Extending the Field Access Pointcuts of AspectJ to Arrays ICS 2006 – Taipei, December 2006 Kung Chen and Chin-Hung Chien* National Chengchi University Hon Hai Precision Industry Co., Ltd*

Agenda Background: Aspect-Oriented Programming (AOP) in AspectJ Motivation Design Considerations Specification of the New Pointcuts Implementation Conclusions

Basic Mechanisms of AOP where Do what aspect Class advice weaving pointcut Aspect Pointcut Advice Weaving Implement Crosscutting concerns modularly

Aspects in AspectJ aspect Name [extends a] { pointcut1; pointcut2; … advice1; advice2; … fields methods } Crosscutting points Actions to take in matched pointcuts Pointcut: selecting a collection of join points

Join Points Model of AspectJ, 1 Join point is a well-defined point in a program’s execution Method call: public void move(int dx, int dy) { setX(_x + dx); setY(_y + dy); } Method call join point:

Join Points Model of AspectJ, 2 Our focus: Field reference and set join points: public void setX(int newx) { x = newx;..println(x); } field set join point: field reference join point:

Agenda Background: Aspect-Oriented Programming (AOP) in AspectJ Motivation Design Considerations Specification of the New Pointcuts Implementation Conclusions

Field Access Join Points of AspectJ Join Points –field reference –field set Pointcut Designators –get(aFieldSignature) –set(aFieldSignature) aspect GuardedX { static final int MAX_CHANGE = 100; before(int newval): set(static int T.x) && args(newval) { if (Math.abs(newval - T.x) > MAX_CHANGE) throw new RuntimeException(); } } Example

Limitations in Using Field Access Pointcuts What if the fields we are interested in are arrays? 01 public class FieldPointcuts { 02 static int ar[]; 03 public static void main( 04 String[] args) { 05 ar = new int[] {100}; //set 06 ar[0] = 200; //get 07 } 08 } set(* *.ar) after() returning (int[] a) : get(* *.ar)

What We Expect Field access pointcuts can also be applied to array elements and further expose information on –the index values of the array element being set or retrieved, and –the value being set to or retrieved from the array element. ar[0] = 200;

Related Work Bruno Harbulot proposed the following arrayset pointcut designator: before(int i, Object s, Object[] a): arrayset() && args(i, s) && target(a) { System.out.println (" Arrayset: ["+i+"/"+(a.length-1)+"] = "+s) ; } The idea is to treat array element set as a call to a “set(int index, object newValue)”

Problems with Harbulot’s Work Ambiguous matching of join points arrayset() && args(i, s) && target(a) arrayset() && args(i1,i2,s) && target(a) ss[0][1] = “two join points”; sss[0][1][2] = “two join points, too”; Mismatch actually two assignments to two 1-D arrays on the byte-code level actually three assignments to three 1-D arrays on the byte-code level Match

Agenda Background: Aspect-Oriented Programming (AOP) in AspectJ Motivation Design Considerations Specification of the New Pointcuts Implementation Conclusions

How We Address This Problem Fix the array target(s) of interest –only need to focus on arrays which are fields of some class –arrays local to a method are irrelevant Include a specification of field signature in our array set/get pointcuts.

Requirements Our arrayset field set pointcut must be flexible enough to select all of them, either individually or as a group. Assignments to a multi-dimensional array can take several forms. 01 class Watch { 02 String [][][] sss= new [2][2][2]; 03 String [][] ss= {{“x”,”y”},{“w”,”z”}}; 04 String [] s= {“abc”, “def”}; 05 sss[0] = ss; 06 sss[0][1] = s; 07 sss[0][1][1] = “change”; 08 ss [0] = s; 09 ss[0][1] = “Me too”; 10 s[0] = ss[1][1]; … }

Agenda Background: Aspect-Oriented Programming (AOP) in AspectJ Motivation Design Considerations Specification of the New Pointcuts Implementation Conclusions

New Field Access Pointcuts Conservative extension of the standard field pointcuts: –arrayset(aFieldSignature) –arrayget(aFieldSignature) Orthogonal to other AspectJ pointcuts – target(), within() and withincode() All array field set join points are treated as having a variable arguments: –the sequence of index values –the value the field is being set to arrayset(* Watch.*) arrayget(* Watch.*)

Context Exposure, 1 Using the args() pointcut aspect Monitor { before(int ix1, int ix2, int newVal): arrayset(* Watch.*) && args(ix1, ix2, newVal) { //advice if (newVal > B.bounds[ix1, ix2]) { ArraySetSignature sig = (ArraySetSignature)tjp.getSignature(); String field = sig.getFieldType() + sig.getName(); throws new RuntimeException("Bad change"+ field) } } 01 class Watch { 02 String [][][] sss= new [2][2][2]; 03 String [][] ss= {{“x”,”y”},{“w”,”z”}}; 04 String [] s= {“abc”, “def”}; 05 sss[0] = ss; 06 sss[0][1] = s; 07 sss[0][1][1] = “change”; 08 ss [0] = s; 09 ss[0][1] = “Me too”; 10 s[0] = ss[1][1]; … } Selective matching – assignments in Line 6 and 9 are captured.

Context Exposure, 2 Can also use the method thisJoinPoint.getArgs() void around() : arrayset( * data.*.* ) { try { ArraySetSignature sig = (ArraySetSignature)tjp.getSignature(); Object[] args = tjp.getArgs(); Integer rhsValue = (Integer)args[args.length-1]; if (rhsValue.intValue() > MAX) { Log.add("Warning: " + sig.getName()); for (int i=0; i<args.length-1; i++) { Log.add("["+ args[i] +"]"); Log.add(" exceeds MAX"); } proceed(); } catch(IndexOutOfBoundException e) {} } Provides great flexibility.

Agenda Background: Aspect-Oriented Programming (AOP) in AspectJ Motivation Design Considerations Specification of the New Pointcuts Implementation Conclusions

Implementation Using the AspectBench Compiler(abc) for AspectJ We follow the standard steps outlined by the abc team to develop this extension. But the following steps are non-trivial: –Identification of join point shadow –Extension of the pointcut matcher of abc

FSM Based Shadow Matcher Cond. #1: instanceOf(getRhs(ca), ArrayFieldRef) Cond. #2: instanceOf(getRhs(ca), ArrayRef) && equalBase(getRhs(ca), ima) && instanceOf (getLhs(ca), Local) Cond. #3: instanceOf(getLhs(ca), ArrayRef) && equalBase(getLhs(ca), ima) Cond. #4: !hasNext() #1: $r2 = r0. #2: $r3 = $r2[0] #3: $r4 = $r3[1] #4: $r5 = new java.lang.String #5: specialinvoke $r5. (java.lang.String)>("foo") #6: $r4[2] = $r5 class C { … Object a[][][] = new Object[2][2][2]; … a[0][1][2] = new String("foo"); … }

Conclusions The field access pointcuts of AspectJ can be extended to expose index-value context for array fields. A FSM-based implementation is presented using the abc compiler for AspectJ. You are welcome to download it and try it yourself.

Q & A