the Crosscutting Concern Problem
Marc EaddyDepartment of Computer
ScienceColumbia University
Motivation□Maintenance dominates software
costs
2
MAINTENANCE
OTHER DEVELOPMENT
50–90% OF TOTAL SOFTWARE COST
3–4 x DEVELOPMENT COSTS
Motivation□>50% of maintenance time spent
understanding the program
3
Motivation□>50% of maintenance time spent
understanding the program□Where are the features,
reqs, etc. in the code?
4
Reqs
Code
Motivation□>50% of maintenance time spent
understanding the program□Where are the features,
reqs, etc. in the code?□What is this code for?
5
Motivation□>50% of maintenance time spent
understanding the program□Where are the features,
reqs, etc. in the code?□What is this code for?□Why is it hard to
understand and changethe program?
6
Main Contributions□Improved state of the
art of concern location
□Innovative metricsand experimentalmethodology
□Evidence of the dangersof crosscutting concerns
7
ConcernTagger
Cerberus
Program Dependency Graphinterface A {
public void foo();}public class B implements A {
public void foo() { ... }public void bar() { ... }
}public class C {
public static void main() {B b = new B();b.bar();
}
Source Code
C AB
foofoomain bar
refscontains contains
calls
containscontains
PDA
8
Improving concern location□ Statement Annotations for Fine-Grained Advising
□ ECOOP Workshop on Reflection, AOP, and Meta-Data for Software Evolution (2006)
□ Eaddy and Aho□ Demo: Wicca 2.0 - Dynamic Weaving using the .NET
2.0 Debugging APIs□ Aspect-Oriented Software Development (2007)□ Eaddy
□ Identifying, Assigning, and Quantifying Crosscutting Concerns□ ICSE Workshop on Assessment of Contemporary Modularization
Techniques (2007)□ Eaddy, Aho, and Murphy
□ Cerberus: Tracing Requirements to Source Code Using Information Retrieval, Dynamic Analysis, and Program Analysis□ IEEE International Conference on Program Comprehension (2008)□ Eaddy, Aho, Antoniol, and Guéhéneuc
9
Innovative metrics & methodology
□ Towards Assessing the Impact of Crosscutting Concerns on Modularity□ AOSD Workshop on Assessment of Aspect Techniques
(2007)□ Eaddy and Aho
□ Do Crosscutting Concerns Cause Defects?□ IEEE Transactions on Software Engineering (2008)□ Eaddy, Zimmerman, Sherwood, Garg, Murphy, Nagappan, and
Aho
Dangers of crosscutting□ Do Crosscutting Concerns Cause Defects?
□ IEEE Transactions on Software Engineering (2008)□ Eaddy, Zimmerman, Sherwood, Garg, Murphy, Nagappan, and
Aho
10
Roadmap□Improved state of the
art of concern location
□Innovative metricsand experimentalmethodology
□Evidence of the dangersof crosscutting concerns
11
ConcernTagger
Cerberus
Program Dependency Graphinterface A {
public void foo();}public class B implements A {
public void foo() { ... }public void bar() { ... }
}public class C {
public static void main() {B b = new B();b.bar();
}
Source Code
C AB
foofoomain bar
refscontains contains
calls
containscontains
PDA
What is a “concern?”
□Feature, requirement, design pattern, code idiom, etc.
