aspect orientated programming © university of liverpoolcomp 319slide 1

ASPECT ORIENTATEDPROGRAMMING

© University of LiverpoolCOMP 319 slide 1

The problem• Code scattering

class BankAccount { public void transfer(int amount,BankAccount

toAccount) {try { Log.transferringMoney(); if (authenticated(username,password) {

// do transfer } } catch (TransactionException texc) { Log.storeExc(texc);}

© University of LiverpoolCOMP319 slide 2

Concerns and Cross cut concerns• Concern examples, banking- Transfer between accounts- Product statement-Make billing payment

• Cross cut concerns- Security/authentication- Logging- Transaction handling


Cross cut concern problems

• Scattering• Duplication- Code cloning

• Tangling- Concerns not properly separated


Cross cutting concern examples and approaches• Persistence-Object-relational mapping

• Internationalization- Resource bundling

• Code mobility- Pre-processing

• Error handling- Chained exceptions, error return codes


Definitions• Cross-cutting – Identify areas of code

where common functionality exists

• Advice – The code to be injected

• Joinpoint – Where one or more aspects can be applied

• Pointcut – A collection of joinpoints

• Aspect – General term for where advice and point-cuts are combined


Terminology

• Weaving – Integrating applications and aspects (e.g. AspectJ is an “aspect weaver”)- Compile-time – Can produce integrated source-code, but typically only produces woven byte-code.

- Run-time – Aspects are applied “on the fly” (typically when classes are loaded)


AOP Platforms

• AspectJ• Spring• JBoss AOP• AspectWerkz-Now working with AspectJ

• Java Aspect Components (JAC)


AspectJ

• Now an Eclipse project (http://eclipse.org/aspectj/)- Developed at Xerox PARC- Released 2001

• Latest version (AspectJ 5) is a collaboration of AspectJ and AspectWerkz

• AJDT - AspectJ Development Tools plug-in for Eclipse


http://eclipse.org/aspectj/

Aspect definition


package com.aspect;

public aspect DemoAspect {

pointcut setterNotification() : call(* *.set*(..));

before() : setterNotification(){System.out.println("setting data...");

}

}

Point-cut

Advice

Aspect

Aspect example (Hello World)public class Main {

public static void main(String[] args) {// TODO Auto-generated method

stubhello();

}public static void hello() {

System.out.println("Hello");}

}


Aspect Example HelloWorld

public aspect Hello { pointcut helloCall() : execution( * Main.hello(..));

after() : helloCall() { System.out.println("World!!!"); }

}


Point cut definitions


//call is the most common joinpoint typecall([access modifier] returnType package.ClassName.method(args));

//Examplescall(* *.*(..)); //Note: “..” is also a wildcardcall(public * *.set*(..));call(void *.set*(..));call(* *.set*(int));call(String com.foo.Customer.set*(..));call(* com.foo.Customer+.set*(..)); //”+” cross-cuts childrencall(public void com..*.set*(int));call(* *.set*(int, ..));call(* *.set*(int, .., String));

Defining a pointcut

• pointcut nameOfPointCut() : definition;

• pointcut setters() :call(public * *.set*(..));


Types of Join point

• call(method expr)-when a method is called

• execution(method expr)-when a method is executed

• handler(exception expr)-when a catch block is executed


Join points

public static void main(String argsv[]) { hello(); call}Public static void hello() { execution try { } catch (Exception e) { handler }}


Types of Join points• get(field expr)-when a field is read

• set(field expr)-when a field is set- pointcut settingLength() : set( int SetterMonitorExample.length);

• staticinitialization(class expr)-when a static block of a class is executed


Types of Join points

• initialization(constructor expr)-when a constructor is executed

• preinitialization(constructor expr)-when inherited constructors are executed

• adviceexecution()-when an advice block is executed


Adding Advice


pointcut everyCall() :

call(* *.*(..));

before() : everyCall(){

System.out.println(“Advice called before method");

}

after() : somePointCut(){

System.out.println("Advice called after each method");

}

Anonymous point cuts in Advice

before() : call(* *.*(..)) {

System.out.println("Injecting advice...");

}

// point cut doesn’t have name


Types of Advice• before()-Before call or body

• after() returning• after() throwing• after()• around()


