Java Interface To Root

Tony Johnson

SLAC

tony_johnson@slac.stanford.edu

Introduction

• This is not a talk on building Java GUI’s for root

• It is a talk about a language level binding from Java to Root,
  • Goal
    • Make it possible to access Root objects from a Java program as if they were Java Objects

Why?

• Could be used to:
  • Build Java based GUI applications with access to Root data (e.g. for online control/monitoring).
  • Write data analysis in Java as an alternative to CINT
  • Write data analysis in Java based scripting language (JPython, NetRexx etc.)
  • Build Root interface to Java Analysis Studio
    • Data format independent physics analysis tool

Why Care about Java?

Example Java Code

Problem Analysis

• Java code is compiled into machine independent bytecodes
  • Converted to machine code at runtime
  • No link step, so cannot link to modules in other languages
    • Java uses Java Native Interface (JNI) instead
      • Straightforward to call C, or Fortran
      • Not so easy to transparently connect to C++ objects.

C++ + Root = Java?

• Root provides run-time information system similar to that built-in to Java
  • Could be used to automate generation of glue code to tie Root Objects to C++ objects.
  • Root already builds dynamic load libraries for user-defined objects
    • Include C++ part of glue code in same DLL

Multiple Inheritance

• C++ supports multiple inheritance
  • Java does not support multiple inheritance for classes
  • Does support multiple inheritance for interfaces
    • Map root classes to Java interfaces
    • Automatically build implementation classes that implement all necessary interfaces

Class/Interaction Diagram

Root

Track

4Vector

Swimable

Java

Track

4Vector

Swimable

TrackImpl

JNI

Proposed Solution

• Built Java library with binding to common root objects

• Modify rootcint to automatically generate glue code for user defined classes
  • rootcint -java Event.h

Status

• “Proof of principle” done
  • Library contains trivial implementations of
    • TTree, TFile, TROOT, TObject
  • Generated code (by hand) for user defined class
    • Event, Track
  • It works!

Root Data in Java Analysis Studio

• An experiment independent HEP Analysis Application
  • Runs on Windows, Unix (inc. Linux) et al
  • Analysis code written in Java
  • Histogram filling + manipulation package
  • Access to data in
    • PAW, SQL Database (Oracle, Access etc), hippo, histoscope, stdhep, objectivity, flat-file, user-defined, + ROOT
  • Remote or Local access to data
  • Powerful GUI
    • Viewing manipulation, fitting, histograms
    • Built in Editor and Compiler
    • User Extensible

Unsolved Problems

• Templates
  • Use solution like Java binding to objectivity

• Type mapping
  • Map Root types to Java types
    • Many mappings obvious
    • Some may be problematic (pointers to pointers etc).

• Root (user code) can crash! (unlike Java)

• Java has garbage collection, Root doesn’t

• Anyone interested in helping?
  • I like coding in Java, but I’m not a root or cint expert

References

• Write up of this talk
  • http://www-sldnt.slac.stanford.edu/tony/root/

• More About
  • Java - http://java.sun.com
  • JPython - http://www.python.org/jpython/

• Java in HEP
  • http://www-sldnt.slac.stanford.edu/java

• Java Analysis Studio
  • http://www-sldnt.slac.stanford.edu/jas/