□Raison d'être for code□Every line of code exists to satisfy some
concern□Existing definitions are poor
□Concern domain must be “well-defined set”
12
Anything that affects the implementation of a program
13
Concern location problem
ConcernsProgram Elements
Concern–code relationship hard to obtain
14
Concern location problem
ConcernsProgram Elements
?□Concern–code relationship
undocumented
Concern–code relationship hard to obtain
15
Concern location problem
ConcernsProgram Elements
□Concern–code relationship undocumented
□Reverse engineer the relationship
Concern–code relationship hard to obtain
Manual concern location□Existing techniques too subjective
□Inaccurate, unreliable□Ideal
□Code affected when concern is changed□My insight
□Prune dependency rule [ACOM’07]□Code affected when concern is pruned
(removed)□i.e., software pruning□Practical approximation 16
Concern–code relationship determined by a human
17
Prune dependency rule□Code is prune dependent on concern if
□Concern pruned code removed or altered □Distinguish between removing and
altering code□Easily determine change impact of
removing code□Code dependent on removed code must
be altered (to prevent compile errors)□Easy for human to approximate
Manual concern location□Existing tools impractical for
analyzing all concerns of a real system□Many concerns (>100)□Many concern–code links (>10K)□Hierarchical concerns
□My solution: ConcernTagger [TSE’08]
18
Concern–code relationship determined by a human
ConcernTagger
19
Automated concern location□Experts look for clues in docs and
code□Existing techniques only consult 1 or
2 experts□My solution: Cerberus [ICPC’08]
1. Information retrieval2. Execution tracing3. Prune dependency analysis
20
Concern–code relationship predicted by an “expert”
21
IR-based concern location□i.e., Google for code□Program entities are documents□Requirements are queries
join Id_join
js_join()
Requirement“Array.join”
SourceCode
22
Vector space model [Salton]□Parse code and reqs doc to extract term vectors
□NativeArray.js_join() method “native,” “array,” “join”□“Array.join” requirement “array,” “join”
□My contributions□Expand abbreviations
□numconns number, connections, numberconnections□Index fields
□Weigh terms (tf · idf)□Term frequency (tf)□Inverse document frequency (idf)
□Similarity = cosine distance between document and query vectors
23
Tracing-based concern location
□Observe elements activated when concern is exercised□Unit tests for each concern□e.g., find elements uniquely activated by a
concern
24
Tracing-based concern location
□Observe elements activated when concern is exercised□Unit tests for each concern□e.g., find elements uniquely activated by a
concernCall
Graph
js_join
var a = new Array(1, 2);if (a.join(',') == "1,2"){ print "Test passed";}else { print "Test failed";}
js_construct
Unit Testfor “Array.join”
25
Tracing-based concern location
□Observe elements activated when concern is exercised□Unit tests for each concern□e.g., find elements uniquely activated by a
concernCall
Graph
js_join
var a = new Array(1, 2);if (a.join(',') == "1,2"){ print "Test passed";}else { print "Test failed";}
js_construct
Unit Testfor “Array.join”
26
Tracing-based concern location
□Elements often activated by multiple concerns
□What is “information content” of element activation?
□Element Frequency–Inverse ConcernFrequency [ICPC’08]
27
Prune dependency analysis□Infer relevant elements based on
structural relationship to relevant element e (seed)□Assumes we already have some seeds
□Prune dependency analysis [ICPC’08]□Automates prune dependency rule
[ACOM’07]□Find references to e□Find superclasses and subclasses of e
PDA example
28
C AB
foofoomain barcalls
contains
refscontainscontains contains
Program Dependency Graphinterface A { public void foo();}public class B implements A { public void foo() { ... } public void bar() { ... }}public class C { public static void main() { B b = new B(); b.bar(); }
Source Code
inherits
29
C AB
foofoomain barcalls
contains
refscontainscontains contains
Program Dependency Graphinterface A { public void foo();}public class B implements A { public void foo() { ... } public void bar() { ... }}public class C { public static void main() { B b = new B(); b.bar(); }
Source Code
inherits
PDA example
30
PDA example
C AB
foofoomain bar
contains
refscontainscontains contains
Program Dependency Graphinterface A { public void foo();}public class B implements A { public void foo() { ... } public void bar() { ... }}public class C { public static void main() { B b = new B(); b.bar(); }
Source Code
calls
inherits
PDA example
31
Program Dependency Graphinterface A { public void foo();}public class B implements A { public void foo() { ... } public void bar() { ... }}public class C { public static void main() { B b = new B(); b.bar(); }
Source Code
inheritsC AB
foofoomain bar
refscontains contains
calls
containscontains
PDA example
32
