1. David Evans http://www.cs.virginia.edu/~evans
Lecture 21: Crosscutting Aspect-Oriented Programming
"...feels like I've died and gone to Java Heaven."
Don Asumu, Planning Tools Technical Development Leader, BT Laboratories on StructureBuilder, commercial tool based on Aspect-Oriented Programming, at
Background
just got here last week
finished degree at MIT week before
Philosophy of advising students
don’t come to grad school to implement someone else’s idea
can get paid more to do that in industry
learn to be a researcher
important part of that is deciding what problems and ideas are worth spending time on
grad students should have their own project
looking for students who can come up with their own ideas for research
will take good students interested in things I’m interested in – systems, programming languages & compilers, security
rest of talk – give you a flavor of the kinds of things I am interested in
meant to give you ideas (hopefully even inspiration!) but not meant to suggest what you should work on
CS655: Programming Languages
University of Virginia
Computer Science
David Evans

2. University of Virginia CS 655
Menu
Self-Application Answer
Sorting in Linda
Aspect-Oriented Programming
“Separation of Concerns”
Examples
D (Distributed Language)
AspectJ
Adaptive Programming
14 November 2018
University of Virginia CS 655

3. Programming Paradigms
All the languages we have seen so far are “Generalized Procedure” languages:
Programs organized into procedures
Procedures may be grouped and associated with data
Some programs have properties (“aspects”) that cross-cut procedures
Examples: communication, synchronization, handling interrupts, security, loop fusion, etc.
14 November 2018
University of Virginia CS 655

4. Aspect-Oriented Programming
Components
(Functional Units)
Aspect
Weaver
Procedural
Program
Aspects
(Non-functional Units)
Compiler
Executable
14 November 2018
University of Virginia CS 655

