Transcript 面向对象技术 - 南京大学计算机科学与技术系

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Aspect Oriented Programming
面向方面的编程
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Nanjing University
2015/7/18
摘要
2
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Background and Motivation
AOP
AspectJ
Summary
Institute of Computer Software
Nanjing University
2015/7/18
摘要
3

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Background and Motivation
AOP
AspectJ
Summary
Institute of Computer Software
Nanjing University
2015/7/18
Background and Motivation
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Where OOP has brought us
 Reusability
of components
 Modularity
 Less
complex implementation
 Reduced cost of maintenance
 ……

Modularity is a universal advancement over
structured programming that leads to clearer and
better understood software
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Nanjing University
2015/7/18
Modularity in Reality
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
Code from org.apache.tomcat
Red shows relevant
lines of code
XML
Parsing
Good
Modularity
Nicely fits in one box
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Nanjing University
2015/7/18
Modularity in Reality
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fits in two boxes
URL
pattern
matching
Pretty Good
Modularity
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Nanjing University
2015/7/18
Modularity in Reality
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Code tangling and
scattering
Red shows lines of code that
handle logging
Logging
Bad
Modularity
Not in just one place
Not even in a small
number of places
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Nanjing University
2015/7/18
History of Programming Paradigms
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In the beginning: Programming in whichever way.
 Monolithic
programs
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2015/7/18
History of Programming Paradigms
9
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Structured Programming
 Functional
decomposition
Modularity
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2015/7/18
History of Programming Paradigms
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Object-Oriented Programming
 Encapsulation
& Inheritance
Modularity
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2015/7/18
Nice things of OOP
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Modular structure
Reuse
 Class
 Design
patterns
 Framework
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Nanjing University
2015/7/18
Limitations of OOP
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Some things cannot be modeled well in object hierarchies!!
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2015/7/18
A motivating example: Tracing
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class Point{
void set (int x, int y){
TraceSupport.traceEntry(“Point.set”);
this.x =x; this.y=y;
TraceSupport.traceExit(“Point.set”);
}
}
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2015/7/18
Java API Example
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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);
Same code repeated
}
16 times in java.io.File
return exits0();
}
public boolean canRead(){
SecurityManager security = System.getSecurityManager();
if(security!=null){
security.checkRead(path);
}
return canRead0();
}
Institute of Computer Software
Nanjing University
2015/7/18
Crossing cutting concerns
15
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Nanjing University
2015/7/18
What is a concern?
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

A particular goal, concept, or area of interest
(requirement)
A software system contains:
 Business
(application) logic concerns
 System-level concerns
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2015/7/18
Crosscutting Concerns
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Crosscutting is how to characterize a concern than
spans multiple units of OO modularity
Crosscutting concerns resist modularization using
normal OO construct
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Separation of Concerns (SoC)
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Object-Oriented Programming separates and
encapsulates some concerns
 Others
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end up tangled and scattered
Basic problem
N
dimensions of concerns
 1 dimension of implementation structure
 Tyranny decomposition
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Nanjing University
2015/7/18
Approaches to SoC
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Composition Filters
Multi-dimensional Separation of Concerns
Adaptive Programming
Aspect-Oriented Programming
 Was
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developed at Xerox PARC （施乐公司 帕洛阿尔托研究中心）
(Related) Meta-Object Protocol (MOP)
 Reflective
programming, meta-object protocol
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Nanjing University
2015/7/18
摘要
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
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Background and Motivation
AOP
AspectJ
Summary
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Nanjing University
2015/7/18
The AOP Idea
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
Many cross-cuts aren’t random!
 They
serve specific purposes
 They have well-defined structure

So…let’s capture the cross-cutting structure in a
modular way to better support for “Separation of
Concerns”
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2015/7/18
Two central problems AOP tries to solve
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
Code tangling
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 One
module, many
concerns
Code scattering