Program Dependency Graphinterface A { public void foo();}public class B implements A { public void foo() { ... } public void bar() { ... }}public class C { public static void main() { B b = new B(); b.bar(); }
Source Code
C AB
foofoomain bar
refscontains contains
calls
containscontains
inherits
Cerberus
33
Cerberus effectiveness
34
0 50 100 150 200 250 300 3500.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 IR + Tracing + PDA("Cerberus")IR + PDAIR + TracingIRTracing + PDATracing
Number of Concerns Located by Technique
F-M
easu
re
Cerberus
PDA IMPROVES IR BY 155%
PDA IMPROVES TRACING BY 104%
CERBERUS MOST EFFECTIVE
Roadmap□Improved state of the
art of concern location
□Innovative metricsand experimentalmethodology
□Evidence of the dangersof crosscutting concerns
35
ConcernTagger
Cerberus
Program Dependency Graphinterface A {
public void foo();}public class B implements A {
public void foo() { ... }public void bar() { ... }
}public class C {
public static void main() {B b = new B();b.bar();
}
Source Code
C AB
foofoomain bar
refscontains contains
calls
containscontains
PDA
The crosscutting concern problem
□Code related to the concern is…□Scattered across (crosscuts) multiple
files□Often tangled with other concern code
36
Some concerns difficult to modularize
ConcernsProgram Elements
Example: Pathfinding in Goblin
□Pathfinding is modularized37
Example: Collision detection
□Collision detection not modularized 38
How to measure scattering?□Existing metrics inadequate□My solution
□Degree of scattering [ASAT’07]□Degree of tangling [ASAT’07]
39
40
Degree of scattering (DOS)□ Measures concern modularity, i.e., distribution of
concern code across multiple classes
□ Average DOS – Overall modularity of concerns□ Summarizes amount of crosscutting present
□ More insightful than traditional metrics□ “class A is highly coupled” vs. “feature A is hard to
change”
[Wong, et al.]
[ACOM’07]
Marc Eaddy 41
□More descriptive than class count□Consider two different concern
implementations
Insight behind DOS
DOS = 1.00 #Classes = 4
DOS = 0.08#Classes = 4
Class #4
Class #1
Class #2
Class #3
Class #197%
Class #2
Class #3 Class #4
Marc Eaddy 42
Degree of tangling (DOT)□Distribution of class code across
multiple concerns
□Average DOT – Overall separation of concerns
[Wong, et al.]
[ACOM’07]
Roadmap□Improved state of the
art of concern location
□Innovative metricsand experimentalmethodology
□Evidence of the dangersof crosscutting concerns
43
ConcernTagger
Cerberus
Program Dependency Graphinterface A {
public void foo();}public class B implements A {
public void foo() { ... }public void bar() { ... }
}public class C {
public static void main() {B b = new B();b.bar();
}
Source Code
C AB
foofoomain bar
refscontains contains
calls
containscontains
PDA
Do crosscutting concerns cause defects? [TSE’08]
□Created mappings1. Requirement–code map (via
ConcernTagger)2. Bug–code map (via BugTagger)3. Bug–requirement map (inferred)
Project Lang. Size (KLOC)
MappedConcerns
MappingTime (hr)
Mylyn–Bugzilla
Java 13 28 31
Rhino Java 44 417 102iBATIS Java 13 173 18
44
Do crosscutting concerns cause defects? [TSE’08]
□Correlated scatteringand bug count□Spearman
correlation□Found moderate
to strong correlation between scatteringand defects
□As scattering increasesso do defects
45
Scat
teri
ngBugs
How widespread is the problem?
□5 case studies of OO programs□Scattering
□Concerns related to 6 classes on average□OO unsuitable for representing these problem domains?
□Most (86%) concerns are crosscutting to some extent□Dispels “modular base” notion□General-purpose solution needed
□Tangling□Classes related to 10 concerns on average□Poor separation of concerns
□Classes doing too much□Crosscutting concerns severely limit modularity
46
Main Contributions□Improved state of the
art of concern location
□Innovative metricsand experimentalmethodology
□Evidence of the dangersof crosscutting concerns
47
ConcernTagger
Cerberus
Program Dependency Graphinterface A {
public void foo();}public class B implements A {
public void foo() { ... }public void bar() { ... }
}public class C {
public static void main() {B b = new B();b.bar();
}
Source Code
C AB
foofoomain bar
refscontains contains
calls
containscontains
PDA
Future work□Further explore new concern analysis field□Techniques to reduce crosscutting□Improve concern location
□Improve PDA generality, precision, and heuristics□Use machine learning to combine judgments□Incorporate smart “grep” and PDA into IDE
□Gather empirical evidence□Impact of reducing crosscutting□Impact of crosscutting on maintenance effort□Impact of code tangling on quality
48
Acknowledgements□Alfred Aho□ConcernTagger/Mapper
□Vibhav Garg□Jason Scherer□John Gallagher□Martin Robillard□Frédéric Weigand-Warr
□BugTagger□Thomas Zimmermann
□Cerberus□Giuliano Antoniol
□Yann-Gaël Guéhéneuc□Andrew Howard
□Gobin□Erik Petterson□John Waugh□Hrvoje Benko
□Wicca□Boriana Ditcheva□Rajesh Ramakrishnan□Adam Vartanian
□Microsoft Phoenix Team49
Questions?
