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SUBJECT
  dev/misc/c++java - How to integrate C++ and Java code

URL
  ftp://ftp.ilog.com/pub/tarfiles/c++java/example.tar.gz     (Native)

DESCRIPTION
  In conjunction with a tutorial I'm giving on 8/7/96 at Java Expo in
  New York, I've written a complete, extended, commented sample
  showing how to access a C++ library from a Java application using
  the original library's API.  Here it is for everyone's benefit.
  
PLATFORM
  Sparc hardware running Solaris 2.4 or above, with JDK 1.0.2 and Sun
  C++ 4.1.
  
SOURCE
  Included
  
AUTHOR
  Harley Davis
  davis@ilog.com
  
CONTACT
  Harley Davis
  Ilog Inc.
  http://www.ilog.com/
  info@ilog.com
  (415) 390-9000
  
BODY
  Next month at the SIGS Object Expo/Java Expo I'm giving a tutorial
  on using C++ and Java together.  I wrote an extended example to
  illustrate how this can be done effectively.  I thought other people
  might be interested in this sample, so I'm making it available via
  FTP.  Here is the README file from the example:

		       Integrating C++ and Java
		       ========================

Introduction
------------

This directory contains the sample code which accompanies the
presentation "Java and C++: Getting the Best of Both Worlds" by Harley
Davis.

What's Included
---------------

This sample includes:

* A small, contrived C++ library with a hierarchy of shapes and some
  methods to manipulate them.

* Java code implementing the API of the C++ library in a parallel
  class hierarchy.

* C++ stub files which forge the link between the Java API and the C++
  library.

* A tiny application written in both C++ and Java to compare the two.
  To run the C++ app, type 'app'.  To run the Java app, make sure the
  JDK is correctly installed and then type 'japp'.

Configuration
-------------

This sample was written for the following configuration:

* Solaris 2.4 or above
* Sparc hardware
* Sun JDK 1.0.2
* Sun C++ 4.1

Before using this sample, make sure that:

* The JDK 1.0.2 is installed and JAVA_HOME is correctly set.
* CLASSPATH contains the directory '.' or the directory in which you
  detar the sample.
* LD_LIBRARY_PATH contains the directory '.' or the directory in which
  you detar the sample.

To download and install the library, follow these steps:

$ ftp ftp.ilog.com
user: anonymous
password: <your email>
ftp> cd pub/tarfiles/c++java
ftp> bin
ftp> get example.tar.gz
ftp> quit
$ gzcat example.tar.gz | tar xvf -
$ cd example
$ app
....
$ java japp
....

Library API
-----------

The library defines the following class hierarchy (showing the
constructor arguments):

Shape()
  Circle(int radius)
  Square(int height)
  Triangle(int side)

Methods on shapes include:

* void draw()	Draws the shape.
* int height()  Returns the height of the bounding box around the
		shape.
* int width()	Returns the width of the bounding box around the shape.
* void move(int dx, int dy)	Moves the shape dx units to the right
				and dy units down.

In addition, circles have a public data member 'int radius'.

The Java class hierarchy has an identical API, except that the data
member 'radius' for Circle is mapped into a method with two
signatures:

* int radius()			Returns the radius.
* void radius(int new_radius)	Sets the radius.

Strategy
--------

Here is how I created the Java binding for the sample library:

* The class CppPeer defines a generic superclass for all C++ classes
  made visible in Java.  It contains a slot for storing the C++ object
  pointer corresponding to the Java peer and a static method called
  '_load' for loading the shared library corresponding to the class's
  stubs.

* For each class in the C++ library, create a Java class which extends
  CppPeer if it has no base class in C++, or the homonymous Java class
  if it does have a C++ base class.

* For each signature of each method of each class, make a Java method.
  If the C++ method is not overloaded, make this method native.  If
  the C++ method is overloaded, make another method for each signature
  with a unique name (prepended by 'native_' and suffixed by a list of
  the types of the arguments) which is declared 'native private
  final'.  Then make a regular Java method signature for the actual
  method name which calls this native method.  This is necessary
  because javah does not allow overloaded native methods.

* Each class has to have a static section which loads a shared library
  implementing the stubs for that class by calling CppPeer's static
  method '_load'.

* Use 'javah' and 'javah -stubs' to generate stubs for these Java
  classes.

* For a class foo, write a C++ file called 'foo_cc.cc' which
  implements the stubs generated by javah.  It does this by calling
  out to the original C++ library.  It includes the javah-generated
  stubs and surrounds everything with 'extern "C"' to avoid link
  problems.

* Compile each of these into a shared library called libfoo.so.  Each
  must be linked with the original library, which must be a shared
  library so Java can load it dynamically.

Files
-----

The files in this sample are:

* README	This file.
* Makefile	Makefile to remake everything.
* sample.h	The interface file for the C++ library
* sample.cc	The implementation file for the C++ library
* libsample.so	The compiled C++ library.
* app.cc	The source code for the C++ application using the
		library.
* app		The compiled C++ application.
* japp.java	The source code for the Java application using the
		library.
* japp.class	The compiled Java application.
* CppPeer.java	Superclass containing generic definition of a C++ peer
		class in Java.
* *.java	Where '*' is one of the classes in the library. The
		source code of the Java class corresponding to *.
* *.class	Where '*' is one of the classes in the library. The
		compiled Java class corresponding to *.
* *.h		Where '*' is one of the classes in the library. The
		javah-generated header stubs for *'s native methods.
* *.c		Where '*' is one of the classes in the library. The
		javah-generated C stubs for *'s native methods.
* *.o		Where '*' is one of the classes in the library. The
		compiled javah-generated C stubs for *.
* *_cc.cc	Where '*' is one of the classes in the library. The
		hand-written C++ stubs for *'s native methods which
		call the original library.
* lib*.so	Where '*' is one of the classes in the library. The
		shared library for * which is loaded when * itself is
		loaded by the JVM.

All handwritten files are stored under RCS in the RCS directory.

Remaking
--------

To remake the whole shebang, do the following:

$ make clean
$ make

Conclusion
----------

Good luck! Please free to send any comments and enhancements (such as
a Windows port) to <URL:mailto:davis@ilog.com>.

-- Harley Davis
   Director of Technology, Ilog Inc.

-------------------------------------------------------------------
Harley Davis                            net: davis@ilog.com
Ilog, Inc.                              tel: (415) 944-7130
1901 Landings Dr.                       fax: (415) 390-0946
Mountain View, CA, 94043                url: http://www.ilog.com/


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