AOP @ PHP The State of Aspect-Oriented Programming for PHP Sebastian Bergmann September, 14 th 2006

Warning It is (at least in my opinion) impossible to discuss „AOP for PHP“ without introducing AOP first. So, I will try to give a short introduction into AOP before I get into „AOP for PHP“. I hope I found a balance between theory and practical examples to not make this boring. There will be no Java / AspectJ examples in this presentation ;-)

It is (at least in my opinion) impossible to discuss „AOP for PHP“ without introducing AOP first.

So, I will try to give a short introduction into AOP before I get into „AOP for PHP“.

I hope I found a balance between theory and practical examples to not make this boring.

There will be no Java / AspectJ examples in this presentation ;-)

Who I Am Sebastian Bergmann. Born 1978. System Developer at eZ systems AS. Committer: PHP, PEAR, Gentoo Linux, ... Creator of Open Source PHP projects such as PHPUnit. Author on PHP and related topics.

Sebastian Bergmann.

Born 1978.

System Developer at eZ systems AS.

Committer: PHP, PEAR, Gentoo Linux, ...

Creator of Open Source PHP projects such as PHPUnit.

Author on PHP and related topics.

Who Are You? What is your background? OOP? PHP 5? AOP?

What is your background?

OOP?

PHP 5?

AOP?

Overview Goals of OOP Make code look like the design. Improve the modularity of code and design.

Goals of OOP

Make code look like the design.

Improve the modularity of code and design.

Overview Goals of AOP Make code look like the design. Improve the modularity of code and design. Deal with cross-cutting concerns. AOP for PHP How can AOP for PHP be implemented? What implementations of AOP for PHP exist? My own implementation of AOP for PHP, GAP: Generic Aspects for PHP.

Goals of AOP

Make code look like the design.

Improve the modularity of code and design.

Deal with cross-cutting concerns.

AOP for PHP

How can AOP for PHP be implemented?

What implementations of AOP for PHP exist?

My own implementation of AOP for PHP, GAP: Generic Aspects for PHP.

Something To Think About

Programming Languages Programming languages are about writing a structure of statements that a compilation or interpretation process will elaborate as a series of primitive directions. This slide uses material by Robert E. Filman . With the first programming languages, each statement was both unitary and local . Unitary in that it ended up having effect in precisely one place in the program. Local in that it was almost always proximate to its neighboring statements.

Programming languages are about writing a structure of statements that a compilation or interpretation process will elaborate as a series of primitive directions.

With the first programming languages, each statement was both unitary and local .

Unitary in that it ended up having effect in precisely one place in the program.

Local in that it was almost always proximate to its neighboring statements.

Programming Languages The history (of this part) of programming languages has been about moving away from purely local and unitary languages. Provide mechanisms that let the programmer separate concepts pragmatically, instead of being tied to saying things just where they happen. This slide uses material by Robert E. Filman .

The history (of this part) of programming languages has been about moving away from purely local and unitary languages.

Provide mechanisms that let the programmer separate concepts pragmatically, instead of being tied to saying things just where they happen.

Programming Languages The first exceptions to locality were subprograms. They enabled the abstracting out of some behavior to someplace else. Still unitary: direct correspondence between one statement written and one sequence of machine instructions executed. This slide uses material by Robert E. Filman .

The first exceptions to locality were subprograms.

They enabled the abstracting out of some behavior to someplace else.

Still unitary: direct correspondence between one statement written and one sequence of machine instructions executed.

Object-Oriented Programming 1961 Ole-Johan Dahl and Kristen Nygaard presented Simula , a syntactical superset of Algol 60 that added features such as objects, classes, subclasses, virtual methods, coroutines, garbage collection, and discrete event simulation. First object-oriented programming language and predecessor of all class-based object-oriented languages. Goal: Make the code look like the model. 1967 The term object-oriented programming is coined.

1961

Ole-Johan Dahl and Kristen Nygaard presented Simula , a syntactical superset of Algol 60 that added features such as objects, classes, subclasses, virtual methods, coroutines, garbage collection, and discrete event simulation.

First object-oriented programming language and predecessor of all class-based object-oriented languages.

Goal: Make the code look like the model.

1967

The term object-oriented programming is coined.

Object-Oriented Programming A way of thinking Objects and Classification Hierarchies that allows us to make code look like the design and improve the modularity of code and design and is supported by language features. classes, encapsulation, polymorphism This slide is based on material by Gregor Kiczales.