Marc EaddyColumbia University
eaddy@cs.columbia.edu
50
Wicca□Extends C# tool chain
□wsc – Wicca# preprocessor/compiler
□Phx.Morph – Post-compile tool [AOSD’06]
□wdbg – Debugger [SC’07]
51
Wicca□Extends runtime environment
□wicca – Command-line shell[AOSD’07]□Dynamic aspect-oriented programming□Dynamic software updating□Edit-and-continue
52
Wicca architecture
53
Wicca transformation pipeline
54
Deltas
WovenProgram
BreakPoints
Wicca#Compiler Phx.Morph
AspectAssemblies
CompiledProgramSource
Files
Frontend Backend
C# Compiler Phoenix
Phx.Morph architecture
55
Phx.MorphEditors
Open Classes, binary and breakpoint weaving
Phoenix-specific AOP
Attribute Handlers
Phoenix
PEREWAssemblyRe-Writer
Phx.AopAOP
Joinpoints, pointcuts, …
AttributesCustom AOP annotations
.NET
MorphPlugin
Example: SimpleDraw□SimpleDraw draws shapes on a
display□Naïve implementation couples Shape
and Display concerns
56
public init() { s = new Shape[3]; s[0] = new Point(10, 10); s[1] = new Point(5, 5); s[2] = new Line(new Point(1, 9), new Point(9, 1)); for (int i = 0; i < s.Length; i++) Display.instance().addShape(s[i]); Display.instance().update();
SimpleDraw.cs public void moveBy(int dx, int dy) { x += dx; y += dy; Display.instance().update();
public void moveBy(int dx, int dy) { a.moveBy(dx, dy); b.moveBy(dx, dy); Display.instance().update();
Point.cs
Line.cs
Example: SimpleDraw□SimpleDraw draws shapes on a
display□Naïve implementation couples Shape
and Display concerns
57
public init() { s = new Shape[3]; s[0] = new Point(10, 10); s[1] = new Point(5, 5); s[2] = new Line(new Point(1, 9), new Point(9, 1)); for (int i = 0; i < s.Length; i++) Display.instance().addShape(s[i]); Display.instance().update();
SimpleDraw.cs public void moveBy(int dx, int dy) { x += dx; y += dy; Display.instance().update();
public void moveBy(int dx, int dy) { a.moveBy(dx, dy); b.moveBy(dx, dy); Display.instance().update();
Point.cs
Line.cs
Display updating aspectclass DisplayUpdating {
[Advice(AdviceType.after, "execution(void Shape.moveBy)")][Advice(AdviceType.after, "execution(void Point.moveBy)")][Advice(AdviceType.after, "execution(void Line.moveBy)")]static public void updateAfterMove() {
Display.instance().update();}…
}
58
Confounding effect of size□Larger concerns are
buggier□Scattering, size, and bug
count strongly interrelated□Is scattering just measuring
the “size effect”?□Control for size using
covariant analysis□Step-wise regression□Principal component analysis
□Scattering still explains some bug count variance 59
DOS
Size
Bugs
.66
.90
.68
Side classesPowerful and disciplined class extension
60
Powerful
• Combines subclasses, aspects, open classes, and refinements• Unified subclass-like notation and semantics• Multiple base classes can be extended simultaneously
Disciplined
• Extensions must be explicitly allowed• Limits quantification
The expression problem□Need to parse simple expression language
□e.g., 3 + 4□Expressions: Literal, Add□Semantics: eval
□Extensibility goals□Easily add new expressions□Easily add new semantics
□Cannot do both easily
61
P. Tarr, H. Ossher, W. Harrison, S. Sutton Jr., “N Degrees of Separation: Multi-Dimensional Separation of Concerns,” ICSE 1999.
