Magic development with Aspects Initiation to Aspect Oriented Programming App Dev Con 2017 - Amsterdam 

About... Xavier F. Gouchet Android Architect Jenkins/Sonar Admin Bash / Python tools dev Bad Puns Advocate @xgouchet on Github, StackOverflow, Twitter, … 

Goal for the afternoon What’s AOP The theory, and motivation How does it work A bit of technical details on byte code and compilation Concrete examples The part where you do the work and I relax… or not. @xgouchet 

“ “Let's start at the very beginning (A very good place to start)” Julie Andrews @xgouchet 

It’s NOT ... ◎ A new programming language ◎ Something to replace OOP ◎ The solution to all your problems ! @xgouchet 

It is a tool to ... ◎ Enhance modularity ◎ Separate cross cutting concerns ◎ Focus on business logic ◎ Reduce noise in the source code @xgouchet 

Cross-cutting concerns ? Any concern that is not business logic and appears everywhere @xgouchet 

Cross-cutting concerns Logging Performances Security Transactions Validation Serialisation Cache Notification Multithreading Memoization @xgouchet 

“ “Let me tell you a story (to chill the bones)” Iron Maiden @xgouchet 

void foo() { Log.v("Tag", "foo"); beginTransaction(); try { bar(); // ... setTransactionSuccessful(); } catch (Exception e) { reportError(e); } finally { endTransaction(); } Log.v("Tag", "foo done"); } Before / After 

void foo() { Log.v("Tag", "foo"); beginTransaction(); try { bar(); // ... setTransactionSuccessful(); } catch (Exception e) { reportError(e); } finally { endTransaction(); } Log.v("Tag", "foo done"); } Before / After 

void foo() { Log.v("Tag", "foo"); beginTransaction(); try { bar(); // ... setTransactionSuccessful(); } catch (Exception e) { reportError(e); } finally { endTransaction(); } Log.v("Tag", "foo done"); } Before / After 

void foo() { Log.v("Tag", "foo"); beginTransaction(); try { bar(); // ... setTransactionSuccessful(); } catch (Exception e) { reportError(e); } finally { endTransaction(); } Log.v("Tag", "foo done"); } Before / After 

void foo() { Log.v("Tag", "foo"); beginTransaction(); try { bar(); // ... setTransactionSuccessful(); } catch (Exception e) { reportError(e); } finally { endTransaction(); } Log.v("Tag", "foo done"); } Before / After 

void foo() { Log.v("Tag", "foo"); beginTransaction(); try { bar(); // ... setTransactionSuccessful(); } catch (Exception e) { reportError(e); } finally { endTransaction(); } Log.v("Tag", "foo done"); } Before / After 

@Trace @SafeTransaction void foo() { bar(); // ... } Before / After 

“ “I've been browsing… (... inspectin’)” Weird Al Yankovic @xgouchet 

input[type=text] { font-family: monospace; } blockquote:before { content: '\201C'; font-size: 300%; font-family: serif; } #nav-home p { color: #FF4630; cursor: pointer; } .contact img:hover { border: 1px #004080; } CSS + HTML 

CSS Attributes Changes in appearance applied to … Elements Nodes in the source code , defined by … Selectors Rules and patterns @xgouchet 

“ “If you know how to use CSS, Then you know how to use AOP” …me @xgouchet 

Pointcuts Rules and patterns JoinPoints Positions in the source code , defined by … Advices Changes in behavior applied to … Aspects @xgouchet 

Advice New behavior (=code) @xgouchet 

JoinPoint Almost every selectable step in bytecode execution @xgouchet 

Pointcuts Description of a (list of) joinpoints where an advice should be applied @xgouchet 

