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I have found large amounts of documentation on how to generate a 2D primitive array in JNI and returning it to Java. But these pieces of information fail to describe how to pass an already existing 2D float array (float**) given a context in C.

To describe my issue explicitly, I'll add some C pseudo code of what I would like to implement:

// Returns a 2D float array from C to Java jfloatArray ndk_test_getMy2DArray(JNIEnv* env, jobject thiz, jlong context) { // Cast my context reference MyContextRef contextRef = (MyContextRef) context; // In case we need it below unsigned int length = MyContextGet1DLength(contextRef); // Get the 2D Array we want to "Cast" float** primitive2DArray = MyContextGet2DArray(contextRef); // Hokus pokus... // We do something to create the returnable data to Java // // Below is the missing piece that would convert the primitive // 2D array into something that can be returned consumed and consumed // by Java jfloatArray myReturnable2DArray return myReturnable2DArray; }

I'm assuming this is not straight forward, given I haven't been able to find anything describing this scenario.

Thanks for any helpful information.

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3 Answers 3

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Thanks Timo for your help and link. For posterity, I'm adding a complete code set that would go through the process of generating a 2D primitive array consumable by Java, from an existing C 2D primitive array.

// Returns a 2D float array from C to Java jobjectArray ndk_test_getMy2DArray(JNIEnv* env, jobject thiz, jlong context) { // Cast my context reference MyContextRef contextRef = (MyContextRef) context; // Get the length for the first and second dimensions unsigned int length1D = MyContextGet1DLength(contextRef); unsigned int length2D = MyContextGet2DLength(contextRef); // Get the 2D float array we want to "Cast" float** primitive2DArray = MyContextGet2DArray(contextRef); // Get the float array class jclass floatArrayClass = (*env)->FindClass(env, "[F"); // Check if we properly got the float array class if (floatArrayClass == NULL) { // Ooops return NULL; } // Create the returnable 2D array jobjectArray myReturnable2DArray = (*env)->NewObjectArray(env, (jsize) length1D, floatArrayClass, NULL); // Go through the firs dimension and add the second dimension arrays for (unsigned int i = 0; i < length1D; i++) { jfloatArray floatArray = (*env)->NewFloatArray(env, length2D); (*env)->SetFloatArrayRegion(env, floatArray, (jsize) 0, (jsize) length2D, (jfloat*) primitive2DArray[i]); (*env)->SetObjectArrayElement(env, myReturnable2DArray, (jsize) i, floatArray); (*env)->DeleteLocalRef(env, floatArray); } // Return a Java consumable 2D float array return myReturnable2DArray; }
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3

I've made a simple function for conversion:

jobjectArray getJNIArray(JNIEnv *env, jobject obj, const vector<vector<float> >& arr) { jclass floatClass = env->FindClass("[F"); // jsize height = arr.size(); // Create the returnable 2D array jobjectArray jObjarray = env->NewObjectArray(height, floatClass, NULL); // Go through the first dimension and add the second dimension arrays for (unsigned int i = 0; i < height; i++) { jfloatArray floatArray = env->NewFloatArray(arr[i].size()); env->SetFloatArrayRegion(floatArray, (jsize) 0, (jsize) arr[i].size(), (jfloat*) arr[i].data()); env->SetObjectArrayElement(jObjarray, (jsize) i, floatArray); env->DeleteLocalRef(floatArray); } return jObjarray; }
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Unfortunately I don't think you can pass C floats up to Java, you'll have to turn the array into a 2D array of jfloats by converting every member into a jFloat.

Essentially, you'll have to create the multidimensional jFloatArray, then iterate through the native C array, convert every element into its jFloat representation and store it into the same position in the jFloatArray you just created.

This part of the documentation should explain it in a bit more depth.

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Yes, I'm aware of this limitation. I'm sorry if it wasn't clear in my question, but what I'm asking is about that process. How can I convert that primitive 2D array into something that can be consumed by Java.
@Claudia, I've just added a link to an example in the JNI documentation.
I've added a full code set that goes through the process. Thanks for your help.

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