Introduction to Aspect Oriented Programming

Introduction to Aspect
Oriented Programming
By: Amir Kost

Agenda
•What is Aspect Oriented Programming?
•Usage
•A short dive to AspectJ
•Demos

What is Aspect Oriented
Programming (AOP) ?
From Wikipedia: ”A programming paradigm
that aims to increase modularity by allowing
the separation of cross-cutting concerns. It
does so by adding additional behavior to
existing code (an advice) without modifying the
code itself”

Usages of AOP
AOP can be used for a multitude of purposes,
but the most common are:
• Security.
• Transactions.
• Logging.

How does AOP work?
AOP works by intercepting methods that
comply to certain parameters (pointcuts), and
executing a callback wrapper (advice).

AOP – Basic Terminology
• Joinpoint - a point of execution.
• Pointcut – a set of joinpoints.
• Advice - Advice brings together a pointcut (to
pick out joinpoints) and a body of code.

AOP – Types of Advice
There are five major types of advice:
• Before
• AfterReturn
• AfterRunning
• AfterThrowing
• Around

AOP Libraries - AspectJ
•Allows interception of all classes and
methods, including 3rd party jars.
•Requires weaving during compile time or load
time.

AOP Libraries – Spring AOP
•Has Spring capabilities (IoC).
•Allows interception of Spring beans only.
•Is proxy based.
•Has limited capabilities – no private methods.
•Intercepts only methods called from one class
to another.

Perfomance Impact - AspectJ
•Since AspectJ modifies the bytecode itself
through weaving, there is negligible overhead
on performance.

Perfomance Impact – Spring AOP
•Adds a constant delay of a few microseconds
for each pointcut.
•Perfomance might be affected if:
• Advices are lengthy.
• There are several aspects for a single
pointcut.
•See the following blog post.

Example 1 – Before Aspect
@Before("call(* org.aspectprogrammer.*.*(..))")
public void callFromFoo(JoinPoint joinPoint) {
System.out.println("Call from : " + joinPoint.getSignature
().getName());
}

Example 1 – Pointcut Analysis
@Before("call(* org.aspectprogrammer.*.*(..))")
Return
Type
Package
Advice
Type
Class
Name
Method
Name
Method
Params
AspectJ
Keyword

Example 2 – Around Aspect
@Around("call(* org.aspectprogrammer..*(..))")
public Object performance(ProceedingJoinPoint thisJoinPoint)
{
// do some business logic
Object res = thisJoinPoint.proceed();
// do some more business logic
return res;
}

A Short Dive into AspectJ
AsepctJ comes in 2 flavors:
1. Annotations (like the ones mentioned
above).
2. An extentsion of Java, with additional
keywords like pointcut and aspect.

AspectJ – Under the hood
•AsepctJ uses a process called weaving to
identify joinpoints and apply the advices.
•It analyzes and manipulates the bytecode by
using ASM and BCEL libraries.

AspectJ Weaving Types
•Load Time Weaving.
•Compile Time Weaving.
•Post Compilation Weaving.

Using AspectJ
AspectJ can be used in the following ways:
1. Command line launcher.
2. Maven plugin.
3. Via javaagent.

Using AspectJ – Command Line
1. ajc for compile or post compile weaving.
Examples:
•ajc HelloWorld.java Trace.java
•ajc -inpath app.jar -aspectpath
tracelib.jar -outjar
tracedapp.jar

Using AspectJ – Command Line –
Load Time Weaving
2. aj for load time weaving:
•Compile your java code.
•Compile your aspects using ajc.
•Run your application with aj:
aj com.mycompany.Myclass

Using AspectJ – Load Time
Weaving - Using javaagent
Compile your classes and aspects, and add
the following option to the JVM:
javaagent:<path_to>/aspectjweaver.jar

Using AspectJ – Load Time Weaving –
configuration
In order to configure to which classes and aspects to
weave, write an aop.xml file:
<aspectj>
<aspects>
<aspect name="com.aspects.MyAspect"/>
</aspects>
<weaver options="-showWeaveInfo">
<include within="com.app.MyController"/>
<include within="org.3rdparty.beans..*"/>
</weaver>
</aspectj>

Using AspectJ - Maven
• Import AOP libraries to your pom file:
<dependency>
<groupId>org.aspectj</groupId>
<artifactId>aspectjrt</artifactId>
<version>1.8.7</version>
</dependency>
<dependency>
<groupId>org.codehaus.mojo</groupId>
<artifactId>aspectj-maven-plugin</artifactId>
<version>1.8</version>
</dependency>

Using AspectJ – Maven – AspectJ Plugin
Add AspectJ weaving plugin to your pom:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>aspectj-maven-plugin</artifactId>
<version>1.8</version>
<executions>
<execution>
<goals>
<goal>compile</goal>
</goals>
</execution>
</executions>
</plugin>

Using AspectJ – Maven – AspectJ Plugin –
Cont’d
Note: This plugin is used both for compiling aspects
and weaving aspects in CTW.

Defining Pointcuts using AspectJ
AspectJ defines a broad set of keywords for
defining pointcuts. Among them:
• call, execute – when calling/executing a
method.
• get/set – when accessing a field in a class.
• this – the currently executing joinpoint is
instance of this.

AspectJ – Inter Type Declaration
Besides running advices, AspectJ can do the
following:
1. Inject a field to a class.
2. Inject a method to a class.
3. Inject a constructor to a class.
4. Inject an interface to a class.

Other AOP Libraries – Guice AOP
• AOP library for Guice framework.
• Does not use ApsectJ annotations.
• Works with Matchers.

Other AOP Libraries – Javassist
• Bytecode injection library.
• Injects code as strings.

Why did we need AOP in eBay?
• In one of our RESTful web services, we
needed to add a certain request param to
each error message in the log.
• The classes writing the error messages are
not aware of that param.
• We did not want to change all calls to logger
api in our modules, or be concerned with
future calls.

Demo 1 – Spring Demo
https://github.com/amirko/spring-app-aop-
example

Demo 2 – AspectJ LTW
https://github.com/amirko/aop-webapp-ltw.git

Demo 3 – AspectJ CTW
https://github.com/amirko/aop-webapp-ctw.git

Aspects Library for Demos 2 + 3
https://github.com/amirko/aspects.git

Questions?
https://www.linkedin.com/pub/amir-kost/0/450/11
https://github.com/amirko

Introduction to Aspect Oriented Programming

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