Claudia Claudia - 4 months ago 27
Java Question

Return a 2D primitive array from C to Java from JNI/NDK

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.

Answer

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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