Extensibility

1
Extensibility

• Pang Wai Man (Raymond)
• 10 Feb 2006

2
Extensibility

• Designator
• Context exposure
• Inter-type declaration
• Body

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Define new pointcut designator

• Using static method in Java with Javassist
• Method have to return a boolean value
• aspect compiler translate into call of the static
  method of the new designator
• Example
• paramType1("ColorPoint")
• static boolean paramType1(CtMethod m, String
  args, JoshContext jc) ..

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Example

• static boolean paramType1(CtMethod m,
• String args, JoshContext jc)
• CtClass parType
• m.getMethod().getParameterTypes()0
• CtClass argType jc.getType(args0)
• if (parType.subtypeOf(argType))
• return true
• if (argType.subtypeOf(parType))
• jc.setIf("1 instanceof " argType.getName())
• return true
• else
• return false

5
Dynamic designator

• Dynamic designator run-time type check
• Weaver insert if statement surround the inserted
  advice body
• Two if statements
• At weaving time
• At runtime
• Insert if at runtime setIf in JoshContext

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Context exposure

• expose method in JoshContext
• E.g jc.expose("String cname \"" c.getName()
  "\"")
• Inserted at beginning of advice body
• String cname "Point"
• / advice body /
• Dynamic context is also possible

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Inter-type declaration

• Similar to Designator, but void return
• static void addThrows(CtMethod m,String args,
  JoshContext jc)
• CtClass type jc.getType(args0)
• m.addExceptionType(type)
• Appends exception type to throws list

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Body

• Addressing the problem of Section 2.2 of paper
  (Visitor pattern)
• intro() within("Expr")
• void accept(Visitor v)
• v.visitlt josh.getCtClass().getName() gt(this)
• Substitute at compile time and inserted by weaver
• "void accept(Visitor v) v.visit"
• josh.getCtClass().getName()
• "(this)"

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Javassist

• Compile time reflection library
• Provide a set of join-point objects for inspect
  and alter type signature of class
• obtain set of join-point objects included in the
  method body (CtMethod)
• Provide various class information including name,
  signature, exceptions etc of a method
  (MethodCall)
• Replace method to substitutes given source text

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Javassist

• Limitations
• Cant define a new kind of join-point object
• Advice body can be only inserted either before,
  after, or around.
• Limited inspect and alter ability
• No Control or data flow information among join
  points
• It is enough for implementing relatively complex
  designator or etc.