Summarized class part of ASM library after reading ASM 4.0 A Java bytecode engineering library

1. ASM Core API - Class
Taegeon Lee
t76lee@uwaterloo.ca
Advisor: Prof. Doo-Hwan Bae
bae@se.kaist.ac.kr
Teaching Assistant: Jiyoung Song
jysong@se.kaist.ac.kr

2. ASM Introduction
• Motivations
• Overview
Scope
Model
Architecture
Organization
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3. Motivations
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 ASMlibrarydesignedto work on compiledjavaclasses.
 There are some techniquessuch as programanalysis,generationand
transformation.
 It is supposedto be fast andsmall.
 Advantagesof working on compiledcode
 Thesourcecodeisnotneeded.So,programtransformationscan beusedonany
applications.
 Itbecomespossibletoanalyze,generateortransformclassesat runtimelodedin
JVM.
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4. Overview
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 Scope
 ASMisstrictlylimitedtoreading,writing,transformingand analyzingclasses.
Theclassloading processisoutofscope.
 Model
 ThecoreAPI–an eventbasedrepresentationofclasses
 ThetreeAPI-an objectbasedrepresentation
ASMprovides bothAPI.
 Architecture
 Usingtheevent basedAPIisatwostepprocess:
Assembling event producer,filter andconsumer components intopossiblycomplex
architectures
Thenstartingtheevent producerstorunthegeneration ortransformationprocess
 Organization
 Severalpackagesthat are distributedinseveraljar files
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5. Classes
• Structure
Overview/ Internal names/ Type descriptors/ Method descriptors
• Interfacesandcomponents
Parsing classes / Generating classes/ Transforming classes/ Removing
class members/ Adding class members
• Tools
Type/ TraceClassVisitor/ CheckClassAdapter/ ASMifier
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6. Structure
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 Overview
 Compiledclasscontainsmodifiers,name,superclass,interfacesand annotations.
 Differencesbetweensource andcompliedclasses
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Source file Compiled class
• Contains several classes • Describes only one class
• Doesn’t contain comments,
but can contain class, field,
method and code attribute
s
• Doesn’t contain a package
and import section
• Contains a constant pool se
ction [e.g) array]
Another important difference is that Java types are represented different in compiled
And same source classes.

7. Structure
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
[1]Overallstructure ofacompiledclass
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8. Structure
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 Internalnames
 Theinternalnameofaclassisjustthefullyqualifiednameofthisclass,where dotsare
replaced withslashes.i.e)java/lang/string
Only fortypesthatareconstrained tobeclass orinterface
 Typedescriptors
 Thedescriptorofaclasstypeistheinternalnameofthisclass,precededbyLand
followedbyasemicolon.i.e)thetypedescriptorofstringisLjava/lang/string
 Method descriptors
 Alistoftypedescriptorsthat describetheparametertypesand thereturntypeofa
methodinasinglestring.
Amethoddescriptordoesn’tcontain themethod’s name orargumentnames.
i.e) (I)Idescribes amethodthattakesoneargumentoftypeintand returnanint.
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9. Interfaces andComponents
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 ASMcomponents: classReader,classWriter,classVisitor
 Parsingclasses
 classReader–onlyrequiredcomponenttoparsean exisitingclass
 Generatingclasses
 classWriter–onlyrequiredcomponenttogenerateaclass
Generating class canbeloaded dynamically withaclassLoader
 Transformingclass
 It’spossibletouseseveraladaptersinordertocomposeseveralindependentclass
transformationstocomplextransformation
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Reader
Adapter
(classVisitor)
Writer

10. Interfaces andComponents
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 Usingtransformedclasses
 Ifyouwant totransformallclasses,youwillhave toputyourtransformationinsidea
classFileTransformerasdefinedinthejava.lang.instrumentpackage.
 Removingclassmembers
 Removingmeansnotforwardingthecallat all.
 Inordertoremoveafieldormethod,youmustnotforwardthemethodcall,and
returnnulltothecaller.
 Addingclassmember
 It’sbasicallyforwarding.
 IfyouputthenewcallinvisitEndmethod,thefieldwillalwaysbeadded sincethis
methodisalwayscalled.
 IfyouputitinvisitFieldorvisitMethod,severalfieldswillbeadded:oneperfieldor
methodintheoriginalclass
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11. Tools
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
 In additionto the classVisitorclass,ASM providesorg.objectweb.asm.util
package.(not neededat runtime)
 Type
 AtypeobjectrepresentsaJava type,andcan beconstructedeitherfromatype
descriptororclassobject.
 Itcan alsorepresentamethodtype.
 TraceClassVisitor
 Itprovidesatextualrepresentationofthevisitedclass.
 classWriterisnotreadable byhumans.So,inordertogetareadable traceofwhat is
actuallygenerated,TraceClassVisitorcanbeused.
 CheckClassAdapter
 classWriterdoesn’tcheckthatitsmethodsarecalledintheappropriateorderand
withvalidarguments.
 Inordertodetectsomeerrors,CheckClassAdapterisused
 ASMifier
 Itprovidesan alternativebackendfortheTraceClassVisitortool.
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12. Reference
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13. Reference
Introduction Background Overall Approach Methodology Experiments Results Conclusion Discussion
[1]E. Bruneton, “ASM 4.0 A Java bytecode engineering library,
” ASM, Sep-2011. [Online].Available: http://asm.ow2.io/asm
4-guide.pdf.
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14. Thank You.
1st June, 2018
t76lee@uwaterloo.ca