Java to C++What could be done

LC Software Workshop, 28-29 May 2009, CERNP. Mato /CERN

What could be done depends strongly on what are the real needs and use cases (previous discussion by Norman Graf) and what we are ready to invest

Basic assumptions◦ Existing Java code needs to be preserved◦ Development of Java applications will continue

Interoperability◦ Until now the interoperability has been achieved

by throw persistent ‘data’ (LCIO, xml files, etc.)◦ True interoperability implies collaboration of Java

and C++ classes in the same running program

Introduction

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Most of the flow from is from Java to C++◦ ‘The main program is Java program’◦ Using existing C++ libraries from Java

Leverage from existing code (e.g. Geant4)◦ Producing C++ data structures that can be then

used by C++ HepMC, Geant4 Geom, LCIO event, ROOT trees and

histograms, etc. Little reverse flow should also be possible

◦ C++ callbacks written in Java◦ Implementation of C++ interfaces in Java

(framework interfaces)

Calling C++ from Java

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Developed PyROOT to enable the usage of any C++ class from Python

Making use of dictionaries (non intrusive!)◦ generated by rootcint or gccxml

Dynamic◦ Python classes are created when needed

Mapping of the C++ constructs to Python equivalent◦ Found an adequate solution for each situation◦ Additional useful ‘pythonizations’

Equivalent Experience with Python

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Julius Hrivnac provided me with very useful information and pointers

The most promising is a solution based on JNI (Java Native Interface)◦ E.g. JavaRoot from Subir Sarkar◦ Automatically generating Java classes from the C++

definitions (header files) at ‘build-time’◦ Java objects are ‘proxies’ to C++ objects

What can be done?

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Java

JNI(generated) C++

Callback

Positive experience with GCCXML◦ Using it to generate Reflex dictionaries (any C++ can be

‘digested’ by GCCXML) Some reasonable mappings of C++ to Java have already be

found◦ E.g. namespaces, operator overloading, default method arguments,

abstract classes, etc. Some of the C++ constructs need to be further studied

◦ True multiple-inheritance, templated classes/methods, function pointers, void*, etc.

For the object ‘ownership’ the same strategy as PyROOT can be applied◦ Objects ‘constructed’ by the Java side are ‘owned’ by Java (garbage

collected). Fine tuning is always possible.

Code generation and language mapping issues

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Extending what exists of JavaRoot (based on JNI) to any C++ class seems feasible◦ This would enable Java applications to use C++ libraries

and/or produce C++ objects runtime interoperability◦ Automated code generation using well known tools

It would require some sizable effort◦ A toy-prototype could be done rather quickly (weeks)◦ A proper and complete implementation would require 1-2

FTE years effort Is this worth?

Summary

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