Aspect-Oriented Programming in Eclipse with AspectJ Dr Helen Hawkins and Sian January

1. Aspect-Oriented Programming in Eclipsewith AspectJDr Helen Hawkins and Sian January

2. Outline • The need for Aspect-Oriented Programming (AOP) • An introduction to AspectJ • AspectJ Demos • Adopting AOP • Further information

3. The 1-to-1 idea: Good modularity • One requirement in your design maps to one module in your implementation • For example – Chess Piece, Chess Board, Player • Clear, simple, direct mapping • Modules are • easy to add • easy to remove • easy to maintain

4. The 1-to-1 idea: Good modularity • Socket Creation in Apache Tomcat • Red shows the relevant lines of code • Nicely fits into one package – 3 classes

5. When things go wrong… • Logging in Tomcat • Scattered and tangled throughout all the packages • Bad Modularity – 1-to-n mapping

6. The trouble with 1-to-n • Redundant code • Same fragment of code in many places • e.g. trace.entry(); trace.exit(); • Difficult to understand • Mapping from the requirement is unclear • Structure is not explicit • Difficult to change • Have to find all the code involved • And be sure to change it consistently • And be sure not to break it by accident

7. n-to-1 and n-to-n • A consequence of the 1-to-n problem • Modules are no longer well defined

8. try { if (!removed) entityBean.ejbPassivate(); setState( POOLED ); } catch (RemoteException ex ) { FFDCEngine.processException( ex,”EBean.passivate()”,”237”, this); destroy(); throw ex; } finally { if (!removed && statisticsCollector != null) { statisticsCollector. recordPassivation(); } removed = false; beanPool.put( this ); if (Logger.isEnabled) { Logger.exit(tc,”passivate”); } } Code View of n-n Tangled Core Logic try { if (!removed) entityBean.ejbPassivate(); setState( POOLED ); } catch (RemoteException ex ) { destroy(); throw ex; } finally { removed = false; beanPool.put( this ); } Example: Code to handle EJB Entity bean passivation

9. Aspect-Oriented Software Development … • Recognises crosscutting concerns: • Are inherent in any complex system • Have a clear purpose • Have a natural structure • Captures crosscutting concerns explicitly: • In a modular way • With linguistic and tool support

10. AspectJ • An aspect oriented programming (AOP) language • Java language extension • Divides system into • core concerns (classes) • crosscutting concerns (aspects) • Broad IDE support • Eclipse, Emacs, JBuilder, NetBeans, …

11. AOP Concepts • Join points • Pointcuts • Advice • Aspects

12. Join Points • Events in the execution of a program • Types of Join Points: • Method & Constructor call • Method & Constructor execution • advice execution • Field get & set • Exception handler execution • Static & dynamic initialization

13. What’s a Pointcut? • Pointcut • Picks out join points in a particular system • e.g. call of method ‘foo()’, set of field ‘x’, constructor for object ‘Fred’ • Can also expose context from the matched join point

14. Types of Pointcuts • Event based - well defined execution points, e.g. • method calls, executions • field gets / sets • exception handling • object and class initialisation • Scoping - only select join points within a certain scope, e.g. • within a set of packages, • within the implementation of a method, • in the control flow of some event (e.g. downstream of an unsecured call) • Context matching - expose context at the join point, e.g. • identity of the caller or target object • exception being handled • value being put into a field

15. What’s Advice? • Java code to execute when conditions of a pointcut are met. • Can be parameterized to process context exposed by the pointcut

16. Types of Advice • before() • Executes before the join point • after() throwing/returning • Executes after the join point, execution can be made conditional on how join point exits (i.e. with/without exception) • around() • Executes ‘instead of’ the join point – has a choice about whether to invoke the original logic

17. What is an Aspect? • Aspects are... • At the design level … concerns that crosscut • At the implementation level … a programming construct Aspect = pointcut + advice

18. Demos – AJDT • AspectJ Development Tools (AJDT) for Eclipse • Open Source • Developed in Hursley • Partnership with AspectJ team • http://www.eclipse.org/ajdt

19. Demos • Demo 1: A Simple Figure Editor • Demo 2: Web Services Invocation Framework

20. Web Services Invocation Framework (WSIF) • Middleware component • Simple Java API for invoking web services, no matter how or where they are provided • Released to Apache • But IBM wants a version tightly coupled to IBM’s normal ‘qualities of service’ • IBM tracing/monitoring/management • How do we manage this?

21. Exploring Re-Use: The WSIF Story WSIF for Open Source Community org.apache.wsif + WebSphere RAS Composition WebSphere FFDC WSIF for WebSphere WebSphere PMI

22. Applications of AOP • Problem determination • Logging, FFDC, performance monitoring • Architectural rule enforcement • Contracts, encapsulation, separation (no “up calls”) • Other concerns • Security, transactions, persistence, caching/pooling, locking • Open source integration

23. Adopting AOP reward AO Analysis,AO Design, AO Strategy auxiliary / infrastructure core / business explorationenforcement time & confidence

24. Further information • AspectJ home page: http://www.eclipse.org/aspectj • AJDT home page: http://www.eclipse.org/ajdt • Pick up a book … • Email us: • Helen Hawkins: hawkinsh@uk.ibm.com • Sian January: sjanuary@uk.ibm.com