5. AO programming vs. idioms
Slide adapted from Gregor Kiczales ICSE Workshop Talk
AO programming vs. idioms
class Book {
private String title;
private PostScript ps;
private String isbn;
private String author;
private User borrower;
author = a;
title = t;
public Book(String t, String a, String i, PostScript p) {
isbn = i; }
ps = p;
public void set_borrower(User u) {borrower = u;}
public User get_borrower() {return borrower;}
public PostScript get_ps() { return ps; }
public class PrinterImpl {
Vector jobs;
String status = “Idle”
pubilc get_status() { return status }
public PrinterImpl() {} }
jobs.add(j);
public add_job(int j) {
class Book {
private UserID borrower;
private PostScript ps;
private BookID id;
id = new BookID(t,a,i);
public Book(String t, String a,
String i, PostScript p) { }
ps = p;
public PostScript get_ps() { return ps; }
public void set_borrower(UserID u) {borrower = u;}
public UserID get_borrower() {return borrower;}
public BookID get_bid() { return id; }
private String title;
class BookID {
public BookID(String t, String a, String i) {
private String isbn;
private String author;
isbn = i;
author = a;
title = t;
public String get_title() {return title;}
class User {
Printer thePrinter;
Library theLibrary;
private UserID id;
public boolean getBook (String title) {
public User(String n) { id = new UserID(n); }
BookID aBook=null;
try{
try {
} catch (RemoteException e) {}
aBook = theLibrary.getBook(id, title);
thePrinter.print(id, aBook); }
public UserID get_uid() { return id; }
return true;
private String name;
class UserID {
public String get_name() { return name; }
public UserID(String n) { name = n; }
Classes
Aspect Weaver
Library theLibrary;
private String name;
class User {
Printer the; Printer
public User(String n) { name = n; }
public boolean getBook (String title) {
return true;
thePrinter.print(this,aBook);
Book aBook = theLibrary.getBook(this, title); }
class Library {
Hashtable books; }
books = new Hashtable(100);
Library(){
public Book getBook(User u, String title) {
Book b = (Book)books.get(title);
if(books.containsKey(title)) {
System.out.println("REQUEST TO GET BOOK " + title);
System.out.println("getBook: Found it:" + b);
b.set_borrower(u);
if (b.get_borrower() == null)
if (b != null) {
return b;
return null;
interface PrinterInterface extends Remote { }
throws RemoteException;
public boolean print (UserID u, BookID b)
implements PrinterInterface {
public class Printer extends UnicastRemoteObject
private Vector jobs = new Vector(10, 10);
private Library theLibrary;
public Printer() throws RemoteException{}
PostScript ps=null;
throws RemoteException{
try{
ps = theLibrary.getBookPS(b);
return queue(newJob);
Job newJob = new Job (ps, u);
} catch (RemoteException e) {}
//...
boolean queue(Job j) {
return true;
interface LibraryInterface extends Remote { }
public PostScript getBookPS(BookID bid) throws RemoteException;
public BookID getBook(UserID u, String title) throws RemoteException;
Library() throws RemoteException {
Hashtable books;
class Library extends UnicastRemoteObject implements LibraryInterface {
books = new Hashtable(100);
throws RemoteException {
public BookID getBook(UserID u, String title)
Book b = (Book)books.get(title);
if(books.containsKey(title)) {
System.out.println("REQUEST TO GET BOOK " + title);
if (b.get_borrower() == null)
System.out.println("getBook: Found it:" + b);
if (b != null) {
return b.get_bid();
b.set_borrower(u);
return null;
public PostScript getBookPS(BookID bid)
Book b = (Book)books.get(bid.get_title());
if (books.containsKey(bid.get_title())) {
return b.get_ps();
if (b != null)
book: Book: {direct pages;}
void print(Book book) {
portal Printer { }
portal Library {
Book: {copy title, author, isbn;}
return:
Book find (String title){ }
Aspects
class Book {
private UserID borrower;
private PostScript ps;
private BookID id;
String i, PostScript p) {
public Book(String t, String a, }
ps = p;
id = new BookID(t,a,i);
public PostScript get_ps() { return ps; }
public void set_borrower(UserID u) {borrower = u;}
public UserID get_borrower() {return borrower;}
public BookID get_bid() { return id; }
private String title;
class BookID {
private String isbn;
private String author;
author = a;
title = t;
public BookID(String t, String a, String i) {
isbn = i;
public String get_title() {return title;}
Library theLibrary;
private UserID id;
class User {
Printer thePrinter;
public User(String n) { id = new UserID(n); }
try{
BookID aBook=null;
public boolean getBook (String title) {
try {
} catch (RemoteException e) {}
aBook = theLibrary.getBook(id, title);
thePrinter.print(id, aBook);
return true;
public UserID get_uid() { return id; } }
private String name;
class UserID {
public String get_name() { return name; }
public UserID(String n) { name = n; }
interface PrinterInterface extends Remote { }
throws RemoteException;
public boolean print (UserID u, BookID b)
implements PrinterInterface {
public class Printer extends UnicastRemoteObject
private Vector jobs = new Vector(10, 10);
private Library theLibrary;
public Printer() throws RemoteException{}
PostScript ps=null;
throws RemoteException{
try{
ps = theLibrary.getBookPS(b);
return queue(newJob);
Job newJob = new Job (ps, u);
} catch (RemoteException e) {}
//...
boolean queue(Job j) {
return true;
interface LibraryInterface extends Remote {
public BookID getBook(UserID u, String title) throws RemoteException; }
public PostScript getBookPS(BookID bid) throws RemoteException;
Library() throws RemoteException {
Hashtable books;
class Library extends UnicastRemoteObject implements LibraryInterface {
books = new Hashtable(100);
throws RemoteException {
public BookID getBook(UserID u, String title)
Book b = (Book)books.get(title);
if(books.containsKey(title)) {
System.out.println("REQUEST TO GET BOOK " + title);
if (b != null) {
System.out.println("getBook: Found it:" + b);
if (b.get_borrower() == null)
return b.get_bid();
b.set_borrower(u);
return null;
public PostScript getBookPS(BookID bid)
Book b = (Book)books.get(bid.get_title());
if (books.containsKey(bid.get_title())) {
return b.get_ps();
if (b != null)
14 November 2018
University of Virginia CS 655

6. University of Virginia CS 655
Join Points
Aspects defined at “Join Points”
Typically, method invocations, entry and exits
Not limited to that – defined by aspect language
Aspect languages are domain specific
Define code or transformations to occur at join points
14 November 2018
University of Virginia CS 655

7. University of Virginia CS 655
What is an aspect?
Definition 1:
A modular unit that has no stand-alone function
That is, it requires a context in order to function
And, that effects other modular units
Definition 2:
Something that implements properties that are not well-captured by functional units
14 November 2018
University of Virginia CS 655

8. University of Virginia CS 655D
Jcore – component language (Java subset, removed synchronization)
Cool – aspect language for expressing coordination of threads
Ridl – aspect language for expressing remote access strategies
Join Points – method invocations
Weaver just needs to put code at method entrances and exits
Can produce Java source code to compile normally
14 November 2018
University of Virginia CS 655

9. University of Virginia CS 655
Cool
Define coordinators that specify locking semantics:
selfexclusive (method executed by at most one thread at a time)
mutexclusive (methods in same set cannot be executed concurrently)
Guarded suspension – define monitor-like abstractions separate from implementations
Can access, but not modify instance variables
14 November 2018
University of Virginia CS 655

10. Cool Example coordinator BoundedStackCoord : BoundedStack {
selfexclusive { pop, push };
mutexclusive { pop, push };
cond boolean full = false;
cond boolean empty = true;
push : requires !full;
on_exit { empty = false; if (sp == MAX) full = true; }
pop : requires !empty;
on_exit { full = false; if (sp == 0) empty = true; } }
Ties aspect to specific implementation!
14 November 2018
University of Virginia CS 655

