Presentation about byte code engineering at Java user group Madras (Chennai)

1. BytecodeEngineering
Sarath Soman
nsarathsoman@gmail.com
https://sarathsoman.wordpress.com/
@sarathsom4

2. ByteCodeengineering
● Modifying existing bytecode - Proxy Obj, AOP
● Creating bytecode from scratch - Scala, Spring Data JPA

3. SLAngbyPraseedPaiKT
● Originally a language designed for .NET CLR
● Used in production as rule engine generating JavaScript
● Been ported to C++ with LLVM backend, Python, JS, VB and
Java
● Experiments going on to support modules → Higher order
functions → Object orientation etc

4. PortingSlangtotheJVM
● Code generation strategy(Choosing the bytecode
engineering library)
● Decided to go with Javassist, BCEL and ASM

5. Prerequisitestodobytecodegeneration
● Know enough about the structure of a class file
● Know enough about the JVM

6. 0xCAFEBAE
● Minor & Major version
● Access Flags
● Constant Pool
● Field Instructions
● Method Instructions

7. Bytecodeexecution
● Heap
● Java Stack
● Method Frame - Operand Stack, Local Variable array
● Method Area
● Execution engine
● Program Counter

8. WhathappenswhenMethodA->x()callsMethodb->y()
● Instance of B is pushed to operand stack of X()
● Parameters of Y() is pushed to operand stack of X() in
reverse order
● When invoke instruction is seen → Stack frame of Y() is
pushed to Java stack
● Parameters (including instance of B) is popped from
operand stack of X() one by one and are pushed to operand
stack of Y()
● Starts executing Y()
● Pops the frame of Y() when return instruction is seen

9. MethodInvocation
● Invokevirtual
● Invokestatic
● Invokeinterface
● Invokespecial

10. How‘IF’worksinJVM?
● You know it if you know ‘goto’

11. How‘loop’worksinJVM?
● You know it if you know ‘goto’

12. ThankYou

13. CodeLinks
● https://github.com/nsarathsoman/Madras-JUG-Byte-Code-
Engineering
● Slang Fork → https://github.com/nsarathsoman/slang4JVM
● Main Repo → https://github.com/bvaisakh/slang4JVM