M. Torgersen, “The Expression Problem Revisited,” ECOOP 2004.
Easy in OO Easy in FP
Parser implementation in OOabstract class Expr {
abstract int eval();}
class Add : Expr {public Expr l, r;public Add(Expr l, Expr r) { this.l = l; this.r = r; }public int eval() { return l.eval() + r.eval(); }
}
62
• New concern: Add printing semantics– Concern is crosscutting!
Add printing using subclassing
abstract class PrintingExpr : Expr {abstract void print();
}
class PrintingAdd : PrintingExpr, Add {public void print() { Console.WriteLine(l + “+” + r); }
}
63
• Invasive• Inflexible
Printing the side class wayabstract class PrintingExpr + Expr {
abstract void print();}
class PrintingAdd + PrintingExpr, Add {public void print() { Console.WriteLine(l + “+” + r); }
}
64
Side class compositi
onoperator
• Modularizes the printing concern• Side class implicitly extends base
class• Clients (and other sub/side classes)
oblivious
Caching result of eval()□Concern: Cache expression
evaluation□Invalidate cache when expression
changes
65
Intertype declarationsinterface IObserver {
void onChanged();}
class CachableExpr + Expr : IObserver {protected IObserver obs = null;
protected int cache;protected bool isCacheValid = false;
void onChanged() { isCacheValid = false; if (obs != null) obs.onChanged();
}} 66
Intertype declarationsinterface IObserver {
void onChanged();}
class CachableExpr + Expr : IObserver {protected IObserver obs = null;
protected int cache;protected bool isCacheValid = false;
void onChanged() { isCacheValid = false; if (obs != null) obs.onChanged();
}} 67
Memberintertype declaratio
ns
Interfaceintertype declaratio
n
Method chainingclass CachableAdd + Add {
public CachableAdd(CachableExpr l, CachableExpr r) {
base(l, r);this.l.obs = this;this.r.obs = this;
}
override public int eval() {if (!isCacheValid) {
cache = base.eval();isCacheValid = true;
}return cache;
} 68
Method chainingclass CachableAdd + Add {
public CachableAdd(CachableExpr l, CachableExpr r) {
base(l, r);this.l.obs = this;this.r.obs = this;
}
override public int eval() {if (!isCacheValid) {
cache = base.eval();isCacheValid = true;
}return cache;
} 69
Call ‘next’ delegate
Virtual fieldsoverride Expr l {
set { base.l = value; this.l.obs = this; this.onChanged();
}}override Expr r {
set { base.r = value; this.r.obs = this; this.onChanged();
}}
70
Virtual fields
71
Virtualfield
override
override Expr l {set {
base.l = value; this.l.obs = this; this.onChanged();
}}override Expr r {
set { base.r = value; this.r.obs = this; this.onChanged();
}}
Metaprogramming
override Expr [$which = l | r] {set {
base.$which = value; this.$which.obs = this; this.onChanged();
}}
72
Metavariable
Limited quantificati
on
However, base class mustpermit extension
73
class Add : Expr {public virtual Expr l, r;public Add(Expr l, Expr r) { this.l = l; this.r = r; }public virtual int eval() { return l.eval() + r.eval(); }
}
Addresses issues of existing mechanisms
□Subclasses□Inflexible composition□Only extend one base class□Only extend instance methods□Requires invasive changes in clients
□Open classes/refinements/mixins□Need for composition ordering□Need for constructor and static member overriding
□Aspects□Lack of modular reasoning□Poor integration with existing OO syntax and semantics□Need for symmetric model□Matching wrong join points
74
Statement annotations [RAM-SE’06]
□Concerns with “irregular” implementations□Error handling, assertions,
optimizations, logging
□Hard to modularize using regular aspects
□Apply aspects to specific locations,statements, or object instances
75
Difficult to modularize irregular concern code
[NonNull] AuthorizationRequest ar =new
AuthorizationRequest(this, dest);
76
77
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