Introduction to ASM - ByteCode Manipulating Tool

Transcription

1 Introduction to ASM - ByteCode Manipulating Tool nadeesh.tv@oracle.com ORACLE India Pvt Ltd 19 March 2016

2 Outline 1 Why do we need a byte code manipulating tool? 2 Compiled Class File Structure 3 ASM

3 Why do we need a byte code manipulating tool? Java refers to pair of technologies Java Programming Language Java Virtual Machine More than 200 JVM languages exist Number of other JVM languages increased due to Cross platform nature Improved performance etc Need a tool which helps in exploring JVM

4 Compiled Class File Structure Figure: Class File Structure

5 Compiled Class File Structure Does not contain import and package statements Magic Number - CAFEBABE Major and Minor Version Jdk 8-52, 0 Constant Pool Class names (in internal form) Variable names, Method names Signature (I, B, Z, L;, [) String literals Access Flags ACC PUBLIC, ACC FINAL ACC SUPER, ACC INTERFACE ACC ABSTRACT

6 Compiled Class File Structure

7 Compiled Class File Structure Method invocation create a stack frame Frame consist of Operand stack and Local variables array Figure: Frame Structure Debugging attribute LineNumberTable LocalVariableTable

8 ASM ASM library is a project of the OW2 Consortium Developed by Eric Bruneton Written in Java Become the defacto standard Goals Parsing Generate Transform Analyze Figure: ASM Package Structure

9 ASM : Visitor Based Pattern ASM follows Visitor Based Pattern for it s operation Figure: Visitor Based Pattern

10 Visitor Based Pattern Visitor.java p u b l i c i n t e r f a c e V i s i t o r { p u b l i c v o i d v i s i t C o l l e c t i o n ( C o l l e c t i o n c o l l e c t i o n ) ; p u b l i c v o i d v i s i t S t r i n g ( S t r i n g s t r i n g ) ; } p u b l i c v o i d v i s i t F l o a t ( F l o a t f l o a t ) ; Vistable.java p u b l i c i n t e r f a c e V i s i t a b l e { p u b l i c v o i d a c c e p t ( V i s i t o r v i s i t o r ) ; }

11 Visitor Based Pattern VisitableString.java p u b l i c c l a s s V i s i t a b l e S t r i n g implements V i s i t a b l e { p r i v a t e S t r i n g v a l u e ; p u b l i c V i s i t a b l e S t r i n g ( S t r i n g s t r i n g ) { v a l u e = s t r i n g ; } } p u b l i c v o i d a c c e p t ( V i s i t o r v i s i t o r ) { v i s i t o r. v i s i t S t r i n g ( t h i s ) ; }

12 Visitor Based Pattern PrintVisitor.java p u b l i c c l a s s P r i n t V i s i t o r implements V i s i t o r { p u b l i c v o i d v i s i t C o l l e c t i o n ( C o l l e c t i o n c o l l e c t i o n ) { I t e r a t o r i t e r a t o r = c o l l e c t i o n. i t e r a t o r ( ) w h i l e ( i t e r a t o r. hasnext ( ) ) { O b j e c t o = i t e r a t o r. n e x t ( ) ; i f ( o i n s t a n c e o f V i s i t a b l e ) ( ( V i s i t a b l e ) o ). a c c e p t ( t h i s ) ; } p u b l i c v o i d v i s i t S t r i n g ( S t r i n g s t r i n g ) { System. out. p r i n t l n ( +s t r i n g+ ) ; } } p u b l i c v o i d v i s i t F l o a t ( F l o a t f l o a t ) { System. out. p r i n t l n ( f l o a t. t o S t r i n g ()+ f ) ; }

13 ASM : Class Visitor ClassVisitor.java p u b l i c a b s t r a c t c l a s s C l a s s V i s i t o r { p u b l i c C l a s s V i s i t o r ( i n t a p i ) ; p u b l i c C l a s s V i s i t o r ( i n t api, C l a s s V i s i t o r cv ) ; p u b l i c v o i d v i s i t ( i n t v e r s i o n, i n t a c c e s s, S t r i n g name, S t r i n g s i g n a t u r e, S t r i n g supername, S t r i n g [ ] i n t e r f a c e s ) ; p u b l i c v o i d v i s i t S o u r c e ( S t r i n g s o u r c e, S t r i n g debug ) ; p u b l i c v o i d v i s i t O u t e r C l a s s ( S t r i n g owner, S t r i n g name, S t r i n g d e s c ) ; A n n o t a t i o n V i s i t o r v i s i t A n n o t a t i o n ( S t r i n g desc, b o o l e a n v i s i b l e ) ; p u b l i c v o i d v i s i t A t t r i b u t e ( A t t r i b u t e a t t r ) ; p u b l i c v o i d v i s i t I n n e r C l a s s ( S t r i n g name, S t r i n g outername, String innername, i n t access ) ; p u b l i c F i e l d V i s i t o r v i s i t F i e l d ( i n t a c c e s s, S t r i n g name, S t r i n g desc, String signature, Object value ) ; p u b l i c MethodVisitor visitmethod ( i n t access, String name, String desc, String signature, String [ ] exceptions ) ; v o i d v i s i t E n d ( ) ; }

14 ASM : Class Visitor Methods of a Class Visitor must call in the following order visit visitsource? visitouterclass? (visitannotation visitattribute) * (visitinnerclass visitfield visit Method )* visitend

15 ClassWriter : Generating Classes Based on ClassVisitor Produce as output a byte array that contains the compiled class

16 ClassReader : Parsing Classes Calling the corresponding visitxxx methods on ClassVisitor instance passed as an argument to its accept method

17 ClassAdapter : Transforming Classes Figure: Class Adapter

18 Other important Classes MethodVisitor CheckClassAdapter Asmfier

19 References

20 Thank you