JNI C++ object

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M. Fernandez - 28 Mar 2006 15:08 GMT

Hello,

I am working with JNI and I have a problem with my program.

In my java application there are some objects in C++ that I would like
to link them with a java object, is it possible to do?
Should I use or manage the VM?

thanks a lot,

Marcelo

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MetallimaX - 28 Mar 2006 15:53 GMT

M. Fernandez a écrit :

> Hello,
>
[quoted text clipped - 7 lines]
>
> Marcelo

Hello,
I think this is possible but be carefull of using C++ libraries. If I
remember, it is the signature of the method that determine the way it
is called from an outter program.
- Try to load a simple C library to check if your JNI is well
managed.
- Focus on the method calling in dynamic libraries en it will help
you.

Max

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M. Fernandez - 28 Mar 2006 16:28 GMT

> Hello,
>
[quoted text clipped - 7 lines]
>
> Marcelo

I have read that it is possible to create a Java object from C++
(creating a VM). However, I would like to do it the other way around. I
need to create c++ objects from java.

What are the usual methods to do this? ()
thanks a lot,

Marcelo

PS: I just need some directions for learning by myself...

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Gordon Beaton - 28 Mar 2006 17:50 GMT

> I have read that it is possible to create a Java object from C++
> (creating a VM). However, I would like to do it the other way
> around. I need to create c++ objects from java.
>
> What are the usual methods to do this? ()

The only way to do it is to add one or more native methods to a Java
class.

From one of those native methods, create your C++ objects. You cannot
create a C++ object directly from Java, nor can you access the object,
its methods or its fields directly from Java.

Read about JNI to learn how to create the necessary native methods:

http://java.sun.com/j2se/1.5.0/docs/guide/jni/index.html
http://java.sun.com/docs/books/jni/

/gordon

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Roedy Green - 28 Mar 2006 18:26 GMT

On Tue, 28 Mar 2006 16:08:36 +0200, "M. Fernandez"
<mache_1999@hotmail.com> wrote, quoted or indirectly quoted someone
who said :

>In my java application there are some objects in C++ that I would like
>to link them with a java object, is it possible to do?
>Should I use or manage the VM?

see http://mindprod.com/jgloss/jni.html

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Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.

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Roedy Green - 28 Mar 2006 18:30 GMT

On Tue, 28 Mar 2006 17:26:50 GMT, Roedy Green
<my_email_is_posted_on_my_website@munged.invalid> wrote, quoted or
indirectly quoted someone who said :

>>In my java application there are some objects in C++ that I would like
>>to link them with a java object, is it possible to do?
>>Should I use or manage the VM?
>
>see http://mindprod.com/jgloss/jni.html

In your JNI code you will need to write some glue code. The Java
objects could be allocated by Java or by C++ with JNI. The C++ objects
are allocated by C++. You have to write C++ code that copies fields
across or that invokes the corresponding method on the other object

The important point is that you can't do anything with C++ objects
directly from Java. You must do it indirectly via Java methods and
objects.

Signature

Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.

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M. Fernandez - 29 Mar 2006 01:26 GMT

> On Tue, 28 Mar 2006 17:26:50 GMT, Roedy Green
> <my_email_is_posted_on_my_website@munged.invalid> wrote, quoted or
[quoted text clipped - 13 lines]
> directly from Java. You must do it indirectly via Java methods and
> objects.

Thanks for your help, I have just one more question.

According to your explanation, it is impossible to have c++ objects
running at the same time as Java objects when JAVA is the main program
and has the control, for instance:

//Imposible jni program...
MAIN_JAVA
java declarations
Creating c++ object.

more java declarations.
using created c++ object.
continue with java.

I have understood that i should "transfer" my c++ declaration class to a
Java class in order to have java objects that may do some "native
method" calls. Is this right?

Thanks a lot,

Marcelo

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Roedy Green - 29 Mar 2006 03:54 GMT

On Wed, 29 Mar 2006 02:26:54 +0200, "M. Fernandez"
<mache_1999@hotmail.com> wrote, quoted or indirectly quoted someone
who said :

>According to your explanation, it is impossible to have c++ objects
>running at the same time as Java objects when JAVA is the main program
>and has the control, for instance:

not at all. It is just that you can't manipulate or create C++ objects
DIRECTLY from Java. You need to write native method implementation
code in C++ to allow java objects and methods to indirectly create and
manipulate the C++ objects. Your c++ code has to do the object
creating an manipulating based on clues passed from Java by native
methods and parameters.

The following is not code. So I can't comment on whether it could
work.

>//Imposible jni program...
>MAIN_JAVA
[quoted text clipped - 4 lines]
> using created c++ object.
>continue with java.

Signature

Canadian Mind Products, Roedy Green.
http://mindprod.com Java custom programming, consulting and coaching.

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M. Fernandez - 29 Mar 2006 08:37 GMT

> On Wed, 29 Mar 2006 02:26:54 +0200, "M. Fernandez"
> <mache_1999@hotmail.com> wrote, quoted or indirectly quoted someone
[quoted text clipped - 22 lines]
>> continue with java.
> v

Thank your very much, this discussion was very helpful.

Marcelo

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JNI C++ object

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