Checking LoD in AspectJ

1. Checking LoD in AspectJ • Show the idea, not the details. • How can we precisely express it in a programming language?

2. An adaptive aspect:Law of Demeter Checker (Object Form) aspect Check { … after(): MethodCallSite{ // call (* *(..)); // check whether // thisJoinPoint.getTarget() // is a preferred supplier // object }

3. How can we capture all calls? pointcut MethodCallSite(): scope() && call(**(..)); BUT: how to avoid checking calls in Java libraries?

4. Avoiding the Java libraries pointcut IgnoreCalls(): call(* java..*.*(..)); pointcut MethodCallSite(): scope() && call(**(..)) && !IgnoreCalls();

6. Complex Request Interfaces • We distinguish between two kinds of complex request interfaces • Aspectual: They define code that will be invoked implicitly. Visitor. • Normal: They define code that will be invoked explicitly. Example: Traversal strategy.

7. CRI 1 and 3 are of the same kind Usage of CRI S SS1 1:CRI to send request to S 2: CRI for Implicit Invocation 3: CRI to send request to SS2 SS2

8. LoD paper Karl’s viewgraphs From a talk with a different title Some adjustment is needed

9. An adaptive aspect:Law of Demeter Checker (Object Form) aspect Check { … after(): Any.MethodCall{ // call (* *(..)); // check whether // thisJoinPoint.getTarget() // is a preferred supplier // object }

10. Observation • Many AspectJ programs are adaptive (designed for a family of Java programs) • Context: Java program or its execution tree (lexical joinpoints or dynamic join points) • Features enabling adaptiveness: • *, .. (wildcards) • cflow, + (graph transitivity) • this(s), target(s), args(a), call (…), … • inheritance as wild card • pc(Object s, Object t): this(s) && target(t) && call(… f …)

11. AspectJ crosscutting used in Law of Demeter checker Dynamic call graph cflow(…) target(Object) Connected join points Isolated join points

12. Aspects and lexical join points • Going to the roots of the Northeastern branch of AOP: Law of Demeter. • Closing the circle: Write an ultimately adaptive program in AspectJ: • Works with all Java programs • Checks the object-form of the Law of Demeter: “talk only to your friends”

13. Instrumentation of Java programs with Aspects Aspect framework Aspect Diagram Supplier ImmediatePartBin TargetBinStack ArgumentBin Checker LocallyConstructedBin uses pointcuts ReturnValueBin Requirements: Statistics GlobalPreferredBin Good Separation of Concerns in Law of Demeter Checker

14. Explanation • The *bin* aspects collect potential preferred supplier objects that represent good coupling in the context of a method body. • The Checker aspect checks each method call whether the receiver is a preferred supplier object. • The Statistics aspect counts events generated by the Checker aspect.

15. Law of Demeter(Join Point Form) JPT(ID) = [<target> ID] <args> List(ID) <children> List(JPT) [<ret> ID]. List(S) ~ {S}.

16. JPT(ID) = [<target> ID] <args> List(ID) <children> List(JPT) [<ret> ID]. List(S) ~ {S}. E target t2 args {a1,a2} target t2 ret r1 J r1.foo1() a1.bar() t2.foo2() r3.foo2() target null ret r3

17. Generic Law of Demeter(Join Point Form) Definition 1: The LoD_JPF requires that for each join point J, target(J) is a potential preferred supplier of J. Definition 2: The set of potential preferred suppliers to a join point J, child to the enclosing join point E, is the union of the objects in the following sets:

18. Generic Law of Demeter(Join Point Form) • Argument rule: the args of the enclosing join point E, including the target • Associated rule: the associated values of E: the ret values of the children of E before J whose target is the target of E or whose target is null.

19. aspect LoD extends Violation { pointcut LoD_JPF(): //LoD definition ArgumentRule() || AssociatedRule(); pointcut ArgumentRule(): if(thisEnclosingJoinPoint.getArgs() .contains(thisJoinPoint.getTarget()); pointcut AssociatedRule(): if(thisEnclosingJoinPoint .hasSelfishChild(thisJoinPoint .getTarget())); }

20. Pseudo Aspect • LoD is a ``pseudo'' aspect because it cannot run in the current implementation of AspectJ, which doesn't allow declare warning to be defined on any pointcut with an if expression.

21. Join Point Form • The pointcuts ArgumentRule and AssociatedRule select the ``good'' join points. • ArgumentRuleselects those join points whose target is one of the arguments of the enclosing join point;

22. Join Point Form • AssociatedRule selects those join points whose target is in the set of locally returned ID's, and the ID's created in the siblings of the current node.