A way of thinking

Objects and Classification Hierarchies

that allows us to

make code look like the design

and improve the modularity of code and design

and is supported by language features.

classes, encapsulation, polymorphism

Good Design This slide uses material by Roger Whitney . Modules should exhibit a high degree of cohesion . The elements in the module exhibit a high degree of functional relatedness. This means that each element in the program unit should be essential for that unit to achieve its purpose. (Sommerville, 1989) be loosely coupled . The interaction or other interrelationship between any two modules at the same level of abstraction within the system should be as weak as possible. Goal Separation of Concerns Locality of Change -> Maintainability Loose Coupling -> Reusability

Modules should

exhibit a high degree of cohesion .

The elements in the module exhibit a high degree of functional relatedness. This means that each element in the program unit should be essential for that unit to achieve its purpose. (Sommerville, 1989)

be loosely coupled .

The interaction or other interrelationship between any two modules at the same level of abstraction within the system should be as weak as possible.

Goal

Separation of Concerns

Locality of Change -> Maintainability

Loose Coupling -> Reusability

Cross-Cutting Concerns Object-Oriented Programming suffers from a limitation called the tyranny of the dominant decomposition . The program can be modularized in only one way at a time, and the many kinds of concerns that do not align with that modularization end up scattered across many modules and tangled with one another. To „break this tyranny“ all kinds of concerns of importance need to be separated simultaneously, including crosscutting concerns.

Object-Oriented Programming suffers from a limitation called the tyranny of the dominant decomposition .

The program can be modularized in only one way at a time, and the many kinds of concerns that do not align with that modularization end up scattered across many modules and tangled with one another.

To „break this tyranny“ all kinds of concerns of importance need to be separated simultaneously, including crosscutting concerns.

Cross-Cutting Concerns Code that deals with XML parsing in Apache Tomcat Object-Oriented Decomposition works for this concern. As a result, the code for XML parsing is not scattered .

Cross-Cutting Concerns Code that deals with logging in Apache Tomcat Object-Oriented Decomposition does not work for this concern. As a result, the code for logging is scattered and tangled .

Aspect-Oriented Programming Late 1970s, early 1980s Cross-Cutting Structure Mechanisms Behavioral Reflection Meta-Object Protocols Subject-Oriented Programming 1997 The term aspect-oriented programming is coined. This slide is based on material by Gregor Kiczales.

Late 1970s, early 1980s

Cross-Cutting Structure

Mechanisms

Behavioral Reflection

Meta-Object Protocols

Subject-Oriented Programming

1997

The term aspect-oriented programming is coined.

Aspect-Oriented Programming This slide uses material by Günter Kniesel . New Modularization Concept. Quantification Ability to declare actions to be applied consistently to many places in a program. Obliviousness The modified program entities do not need to be aware of being subject to aspect activity and do not need to provide special hooks for enabling such activity. Functionality that would otherwise be scattered throughout a program can be encapsulated in one aspect.

New Modularization Concept.

Quantification

Ability to declare actions to be applied consistently to many places in a program.

Obliviousness

The modified program entities do not need to be aware of being subject to aspect activity and do not need to provide special hooks for enabling such activity.

Functionality that would otherwise be scattered throughout a program can be encapsulated in one aspect.

Aspect-Oriented Programming A way of thinking Aspects and Cross-Cutting Structure that allows us to make code look like the design and improve the modularity of code and design and is supported by language features. Aspects, Join Points, Pointcuts, Advice, Inter-Type Declarations This slide is based on material by Gregor Kiczales.

A way of thinking

Aspects and Cross-Cutting Structure

that allows us to

make code look like the design

and improve the modularity of code and design

and is supported by language features.

Aspects, Join Points, Pointcuts, Advice, Inter-Type Declarations

Aspect-Oriented Programming New Object Join Point Method Call Join Point

Aspect-Oriented Programming Join Point A well-defined point in the execution of a program, for instance a method call or attribute access. Pointcut A predicate that matches join points.

Join Point

A well-defined point in the execution of a program, for instance a method call or attribute access.

Pointcut

A predicate that matches join points.

Aspect-Oriented Programming Advice A piece of code that is executed at a join point matched by a pointcut. Inter-Type Declaration An introduction of new code into an existing class. Aspect A module that can consist of pointcuts, advice, and inter-type declarations.

Advice

A piece of code that is executed at a join point matched by a pointcut.

Inter-Type Declaration

An introduction of new code into an existing class.

Aspect

A module that can consist of pointcuts, advice, and inter-type declarations.

Implementing AOP for PHP Why? Good question. While attending a course on AOP at the University I started wondering how to implement AOP for with PHP. Where could it be useful? Enforce rules during development time. This is also the most popular use of AspectJ at the moment. Help with the development of tools such as PHPUnit. Mock Objects, for instance, are easy to implement if you have class posing. An aspect can implement that.

Why?

Good question.

