面向方面的编程 Aspect Oriented Programming 2015-10-4 Institute of Computer Software Nanjing University 1

面向方面的编程

Aspect Oriented Programming

23/4/21Institute of Computer Software

Nanjing University

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摘要 Background and Motivation AOP AspectJ Summary


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Background and Motivation

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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Modularity in Reality

Code from org.apache.tomcat


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XML

Parsing

Red shows relevant lines of code

Nicely fits in one box

Good Modularity



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URL pattern matching

Pretty Good Modularity

fits in two boxes



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Logging

Red shows lines of code that handle logging

Not in just one place Not even in a small number of places

Bad Modularity

Code tangling and scattering

History of Programming Paradigms

In the beginning: Programming in whichever way. Monolithic programs


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Structured Programming Functional decomposition


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Modularity


Object-Oriented Programming Encapsulation & Inheritance


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Modularity

Nice things of OOP

Modular structure Reuse

Class Design patterns Framework


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Limitations of OOP


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Some things cannot be modeled well in object hierarchies!!

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”); }}

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); } return exits0(); } public boolean canRead(){ SecurityManager security = System.getSecurityManager(); if(security!=null){ security.checkRead(path); } return canRead0(); }

Same code repeated 16 times in java.io.File

Crossing cutting concerns


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What is a concern?

A particular goal, concept, or area of interest (requirement)

A software system contains: Business (application) logic concerns System-level concerns


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Crosscutting Concerns

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)

Object-Oriented Programming separates and encapsulates some concerns Others end up tangled and scattered

Basic problem N dimensions of concerns 1 dimension of implementation structure Tyranny decomposition


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Approaches to SoC

Composition Filters Multi-dimensional Separation of Concern

s Adaptive Programming Aspect-Oriented Programming

Was 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

protocol23/4/21

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The AOP Idea

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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Two central problems AOP tries to solve

Code tangling One module,

many concerns


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Code scattering One concern,

many modules

Example:

logging

Two central problems AOP tries to solve


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AOP approach to SoC


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Prism Analogy


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Crosscutting concerns

Implement each concern separately

Weave concerns together

Basic Mechanisms of AOP


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Aspect

Pointcut

Advice

Weaving

Summary so far

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 AspectJ is the leading implementation of

AOP that extends Java’s OOP model


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What is AspectJ?

A simple and practical aspect-oriented extension to Java

A general purpose AO language Based on over ten years of research at Xerox

PARC http://www.parc.com/research/projects/aspectj/defa

ult.html Launched in 1998

Transferred to Eclipse.org in 2002 http://www.eclipse.org/aspectj/

Latest version: AspectJ 1.6.1023/4/21

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Design assumptions

Real Community Users Writing aspects Reading aspects Idioms How effective for concerns Modular, reusable and easy to develop and

maintain Java compatibility

Upward compatibility Platform compatibility Tool compatibility Programmer Compatibility


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AspectJ Compilation Process


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weavingApplication

System

Applicationmodules

.java or jar files

Aspects.aj files

ajc: javac extension

Dynamic VS Static crosscutting

Dynamic crosscutting define additional behavior to run at certain

well-defined 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

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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Java Program

AspectJ

Advice

pointcut advice body

join point

Join Points Model

Join point is a well-defined point in a program’s execution

Method call:


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Public void move(int dx, int dy){

setX(_x+dx);

setY(_y+dy);

}

Method call join point

More Join Points

public void setX(int x){ _x = x;}


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Method execution join point

Field set join point

All Join Points

method & constructor execution method & constructor call field get & set exception handler execution static & dynamic initialization dynamic joinpoints (cflow, cflowbelow)


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Pointcuts

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

Can be a Named set of join points Capture all the executions of the

<void Point.setX(int)> or <void Point.setY(int)> method


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pointcut move():

execution (void Point.setX(int)) ||

execution (void Point.setY(int));

Name and parameters

Executions of methods with the specified sig.

or

More on Pointcuts

Basic pointcuts and pointcuts composition

Pointcuts can be composed as boolean expressions with “&&”, “||” and “!”

Pointcuts can have arguments


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Pointcut Designators

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

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Advice

Code that runs before, after, or around (instead of) a join point


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after() returning(): move() {

//code here runs after each move

}

Type of advice Pointcut it applies to

Advice Types in AspectJ

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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Aspects

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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Example:

Aspects

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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How it works

Short answer: bytecode modification


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A first example


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What it does


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Sample Code

AspectJ’s Introductions

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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Introduction examples

public int foo.bar(int x); private int foo.counter; declare parents: mammal extends

animal; declare parents: MyThread implements

MyThreadInterface;


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Some Advanced Topics

Aspect Extension (Inheritance) example


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Some Advanced Topics

Aspect Creation (Association) Aspect instance

Single (static) vs. Multiple (dynamic) Default: Singleton, created when program

begins Object (Class) association:

perthis, pertarget Controlflow association

Percflow, percflowbelow Aspect & Advice Precedence & Interactions


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Performance impact

See “Advice Weaving in AspectJ” “The implementation of advice weaving

introduces very little performance overhead when compared to the same functionality coded by hand.”

Does make it easy to create performance destroying code…

But allows powerful optimizations


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AspectJ Terminology

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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Dynamic AOP Systems

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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Other Java AOP sightings

AspectWerkz: load-time/run-time weaving, AOP constructs defined using javadoctags

BEA: AOP framework for Weblogic JBoss AOP framework

AOP with interceptors Spring framework

Limited AOP with proxies J2EE without EJB

AspectJ2EE JasCo …


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Other Aspect-Oriented Languages

AspectC AspectC++ Aspect.Net Lightweight Python AOP AspectL(Lisp) AspectML AspectPHP


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Middleware & System Level Applications

Security Transaction Persistence Mobility Realtime& Embedded Systems OS ……


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AOSD

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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AOSD


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A fast development area

AOP Aspect-Oriented Programming

AOSD Aspect-Oriented Software Design

(Development) AORE (AOR)

Aspect-Oriented Requirements Engineering AOM

Aspect Oriented Modeling


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Where to Get More Information

The AspectJ website http://eclipse.org/aspectj

AOSD website http://www.aosd.net

Books Mastering AspectJ AspectJ in Action

Google


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作业（本次作业不用提交）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？


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Documents

面向方面的编程 Aspect Oriented Programming 2015-10-4 Institute of Computer Software Nanjing University 1