“ “You can knit a sweater by the fireside” The Beatles @xgouchet 

Bytecode weaving AspectJ @xgouchet 

Code Weaving class Foo {…} class Bar {…} class Baz {…} class Bal {…} @xgouchet 

“ “It's time to get our hands dirty” Natalie Grant @xgouchet 

github.com/xgouchet/magic-appdevcon 

Logging The “Hello World” of AOP @xgouchet 

public Foo(); Signature: ()V Code: Stack=1, Locals=1, Args_size=1 0: aload_0 1: invokespecial #1; //Method java/lang/Object."":()V 4: return LineNumberTable: line 1: 0 public java.lang.String getBar(); Signature: ()Ljava/lang/String; Code: Stack=1, Locals=1, Args_size=1 0: aload_0 1: getfield #2; //Field bar:Ljava/lang/String; 4: areturn LineNumberTable: line 5: 0 public void setBar(java.lang.String); Signature: (Ljava/lang/String;)V Code: Stack=2, Locals=2, Args_size=2 0: aload_0 1: aload_1 2: putfield #2; //Field bar:Ljava/lang/String; 5: return LineNumberTable: line 8: 0 line 9: 5 } JD-GUI : http://jd.benow.ca/ But what’s going on really ? @xgouchet 

Monitoring Let’s measure things… @xgouchet 

Retry Because sh*t happens @xgouchet 

Threads Because #PerfMatters @xgouchet 

Compile Checks Too many rules to remember @xgouchet 

Aspects Order Finer control of all your aspects @xgouchet 

Memoization Because computing is hard @xgouchet 

Object Pool Hack your constructors @xgouchet 

It’s just the beginning of the journey - Many other pointcuts - Inter-type declarations - Change class hierarchy - Add fields - Patch jar files @xgouchet 

“ “It’s a kind of magic” Queen @xgouchet 

Wizardry or Witchcraft @Aspect public class PreventNPEAspect { @Pointcut("within(com.example.model.*)") public void withinModelClass() {} @Around("withinModelClass() && execution(* *(..))") public void advice(ProceeJoinPoint jp) { Object[] args = jp.getArgs(); for (int i = 0; i < args.length; i++) { if (args[i] == null) { args[i] = ""; } } jp.proceed(args); } } 

Wizardry or Witchcraft @Aspect public class PreventNPEAspect { @Around("execution(@PreventStringNPE * *(..))") public void advice(ProceeJoinPoint jp) { Object[] args = jp.getArgs(); for (int i = 0; i < args.length; i++) { if (args[i] == null) { args[i] = ""; } } jp.proceed(args); } } 

“ “I wanna find some answers I wanna ask for some help ” Bruce Springsteen @xgouchet 

FAQ ◎ What about stack traces ? Caused by: java.lang.RuntimeException at com.sample.aspectMyAspect.myAdvice(MyAspect.java:666) at com.app.model.Foo.bar(Foo.java:69) at com.app.model.Foo.(Foo.java:42) at java.lang.Class.newInstance(Native Method) <9 more> ◎ What about breakpoints ? Stepping through your code is ok ◎ What about proguard ? Obfuscation performed after weaving @xgouchet 

“ “Forever in debt to your priceless advice” Nirvana @xgouchet 

Bad practices and code smells ◎ Pokemon (gotta catch’em all) ◎ Wizard of Oz (pay no attention to the man behind the curtain) ◎ Dr Octopus (my left tentacle doesn’t know what my right is doin’) ◎ Inception (aspects within aspects …) ◎ MacGyver (the swiss army knife @xgouchet 

Things to remember ◎ Focus on Cross Cutting concerns ◎ If you can do it more easily with OOP don’t use AOP ◎ You can only weave code you own @xgouchet 

Writing Aspects… ◎ Inheritance, abstracts, generics, inner classes… ◎ Unit tests ◎ Android flavors ◎ AOP is real programming @xgouchet 

“ “There’s somethin’ we can use, so don't say no” Toni Basil @xgouchet 

Useful links ◎ Android AspectJ plugin ○ https://github.com/deezer/Android-Aspectj-Plugin ◎ Counsel ○ https://github.com/deezer/Counsel ◎ AspectJ Documentation ○ https://eclipse.org/aspectj/doc/next/progguide/ ◎ Java Bytecode Viewer : JD-GUI ○ http://jd.benow.ca/ @xgouchet 

https://speakerdeck.com/xgouchet Thanks for listening! Any question ? Deezer’s hiring http://jobs.deezer.com