Types of Advice

• after() returning [(type expr)]- called after normal execution of the joinpoint

• after() throwing [(exception expr)]- called if the execution of the joinpoint throws the specified exception

• after() - executes regardless (sort of like finally)


Types of Advice

• type around() [throws exception]-executes in place of a joint point of execution-You can stop or replace code execution-To call the original code, use proceed()


Types of Advice

• withincode(method expr)- true if joinpoint is within defined method

• within(type expr)- true if joinpoint is within defined type (i.e. class definition)


Exception handling in Advice

• after() throwing- Intercepts at method level, before the

catch() block• handler(exception type)- Intercepts at the catch() block

• declare soft : exception type- Wraps any exception as a

RuntimeException (which consequently no longer needs to be declared in the throws clause of the method)


Exception handling advice

before(java.io.FileNotFoundException exc) : handler(java.io.FileNotFoundException ) && args(exc) {System.out.println("Could not find file... message is "+exc.getMessage());}


Point cut operators

• if(expr)- evaluate boolean expression

• withincode(method expr)- true if joinpoint is within defined method

• within(type expr)- true if joinpoint is within defined type


Point cut operators

• this(type expr)- true when joinpoint matches source object

• target(type expr)- true when joinpoint matches target object

• args(type expr)- true for any joinpoint matching argument type list


Getting at the context

• Sometimes we want to get at- The instance of this, which is being used at the point of call- The target instance- Example fred.getX(); // fred is the

target- The arguments (x and y)- Example fred.setPosition(x,y)


Getting the context• Declaring the context, type- before(BankAccount account) :

• Declares the context type in the point cut, for example..

before(BankAccount account) : target(account) && authenticationNeeded() && (!execution(* BankAccount.authenticate(..))) {

account.authenticate(); }

-Question … Why do we have !execute(* BankAccount(authenticate())


Arguments example

pointcut settingLength(int length) : set( int SetterMonitorExample.length) && args(length);

void around(int inputLength) : settingLength1(inputLength) {

System.out.println("Trying to set length to "+inputLength);

if (inputLength<5) { proceed(inputLength); } else { System.out.println("Skipping length setting

length too large"); } }


Point cut logical operators

• &&- Logical AND

• ||- Logical OR

• !- Logical NOT


thisJoinPointthisJoinPoint.toString() thisJoinPoint.getKind() // call, execution, set, getthisJoinPoint.getSignature();

System.out.println("thisJoinPoint.getSourceLocation(): " + thisJoinPoint.getSourceLocation(); // source code line numberthisJoinPoint.getTarget(); // target objectthisJoinPoint.getThis(); // source object


thisJointPoint example

package testaspect;

public class AspectReflectionExample {public void greet() { Greeter greeter=new Greeter("Seb"); greeter.greet("Hello ");}public static void main(String argvs[]) { AspectReflectionExample instance=new AspectReflectionExample();instance.greet();

}

}


thisJointPoint example


package testaspect;

public class Greeter {private String name="";public void goodMorning() { System.out.println("Good morning "+name);}public void greet(String message) { System.out.println(message+name);}public Greeter(String name) { this.name=name;}}

thisJoinPoint example


package testaspect;

public aspect ReflectAspect {pointcut greetings(Greeter example) : call(* *.Greeter.greet(..)) && target(example);

before(Greeter example) : greetings(example) {System.out.println("this: " + this);System.out.println("thisJoinPoint.toString(): " + thisJoinPoint.toString() );System.out.println("thisJoinPoint.getKind(): " + thisJoinPoint.getKind() );System.out.println("thisJoinPoint.toLongString(): " + thisJoinPoint.toLongString() );System.out.println("thisJoinPoint.toShortString(): " + thisJoinPoint.toShortString() );System.out.println("thisJoinPoint.getClass(): " + thisJoinPoint.getClass() );System.out.println("thisJoinPoint.getSignature(): " + thisJoinPoint.getSignature() );System.out.println("thisJoinPoint.getSourceLocation(): " +thisJoinPoint.getSourceLocation() );System.out.println("thisJoinPoint.getTarget(): " + thisJoinPoint.getTarget() );System.out.println("thisJoinPoint.getThis(): " + thisJoinPoint.getThis() );