While attending a course on AOP at the University I started wondering how to implement AOP for with PHP.

Where could it be useful?

Enforce rules during development time.

This is also the most popular use of AspectJ at the moment.

Help with the development of tools such as PHPUnit.

Mock Objects, for instance, are easy to implement if you have class posing. An aspect can implement that.

Implementing AOP for PHP Implement AOP not on the language level. Component / Container based frameworks Seasar2 Graph-Oriented Programming (GOP) Pointcuts on node transitions Workflow component of the eZ components

Implement AOP not on the language level.

Component / Container based frameworks

Seasar2

Graph-Oriented Programming (GOP)

Pointcuts on node transitions

Workflow component of the eZ components

Implementing AOP for PHP Preprocessor A preprocessor can be used to perform source code transformations and statically weave the aspect code into the base program. The result of this weaving is PHP source code that can then be deployed in a standard PHP environment. This slide uses material by William Candillon . PHPAspect

Preprocessor

A preprocessor can be used to perform source code transformations and statically weave the aspect code into the base program.

The result of this weaving is PHP source code that can then be deployed in a standard PHP environment.

PHPAspect

PHPAspect

Implementing AOP for PHP Aspect-Aware PHP Interpreter Zend Extension Modify the executor to intercept events such as method calls and attribute accesses, for instance, and delegate them to an aspect-aware dispatcher. This dispatcher can be either implemented as part of the extension or in userland. Patch for the Zend Engine Allows changes to scanner and parser rules. But hard to maintain.

Aspect-Aware PHP Interpreter

Zend Extension

Modify the executor to intercept events such as method calls and attribute accesses, for instance, and delegate them to an aspect-aware dispatcher.

This dispatcher can be either implemented as part of the extension or in userland.

Patch for the Zend Engine

Allows changes to scanner and parser rules.

But hard to maintain.

Implementing AOP for PHP Meta-Programming Reflection API Runkit __get() , __set() , __call()

Meta-Programming

Reflection API

Runkit

__get() , __set() , __call()

GAP: Generic Aspects for PHP G eneric A spects for P HP. Generic Join Point model for PHP. Framework to implement custom pointcut system. Aspect Generic ity. Advice = Plain PHP method. Inside this method, the same information is available that is used by GAP for advice dispatching. Sample pointcut system uses annotations in code comments to connect join points to aspect effects.

G eneric A spects for P HP.

Generic Join Point model for PHP.

Framework to implement custom pointcut system.

Aspect Generic ity.

Advice = Plain PHP method.

Inside this method, the same information is available that is used by GAP for advice dispatching.

Sample pointcut system uses annotations in code comments to connect join points to aspect effects.

GAP: Generic Aspects for PHP Join Points Object Initialization Field Access Method Call / Method Execution Aspect Effects Advice Intertype-Declarations Custom Errors

Join Points

Object Initialization

Field Access

Method Call / Method Execution

Aspect Effects

Advice

Intertype-Declarations

Custom Errors

GAP: Generic Aspects for PHP Sample Pointcut System initialization( class ( parameters )) get( modifier class -> attribute ) set( modifier class -> attribute ) method( modifier class -> method ( parameters )) source( modifier class -> method ( parameters )) cflow( modifier class -> method ( parameters ))

Sample Pointcut System

initialization( class ( parameters ))

get( modifier class -> attribute )

set( modifier class -> attribute )

method( modifier class -> method ( parameters ))

source( modifier class -> method ( parameters ))

cflow( modifier class -> method ( parameters ))

A GAP Aspect

PHP Base Code

Custom Errors

Implementation of GAP Class Sources Aspect Sources Interception of loaded PHP source Source-Level Transformation Normal Compilation

Implementation of GAP Bytecode-Level Transformation

License These slides are available under the Creative Commons Attribution 2.0 Germany license. You are free: to copy, distribute, display, and perform the work to make derivative works to make commercial use of the work Under the following conditions: Attribution: You must attribute the work in the manner specified by the author or licensor. For any reuse or distribution, you must make clear to others the license terms of this work. Any of these conditions can be waived if you get permission from the copyright holder. Your fair use and other rights are in no way affected by the above. „ The State of AOP for PHP“ Sebastian Bergmann http://www.sebastian-bergmann.de/talks/

These slides are available under the Creative Commons Attribution 2.0 Germany license.

You are free:

to copy, distribute, display, and perform the work

to make derivative works

to make commercial use of the work

Under the following conditions:

Attribution: You must attribute the work in the manner specified by the author or licensor.

For any reuse or distribution, you must make clear to others the license terms of this work.

Any of these conditions can be waived if you get permission from the copyright holder.

Your fair use and other rights are in no way affected by the above.