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Byte Code engineering● Modifying existing bytecode - Proxy Obj, AOP● Creating bytecode from scratch - Scala, Spring Data JPA
SLAng by Praseed Pai K T● 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
Porting Slang to the JVM● Code generation strategy(Choosing the bytecode
engineering library)● Decided to go with Javassist, BCEL and ASM
Prerequisites to do bytecode generation● Know enough about the structure of a class file● Know enough about the JVM
0xCAFEBAE● Minor & Major version● Access Flags● Constant Pool● Field Instructions● Method Instructions
Bytecode execution● Heap● Java Stack● Method Frame - Operand Stack, Local Variable array● Method Area● Execution engine● Program Counter
What happens when Method A->x() calls Method b->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
Method Invocation● Invokevirtual● Invokestatic● Invokeinterface● Invokespecial
How ‘IF’ works in JVM?● You know it if you know ‘goto’
How ‘loop’ works in JVM?● You know it if you know ‘goto’
Thank You
Code Links● https://github.com/nsarathsoman/Madras-JUG-Byte-Code-
Engineering● Slang Fork → https://github.com/nsarathsoman/slang4JVM ● Main Repo → https://github.com/bvaisakh/slang4JVM
