Re: JNI return jobjectArray

Philipp Kraus <>
Mon, 1 Oct 2012 00:23:55 +0200
On 2012-09-30 23:56:57 +0200, markspace said:

On 9/30/2012 1:23 PM, Philipp Kraus wrote:


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

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