One concern,
many modules
Example:
logging
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2015/7/18
Two central problems AOP tries to solve
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2015/7/18
AOP approach to SoC
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Prism Analogy
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Implement each
concern separately
Crosscutting concerns
Weave concerns
together
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2015/7/18
Basic Mechanisms of AOP
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Aspect
Advice
Pointcut
Weaving
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Summary so far
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
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AOP is a software development technique that
complements and extends OOP
Provides new and powerful ways to modularize
“crosscutting concerns”
 Crosscutting
concerns: Behavior that cuts across class
boundaries
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AspectJ is the leading implementation of AOP that
extends Java’s OOP model
Institute of Computer Software
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2015/7/18
摘要
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
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Background and Motivation
AOP
AspectJ
Summary
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Nanjing University
2015/7/18
What is AspectJ?
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A simple and practical aspect-oriented extension to Java
A general purpose AO language
Based on over ten years of research at Xerox PARC
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Transferred to Eclipse.org in 2002
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http://www.parc.com/research/projects/aspectj/default.html
Launched in 1998
http://www.eclipse.org/aspectj/
Latest version: AspectJ 1.6.10
2015/7/18
Design assumptions
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Real Community Users
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Writing aspects
Reading aspects
Idioms
How effective for concerns
Modular, reusable and easy to develop and maintain
Java compatibility
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Upward compatibility
Platform compatibility
Tool compatibility
Programmer Compatibility
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AspectJ Compilation Process
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Application .java or jar files
modules
weaving
Application
System
ajc: javac extension
Aspects
.aj files
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2015/7/18
Dynamic VS Static crosscutting
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Dynamic crosscutting
 define
additional behavior to run at certain welldefined points in the execution of the program

Static crosscutting
 modify
the static structure of a program (e.g., adding
new methods, implementing new interfaces, modifying
the class hierarchy)
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Concepts in AspectJ
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Join Points 连接点
Pointcut 切入点
Advice 通知
Aspect 方面
Introduce 引入
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High level View of AspectJ
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AspectJ
Advice
advice body
pointcut
join point
Java Program
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Join Points Model
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Join point is a well-defined point in a program’s
execution
Method call:
Public void move(int dx, int dy){
setX(_x+dx);
setY(_y+dy);
}
Method call
join point
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More Join Points
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public void setX(int x){
_x = x;
}
Method execution
join point
Field set join point
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All Join Points
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method & constructor execution
method & constructor call
field get & set
exception handler execution
static & dynamic initialization
dynamic joinpoints (cflow, cflowbelow)
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2015/7/18
Pointcuts
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
Pointcut:
 Predicate
on join points: selecting a collection of
joinpoints
 Example: call (void Point.setX(int))

Wildcard characters are allowed. The following
pointcut matches any method call to the methods of
the class Point whose names begin with “set”
 call
(void myPackage..*Point.set*());
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Named Pointcuts
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Can be a Named set of join points
Capture all the executions of the
 <void
Point.setX(int)> or <void Point.setY(int)> method
Name and
parameters
Executions of methods with the
specified sig.
pointcut move():
execution (void Point.setX(int)) ||
or
execution (void Point.setY(int));
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2015/7/18
More on Pointcuts
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Basic pointcuts and pointcuts composition
Pointcuts can be composed as boolean expressions
with “&&”, “||” and “!”
Pointcuts can have arguments
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Pointcut Designators
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call, execution (method or constructor)
get, set (field)
within (type), withincode(method or constructor)
this, target (type or id)
args(type or id list)
cflow, cflowbelow(pointcut)
handler, throwing (exception type)
combine with ||, && and !
use *, + and .. wildcards
bind arguments (autoboxing!), this, target
Advice
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Code that runs before, after, or around (instead of)
a join point
Type of advice
Pointcut it applies to
after() returning(): move() {
//code here runs after each move
}
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2015/7/18
Advice Types in AspectJ
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Before advice: runs at the moment join point is
reached, before method runs
After advice: runs at the moment control returns
through join point, just after method
 After,

