[NTLUG:Discuss] C++ calls Java which calls C++

[Stephen Davidson]

Hi Steve.

I will look at this tonight.  In the meantime, lets move this over to 
the JavaMUG list, as it seems to be a Java specific issue.

Oh, and for gaming and Java?  Check out 
http://www.bytonic.de/html/jake2.html.  If you are a Quake fan, have 
fun.  (Runs on Linux!)

Hey Java Geeks -- any idea what is being missed here?

-Steve

steve wrote:
> Stephen Davidson wrote:
>
>   
>> Having done much JNI programming (mostly to control HW devices), I can 
>> give you a hand, although it has been a while.
>>     
>
> Thanks!  I really need some help here.
>
>   
>> The first question I would have, is the main program accessible to the 
>> .so library?
>>     
>
> That's what I want to happen - but it's not.
>
> If I load the library from within the main C++ application using dlopen
> then entrypoints in the application are accessible from within the
> library - but when Java loads the library they are not.
>
> I presume it's using some kind of loading mechanism that somehow doesn't
> recognise the existance of the application that invoked the JNI/JVM
> environment.
>
>   
>> Specifically, are the functions you are looking for in the 
>> main program exported?
>>     
>
> Well, this is Linux - everything that's not 'static' is exported -
> right?
>
>   
>> Otherwise, I am not sure that the .so can see 
>> them.  If my memory serves, when the native library is loaded, it is 
>> loaded into its own address space, and must dlopen any other libraries 
>> that it needs.
>>     
>
> That's what seems to be the problem.
>
>   
>> If it needs to call back into the VM, it needs to use 
>> the *env pointer -- which may give you the handle you need.
>>     
>
> It's not the JVM I want to call.  It's *my* application.
>
> Essentially, we have the main application in C++ - it's a game
> engine ("Botzilla" - find it at http://botzilla.sf.net) - which
> has a C++ function inside it
>
>    int fetchRobotGameEvent () ;
>
> ...elsewhere in that program, I create the JNI/JVM stuff and load
> a Java '.class' file into it.  The Java class is going to be written
> by the kids in my son's Java class and it's going to look something
> like this:
>
> import java.io.*   ;
> import java.util.* ;
>
> /* My utility stuff - including class GameInterface */
> import botzilla.*   ;
>
> public class MyClass extends GameInterface
> {
>    public MyClass ()
>    {
>      System.out.println ( "==> MyClass constructed OK" ) ;
>    }
>
>    public void update ()
>    {
>      /* getGameEvent is a part of class GameInterface */
>      System.out.println ( getGameEvent () ) ;
>    }
> }
>
> ...I have them extend my base class 'GameInterface' so that
> I can hide all of the nasty uliness associated with interfacing
> to the game engine inside the base class.
>
> So I provide the base class 'GameInterface':
>
> package botzilla ;
>
> import java.io.* ;
> import java.lang.* ;
> import java.util.* ;
>
> public class GameInterface
> {
>    public native int    getGameEvent () ;
>
>    ....lots and lots of other native functions....
>
>    public GameInterface ()
>    {
>      System.load ( "/usr/local/lib/libBotzillaInterface.so" ) ;
>    }
> }
>
>
> ...and then, inside libBotzillaInterface, I have some horrible
> JNI-compatible function written in C++:
>
> extern "C" JNIEXPORT jint JNICALL
>     Java_botzilla_GameInterface_getGameEvent
>     (JNIEnv *env, jobject _jobj)
> {
>    printf ( "Inside Botzilla Interface!\n" ) ;
> }
>
>
> ...OK - so the botzilla application calls the 'MyClass'
> constructor - then repeatedly calls the 'update' function.
> Because MyClass 'extends' the GameInterface class, the
> MyClass constructor function implicitly invokes the GameInterface
> constructor - which in turn loads my C++ library.   When the
> MyClass update function calls 'getGameEvent', that calls the
> Java_botzilla_GameInterface_getGameEvent inside my C++ library
> and everything seems to work just fine.
>
> The problem comes when 'getGameEvent' wants to call a function
> inside the main game engine - for example by trying to call
>
>    int fetchRobotGameEvent () ;
>
> If I change the Java_botzilla_GameInterface_getGameEvent function
> to call 'fetchRobotGameEvent()', I get a runtime error:
>
> symbol lookup error: /usr/local/lib/libBotzillaInterface.so: undefined 
> symbol: _Z13fetchRobotGameEventv
>
> ...which means that the Java 'System.out.load' function was unable
> to resolve the symbols that the library needed from the main C++
> application.
>
> This is odd because if I make the C++ code explicitly load
> the library using
>
>   dlopen("/usr/local/lib/libBotzillaInterface.so". RTLD_NOW);
>
> ...then the symbols all resolve OK - but I still get the runtime
> error from Java's System.out.load call.
>
> So it looks like Java is somehow creating an entirely new
> namespace for things it loads.
>
>   
>> http://java.sun.com/j2se/1.5.0/docs/index.html
>> http://java.sun.com/j2se/1.5.0/docs/guide/jni/index.html
>> http://java.sun.com/j2se/1.5.0/docs/guide/jni/spec/jniTOC.html
>>     
>
> That's where I got my information to get this far.  There doesn't
> seem to be any mention of this kind of thing.
>
> All of the examples I could find were very simple and involved EITHER
> C++ calling Java OR Java calling C++.  My problem is with C++ calling
> Java which in turn calls C++ which in turn calls the main application.
>
> None of the examples I could find were anywhere near that complex.
>
>   
>> Q: Any reason why they can't just port the whole thing into Java?
>>     
>
> Well, I'd have to do that port - and:
>
> a) I'm no good at Java - the kids are still learning what a 'class'
>     is - so they can't do it.
> b) It's a HUGE C++ program (over 150,000 lines of code including all
>     of the game libraries it uses) and porting it to Java would be an
>     insane effort.
> c) Java just isn't fast enough to write things like intense 3D
>     computer games in which robotic dinosaurs rampage through the
>     streets with explosions and stuff.  The only way to make Java
>     fast enough is to pull out the heavy duty stuff and write it
>     in C++...which is what I'm doing.
>
> The idea was just to plug in some SIMPLE Java 'scripting' so the
> kids could write short programs to drive the robotic dinosaurs
> around the city.  This would be a very satisfying thing for them
> to work on compared to the dry problems they are given in their
> Java programming class at school.
>
> All I want here is to dump some really simple 'plugins' into my
> game - without changing the game hardly at all.
>
>   
>> It has been a compiled native language since 1.3 came out, over 5 years 
>> ago.  The Swing (or maybe the SWT - stay away from AWT though, unless 
>> you a sado-masochist!) libraries are probably more than up to the task.  
>> And if they are not, OpenGL has been available since 1.4 (about 3 years 
>> ago).  For me to port a medium size C-Module (for Serial Device Control) 
>> into Java took about two weeks, including debugging (those command codes 
>> and global arrays were the killers!).
>>     
>
> Well, I was hoping that this would be a two evening effort.
>
>   
>> If you are still stuck after this, let me know, and I will be happy to 
>> take a closer look.
>>     
>
> Thanks!  I'd appreciate it.
>
> It really boils down to understanding how Java loads native libraries
> and whether I can control how it does that to any useful extent.
>
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