11. Ridl Control remote method invocations
Gives programmer control over copying semantics in parameter passing (can elect to copy just parts of object graph)
remote Printer {
Static print (Document:
copy *.int, *.String, documentRepository));
... }
Only copy int and String fields, and reference to documentRepository.
14 November 2018
University of Virginia CS 655

12. University of Virginia CS 655
AspectJ
crosscuts identify events
Can be associated with method calls, exception handlers
advice defines code associated with crosscuts
14 November 2018
University of Virginia CS 655

13. Tangled Example: Java API
package java.io;
public class File implements java.io.Serializable {
private String path;
...
public boolean exists() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkRead(path); }
return exists0();
public boolean canRead() {
return canRead0();
Exact same code,
repeated 8 times
in java.io.File
Why not make it a procedure?
Performance (method call overhead)
Better error messages (?)
14 November 2018
University of Virginia CS 655

14. University of Virginia CS 655
Cross-cuts
crosscut name(context): event list
crosscut fileReaders(java.io.File f): java.io.File
& (boolean exists() | boolean canRead()
| boolean canRead() | boolean isDirectory()
| long lastModified() | long length() | String[] list()
| String[] list(FilenameFilter filter))
Wildcards
crosscut nonStatics (): java.io.* & !static * (..);
crosscut intMethods (): MyClass & public * (int);
crosscut returnObject (): * & Object * (..);
crosscut ioHandlers (): Account & * (..) & catch (IOException);
14 November 2018
University of Virginia CS 655

15. University of Virginia CS 655
Advice
advice (Parameters) : crosscut
advice checkRead (java.io.File f): fileReaders (f) {
before {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkRead (path); }
14 November 2018
University of Virginia CS 655

16. AspectJ File Implementation
Aspects shown on previous slides
Simple implementation with security code removed
package java.io;
public class File implements java.io.Serializable {
private String path;
...
public boolean exists() { return exists0(); }
public boolean canRead() { return canRead0(); }
14 November 2018
University of Virginia CS 655

17. Is AspectJ version better?
Advantages
Compact program
Separate concerns without any performance cost
Easier to change and reuse
Easier to understand and analyze policy
Disadvantages
Have to learn a new language
Extra compilation step
Debugging may be difficult (but special-purpose tools)
14 November 2018
University of Virginia CS 655

18. Example: Exception Handling
[Lippert & Lopes, 99]
Exception handling code clutters (and often dwarves) normal code
Tangled exceptions limit reuse
Cannot change exceptional behavior without overriding entire method
Tangled exceptions lose abstraction
All methods must handle network failures by retrying is scattered throughout code (and easy to forget)
14 November 2018
University of Virginia CS 655

19. University of Virginia CS 655
JWAM Experiment
Java framework
Designed around design-by-contract with explicit calls to Contract.require and Contract.ensure
Use AspectJ to abstract out crosscut for contracts and exception handling
11% of original code dealt with exception detection and handling; many handlers do the same thing
14 November 2018
University of Virginia CS 655

20. University of Virginia CS 655
Bank Account
public class Account {
public Account (String owner, int accNo) {
Contract.require (owner != null && owner.length() > 0);
Contract.require (accNo > 0);
this.owner = owner; this.accNo = accNo; }
public void deposit (float amount) {
Contract.require (amount > 0.0);
balance += amount;
...
14 November 2018
University of Virginia CS 655

21. Aspect-Oriented Implementation
class AccountContract {
static advice Account & new(String s, int n) {
before {
Contract.require (s != null && s.length () > 0);
Contract.require (n > 0); } }
static advice Account & deposit (float f) {
before { Contract.require (f > 0.0); } }
14 November 2018
University of Virginia CS 655

22. Handling Library Exceptions
Typical code:
try { registry.put (name, this); }
catch (RemoteException e) {
ErrorLog.print (“Remote call failed: ” + e); }
Replace with:
crosscut remoteHandlers () :
* & * *(..) & catch (RemoteException);
static advice remoteHandlers () {
ErrorLog.print (“Remote call failed in ” thisMethodName + “:” + e); }
14 November 2018
University of Virginia CS 655

23. University of Virginia CS 655
Results
Reduced exception detection and handling code by a factor of 4.5
Removed many simple conditions (null tests of results and arguments in contracts), common exception handlers
Is resulting code likely to be more reliable and maintainable?
14 November 2018
University of Virginia CS 655

24. Adaptive Programming: Demeter
Instance of AOP [Lieberherr92]
Aspects are traversal strategies
Separate the program text and the class structure
Program is independent of class graph
Accomplish tasks by traversals
Specification for what parts of received object should be traversed
Code fragments for what to execute when specific object types are encountered
14 November 2018
University of Virginia CS 655