after returning, after throwing
Around advice: runs when join point is reached and
has control over whether method itself runs at all
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2015/7/18
Aspects
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
Example:
Aspect HistoryUpdating{
pointcut move():
execution(voidPoint.setX(int)) ||
execution(voidPoint.setY(int));
after() returning: move() {
//code here runs after each move
}
}
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2015/7/18
Aspects
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Mix everything we’ve seen up to now and put it one
or more modular units called Aspects.
Looks a lot like a class!
Can contain pointcuts, advice declarations, methods,
variables ….
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2015/7/18
How it works
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
Short answer: bytecode modification
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A first example
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What it does
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Sample Code
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AspectJ’s Introductions
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An introduction is an aspect member that allows to
 Add
methods to an existing class
 Add field to an existing class
 Extend an existing class with another
 Implement an interface in an existing class
 Convert checked exceptions into unchecked exceptions
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2015/7/18
Introduction examples
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public int foo.bar(int x);
private int foo.counter;
declare parents: mammal extends animal;
declare parents: MyThread implements
MyThreadInterface;
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2015/7/18
Some Advanced Topics
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
Aspect Extension (Inheritance)
 example
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Some Advanced Topics
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
Aspect Creation (Association)
 Aspect
instance
 Single
(static) vs. Multiple (dynamic)
 Default:
Singleton, created when program begins
 Object (Class) association:
 perthis,
pertarget
 Controlflow
 Percflow,

association
percflowbelow
Aspect & Advice Precedence & Interactions
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Performance impact
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
See “Advice Weaving in AspectJ”
 “The
implementation of advice weaving introduces very
little performance overhead when compared to the
same functionality coded by hand.”
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Does make it easy to create performance
destroying code…
But allows powerful optimizations
Institute of Computer Software
Nanjing University
2015/7/18
AspectJ Terminology
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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
a compile time declaration introduces a compile time warning or error
upon detection of certain usage patterns
an aspect is a module for handling crosscutting concerns
 Aspects are defined in terms of pointcuts, advice, and introduction
 Aspects are reusable and inheritable
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Nanjing University
2015/7/18
Dynamic AOP Systems
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Creating aspects at run-time
Specifying pointcuts at run-time
Dynamic code translation (or some special feature
of the run-time environment)
Enabling/disabling aspects at run-time
Pointcuts are dynamic: it can be decided only at
run-time whether advices have to be executed or
not (based on run-time values, call-stack, etc.)
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2015/7/18
Other Java AOP sightings
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AspectWerkz: load-time/run-time weaving, AOP
constructs defined using javadoctags
BEA: AOP framework for Weblogic
JBoss AOP framework
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Spring framework
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AOP with interceptors
Limited AOP with proxies
J2EE without EJB
AspectJ2EE
JasCo
…
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2015/7/18
Other Aspect-Oriented Languages
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AspectC
AspectC++
Aspect.Net
Lightweight Python AOP
AspectL(Lisp)
AspectML
AspectPHP
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2015/7/18
Middleware & System Level Applications
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Security
Transaction
Persistence
Mobility
Realtime& Embedded Systems
OS
……
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Nanjing University
2015/7/18
AOSD
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AOSD is a promising approach to encapsulate and
modularize crosscutting concerns
AOSD is not a substitute of the OO paradigm but
an extension to it.
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Nanjing University
2015/7/18
AOSD
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2015/7/18
A fast development area
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
AOP
 Aspect-Oriented

AOSD
 Aspect-Oriented

Software Design (Development)
AORE (AOR)
 Aspect-Oriented

Programming
Requirements Engineering
AOM
 Aspect
Oriented Modeling
Institute of Computer Software
Nanjing University
2015/7/18
Where to Get More Information
62

The AspectJ website
 http://eclipse.org/aspectj

AOSD website
 http://www.aosd.net

Books
 Mastering
AspectJ
 AspectJ in Action

Google 
Institute of Computer Software
Nanjing University
2015/7/18
作业（本次作业不用提交）
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
运行demo程序，研究AspectJ的语法、编译过程
 注意：先安装Eclipse

AJDT插件
思考为什么说AOP不会取代OOP，而是对OOP
的补充？
Institute of Computer Software
Nanjing University
2015/7/18