23. Map Dynamic Object Form (DOF) to LoD_JPF • We use LoD_JPF pointcut to check DOF: • Dynamic join point model is mapped to JPT. • Object is mapped to ID. • Method invocations are mapped to JPF join points. The enclosing join point is the parent in the control flow.

24. Map Lexical Class Form (LCF) to LoD_JPF • We use LoD_JPF to check LCF as follows. • Lexical join point model is mapped to JPT. Lexical join points are nodes in the abstract syntax tree • Class is mapped to ID. • Join points are signatures of call sites. The enclosing join point is the signature of the method in which the call site resides. To run the aspect, a suitable ordering has to be given to the elements of children: • all constructor calls, followed by local method calls, followed by the other join points.

25. AspectJ code • In AOSD 2003 paper with David Lorenz and Pengcheng Wu • DOF: AspectJ works well. Program uses most adaptive ingredients of AspectJ: *, cflow, this, target, etc. • LCF: AspectJ cannot do it. We sketch how to add statically executable advice to AspectJ.

26. package lawOfDemeter; public abstract class Any { public pointcut scope(): !within(lawOfDemeter..*) && !cflow(withincode(* lawOfDemeter..*(..))); public pointcut StaticInitialization(): scope() && staticinitialization(*); public pointcut MethodCallSite(): scope() && call(**(..)); public pointcut ConstructorCall(): scope() && call(*.new (..)); public pointcut MethodExecution(): scope() && execution(**(..)); public pointcut ConstructorExecution(): scope() && execution(*.new (..)); public pointcut Execution(): ConstructorExecution() || MethodExecution(); public pointcut MethodCall(Object thiz, Object target): MethodCallSite() && this(thiz) && target(target);

27. Class Any continued public pointcut SelfCall(Object thiz, Object target): MethodCall(thiz, target) && if(thiz == target); public pointcut StaticCall(): scope() && call(static * *(..)); public pointcut Set(Object value): scope() && set(* *.*) && args(value); public pointcut Initialization(): scope() && initialization(*.new(..)); }

28. package lawOfDemeter.objectform; import java.util.*; abstract class ObjectSupplier { protected boolean containsValue(Object supplier){ return targets.containsValue(supplier); } protected void add(Object key,Object value){ targets.put(key,value); } protected void addValue(Object supplier) { add(supplier,supplier); } protected void addAll(Object[] suppliers) { for(int i=0; i< suppliers.length; i++) addValue(suppliers[i]); } private IdentityHashMap targets = new IdentityHashMap(); }

29. package lawOfDemeter.objectform; public aspect Pertarget extends ObjectSupplier pertarget(Any.Initialization()) { before(Object value): Any.Set(value) { add(fieldIdentity(thisJoinPointStaticPart), value); } public boolean contains(Object target) { return super.containsValue(target) || Percflow.aspectOf().containsValue(target); } private String fieldIdentity(JoinPoint.StaticPart sp) { … } private static HashMap fieldNames = new HashMap(); }

30. package lawOfDemeter.objectform; aspect Check { private pointcut IgnoreCalls(): call(* java..*.*(..)); private pointcut IgnoreTargets(): get(static * java..*.*); after() returning(Object o):IgnoreTargets() { ignoredTargets.put(o,o); } after(Object thiz,Object target): Any.MethodCall(thiz, target) && !IgnoreCalls() { if (!ignoredTargets.containsKey(target) && !Pertarget.aspectOf(thiz).contains(target)) System.out.println( " !! LoD Object Violation !! " + thisJoinPointStaticPart/*[*/ + at(thisJoinPointStaticPart)/*]*/); } private IdentityHashMap ignoredTargets = new IdentityHashMap();}

31. package lawOfDemeter.objectform; aspect Percflow extends ObjectSupplier percflow(Any.Execution() || Any.Initialization()){ before(): Any.Execution() { addValue(thisJoinPoint.getThis()); addAll(thisJoinPoint.getArgs()); } after() returning (Object result): Any.SelfCall(Object,Object) || Any.StaticCall() || Any.ConstructorCall() { addValue(result); } }

32. Conclusions • Aspects and adaptiveness must work closely together to achieve best results. Crosscutting is closely linked to adaptiveness. • AP is a specialization of AOP and AOP is a specialization of AP. It goes both ways. • AspectJ is a really useful language but we are a little concerned about how difficult it was to debug the Law of Demeter checkers.