25. University of Virginia CS 655
Law of Demeter
Law of Demeter: a method should talk only to its friends:
arguments and part objects (computed or stored)
newly created objects
Dilemma:
Small method problem of OO (if followed)
Unmaintainable code (if not followed)
Traversal strategies are the solution to this dilemma
Demeter = Greek Goddess of Agriculture (grow software from small blocks)
14 November 2018
University of Virginia CS 655

26. AP Example: UML Class Diagram
Slide adapted from Karl Lieberherr talk
AP Example: UML Class Diagram
busStops
BusRoute
BusStopList
buses
0..*
BusStop
BusList
waiting
0..*
passengers
Example for developing strategies and writing adaptive programs
Bus
PersonList
Person
0..*
11/14/2018
AOP/Demeter

27. Collaborating Classes
Slide adapted from Karl Lieberherr talk
Collaborating Classes
Find all persons waiting at any bus stop on a bus route
busStops
BusRoute
BusStopList
OO solution:
one method
for each red
class
buses
0..*
BusStop
BusList
waiting
0..*
passengers
Bus
PersonList
Person
0..*
11/14/2018
AOP/Demeter

28. Java Solution (excerpt)
class BusRoute {
BusStopList busstops;
void printWaitingPassengers () {
busstops->printWaitingPassengers (); } }
class BusStopList {
BusStop stops[];
for (int i = 0; i < stops.length; i++)
stops[i].printWaitingPassengers ();
14 November 2018
University of Virginia CS 655

29. University of Virginia CS 655
Java Solution (cont.)
class BusStop {
PersonList waiting;
void printWaitingPassengers () {
waiting.print (); } }
class PersonList {
Person people[];
void print () {
for (int i = 0; i < people.length; i++) people[i].print (); }
class Person {
String name;
void print () { System.stdout.println (name); } }
14 November 2018
University of Virginia CS 655

30. University of Virginia CS 655
Demeter Approach
Devise a traversal strategy
Specify code for different types of objects reached on a traversal
Example: code prints name if object is a Person
Independent of class graph
14 November 2018
University of Virginia CS 655

31. Slide adapted from Karl Lieberherr talk
Traversal Strategy
First try: from BusRoute to Person
busStops
BusRoute
BusRoute
BusStopList
BusStopList
buses
0..*
BusStop
BusStop
BusList
BusList
waiting
0..*
passengers
Bus
Bus
PersonList
PersonList
Person
Person
0..*
11/14/2018
AOP/Demeter

32. Slide adapted from Karl Lieberherr talk
Traversal Strategy
from BusRoute through BusStop to Person
busStops
BusRoute
BusStopList
buses
0..*
BusStop
BusList
waiting
0..*
passengers
Bus
PersonList
Person
0..*
11/14/2018
AOP/Demeter

33. Writing Adaptive Programs with Strategies
Slide adapted from Karl Lieberherr talk
Writing Adaptive Programs with Strategies
strategy: from BusRoute through BusStop to Person
BusRoute {
traversal waitingPersons(PersonVisitor) {
through BusStop to Person; } // from is implicit
void printWaitingPersons() // traversal/visitor weaving instr.
= waitingPersons(PrintPersonVisitor);
PrintPersonVisitor {
before Person () < do printing >}
can we really program with strategies? Here is how.
11/14/2018
AOP/Demeter

34. Robustness of Strategy
Slide adapted from Karl Lieberherr talk
Robustness of Strategy
from BusRoute bypassing Bus to Person
villages
BusRoute
BusStopList
buses
VillageList
busStops
0..*
0..*
BusStop
BusList
Village
waiting
0..*
passengers
Bus
PersonList
Person
0..*
11/14/2018
AOP/Demeter

35. Filter out noise in class diagram
Slide adapted from Karl Lieberherr talk
Filter out noise in class diagram
only three out of seven classes
are mentioned in traversal
strategy!
from BusRoute through BusStop to Person
explain why strategy better
replaces traversal methods for the classes
BusRoute VillageList Village BusStopList BusStop
PersonList Person
11/14/2018
AOP/Demeter

36. University of Virginia CS 655
Summary
Aspect-Oriented Programming and Adaptive Programming provide programmers with new expressive options
Active research area (Separation of Concerns Workshops at OOPSLA, ICSE, ECOOP, etc.)
Many directions to explore
Practical use still remains to be seen
Kiczales paper claims like OO in 1980 (by induction)
But some commercial success
Tendril Software (Adaptive programming tools for Java), bought by BEA Systems.
14 November 2018
University of Virginia CS 655

37. University of Virginia CS 655
Charge
Next time: John Thornley
Project Progress
If your implementation will not be complete by Monday morning, schedule a meeting with me now
14 November 2018
University of Virginia CS 655