Groups > comp.lang.java.programmer > #19001

Re: JNI return jobjectArray

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From	Philipp Kraus <philipp.kraus@flashpixx.de>
Newsgroups	comp.lang.java.programmer
Subject	Re: JNI return jobjectArray
Date	Mon, 1 Oct 2012 00:23:55 +0200
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On 2012-09-30 23:56:57 +0200, markspace said:

> On 9/30/2012 1:23 PM, Philipp Kraus wrote:
>> Hello,
>> 
>> I hope this question is not OT.
>> 
>> I create a JNI call for this Java method
>> 
>> class myclass {
>> 
>> void native mymethod( Double[] x )
>> }
>> 
>> so the parameter x should be a call-by-reference, I would set x in the
>> native JNI function:
>> 
>> void mymethod( JNIEnv *jenv, jclass jcls, jobjectArray& jarg )
>> {
>> // do something
>> jobjectArray t = .....
>> 
>> jarg = t;
>> }
>> 
>> How can I set the jobjectArray& back, so the data is return in the
>> parameter x?
> 
> 
> You can't.  Java doesn't have a pass-by-reference call scheme, at all. 
> Java uses only pass by value.  You could:
> 
> 1. Return the new array as a return value.
> 2. Create a reference to a reference so you can replace the 2nd one:
> 
>  > class myclass {
>  >
>  >     void native mymethod( Double[][] x )
>  > }
> 
> will give you a pointer to an array which you can then modify.
> 
> A Google search for "Java pass by reference" will give you lots of 
> details.  Maybe add "jni" to get some clues more specific results for 
> your particular situation.

You are not right, because the problem is not the Java side, it is the 
JNI (C/C++) side, so under C/C++ exists call-by-reference.

In detail I have got a native (C) methode, that shows in Java:

class myclass {
	native void mymethod( Double[] x, Double[][] y )
}

The JNI glue code generates a

void mymethod (JNIEnv* jnv, jclass jcls, jobjectArray arg1, jobjectArray arg2 )

So the reference to the underlaying heap object is passed in the JNI 
code parameters arg1 & arg2,
so I would modify this two heap adresses.

If I call in Java this code:

Double[] x = null;
Double[][] y = null;

myclass.mymethod(x, y);
// x & y are now filled with data


in x & y should be create a new array object, so I need a 
GlobalReference on the heap of the VM
and push back the reference. I can create the global object on the VM 
heap, but how can I push back
the reference

Thread

JNI return jobjectArray Philipp Kraus <philipp.kraus@flashpixx.de> - 2012-09-30 22:23 +0200
  Re: JNI return jobjectArray markspace <-@.> - 2012-09-30 14:56 -0700
    Re: JNI return jobjectArray Philipp Kraus <philipp.kraus@flashpixx.de> - 2012-10-01 00:23 +0200
      Re: JNI return jobjectArray markspace <-@.> - 2012-09-30 19:21 -0700
        Re: JNI return jobjectArray Lew <lewbloch@gmail.com> - 2012-09-30 21:14 -0700
          Re: JNI return jobjectArray markspace <-@.> - 2012-10-01 08:32 -0700
      Re: JNI return jobjectArray "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2012-10-01 09:39 +0100
  Re: JNI return jobjectArray Roedy Green <see_website@mindprod.com.invalid> - 2012-10-01 10:21 -0700
  Re: JNI return jobjectArray Steven Simpson <ss@domain.invalid> - 2012-10-01 23:46 +0100

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