}

}

thisJoinpointthis: [email protected](): call(void testaspect.Greeter.greet(String))thisJoinPoint.getKind(): method-callthisJoinPoint.toLongString(): call(public void testaspect.Greeter.greet(java.lang.String))thisJoinPoint.toShortString(): call(Greeter.greet(..))thisJoinPoint.getClass(): class org.aspectj.runtime.reflect.JoinPointImplthisJoinPoint.getSignature(): void testaspect.Greeter.greet(String)thisJoinPoint.getSourceLocation(): AspectReflectionExample.java:6thisJoinPoint.getTarget(): [email protected](): testaspect.AspectReflectionExample@2afa3ac1


Modifying target classes• Inter type declarations- Aspect can add methods, field and constructors to external class


aspect SecurityChecksboolean BankAccount.authenticated=false;public void BankAccount.authenticate() {

if (authenticated) return; // TO DO more authentication code } public BankAccount.new(String password,String name, String pin) { }}

Modifying target classes

• You can also use inter-type declarations to change the inheritance, to add new interface or new super class

• New parent must be sub-class of existing parent


public aspect ChangeStructure {

declare parents: BankAccount implements Serializable; declare parents: BankAccount extends SecureObject;

}

Intertype declarations, compile control• declare parent• declare warning- Compiler warnings result from joinpoints

• declare error- Compiler errors result from joinpoints

• declare precedence-Define aspect precedence


Encouraging logging, using warnings


• Simple example- From Matt Chapman, AJDT Committer

public aspect EnforceLogging{

pointcut scope(): within(*);

pointcut printing():

get(* System.out) ||

get(* System.err) ||

call(* Throwable+.printStackTrace());

declare warning : scope() && printing(): "don't print, use the logger";

}

Class watcher example• Monitors all calls to setters• If data changes- Dirty flag=true

• Application can use this to- Save data only if dirty flag=true, saves

unrequired writes- Data can be discarded from cache if

dirty flag=false- Supports lazy writing caches- Help data integraty testing


Watcher Examplepublic aspect WatchSetters { // just to invoke test code below public static void main(String[] args) { Client.handleTimer(new Timer()); } private static final Class[] GETTER_ARG_TYPES = new Class[]{}; private static final Object[] GETTER_ARGS = new Object[]{}; private static final Object NONE = new Object(); public interface IWatched {} /** true if new value sent to any setter */ private boolean IWatched.dirty; /** false if unable to maintain dirty b/c no privileges, no getter... */ private boolean IWatched.dirtyValid = true; /** clients can use dirty flag */ public boolean IWatched.isDirty() { return dirty; } /** clients handle case when dirty flag is invalid */ public boolean IWatched.isDirtyValid() { return dirtyValid; }


Watcher example (Pointcut)

• public pointcut setters(IWatched watched) : target(watched) && execution(void IWatched+.set*(*)); // advice uses args[0]


Watch timer advice


void around(IWatched watched) : setters(watched) && if (watched.dirtyValid) { // get value by invoking getter method Object value = NONE; // no valueString getterName=“”;try { getterName = "g" + thisJoinPoint.getSignature().getName().substring(1); Method method = watched.getClass() .getMethod(getterName, GETTER_ARG_TYPES); value = method.invoke(watched, GETTER_ARGS); } catch (Throwable t) { } ; if (NONE == value) { watched.dirtyValid = false; proceed(watched); return; } // compare value with arg being set - pointcut has exactly 1 parm Object arg = thisJoinPoint.getArgs()[0]; if (!(null == arg ? value == null : arg.equals(value))) { proceed(watched); watched.dirty = true; } }

Watcher Example Persistence


if (!(null == arg ? value == null : arg.equals(value))) { proceed(watched); watched.dirty = true; String tableName=thisJoinPoint.getTarget().getClass().getName(); tableName=tableName.replace(".", "_"); String sql="update "+tableName+" set "+getterName.substring(3)+"='"+arg.toString()+"' where id='"+watched.id+"'"; System.out.println("sql : "+sql); }

aspect orientated programming © university of liverpoolcomp 319slide 1

Documents