Re: Setting up a Java Callback to Ada

[Jim Rogers <jimmaureenrogers@worldnet.att.net>]

Kevin Brugh wrote:

> Yes, a JNI.  I have Aonix 95 that uses JNI to interface with Java
> code.  But, since Ada doesn't have Objects/Method like Java.  Ada
> cannot handle Java interfaces, which Weblogic and many other Java
> Classes use a lot of.  I figured if I could pass the access of the
> procedure to Java and it would save it, then when my method was
> invoked/notified I could use that call back procedure.  It would be
> nice if Java could call a Ada procedure that would call my callback
> procedure directly. I have looked at using a C/C++ wrapper for Java,
> but I would like something more simpler and direct.
> 

The real difficulty is on the Java side. Sun claims that JNI should be
able to work with a wide variety of languages. Currently JNI works only
with C. This means that your Ada dlls or shared libraries must look like
C. It also means that you are responsible for handling any and all
data compatibility issues between Ada and Java. Part of the JNI model
is that Java has no knowledge of other programming languages. All other
languages must support Java data types for JNI to work.

Note that the JNI model of software development ALWAYS starts with Java.
First you define your class containing remote methods. Then you create a
C header file from those methods using the javah tool. You implement the
corresponding C functions. You also implement the Java code to call the
remote methods. The C functions must handle all compatibility issues.

If you want Java to call an Ada subprogram you must build a C wrapper
that Java actually calls. That wrapper will handle all data
compatiblity issues and call the Ada subprogram. Trying to use JNI to
communicate between Java and Ada means programming in three languages.

Jim Rogers
Colorado Springs, Colorado USA