Access array in native code (NDK) from Java (SDK) via JNI

Question

I want to access an array that is created and updated in the native C code efficiently. If need be, i could send a pointer or reference from Java code to the native side and have the C-code populate it so that i can read it from SDK side when it's ready to be consumed.

Currently, this is how i am doing it. But i think there can be better ways to do it, since i am doing one copy in C-side and then there an object that is created every time i issue a read on the Java-side.

My Java code:

double[] valuesFromNative = getValues();
public static native double[] getValues();
static { System.loadLibrary("test-jni"); }

My native (C and not C++) code:

#define LEN 18
double testDoubleArr[LEN];
jdoubleArray Java_com_test_testActivity_getValues(JNIEnv *env, jclass clazz) {
    jboolean isCopy;
    int i;
    jdoubleArray result = (*env)->NewDoubleArray(env, LEN);
    jdouble* destArrayElems = (*env)->GetDoubleArrayElements(env, result, &isCopy);
    for (i = 0; i < LEN; i++) {
       destArrayElems[i] = testDoubleArr[i];
    }
    if(isCopy == JNI_TRUE) {
        //  isCopy should NEVER be JNI_TRUE in this case, right?
        // so, i could as well replace this condition with
        // assert(isCopy == JNI_FALSE)?
    }
return result;
}

This code snippet works - so, i am looking at more efficient or rather correct way to achieve the same thing.

Thanks for sharing your thoughts.

Answer 1

I think SetDoubleArrayRegion() would be faster. Less code and less JNI calls, that's for sure.

jdoubleArray result = (*env)->NewDoubleArray(env, LEN); 
(*env)->SetDoubleArrayRegion(env, result, 0, LEN, testDoubleArr);

You don't even have to create the array on the C++ side. Declare the method like this:

public static native void getValues(double[] a);   

Implement like this:

void Java_com_test_testActivity_getValues(JNIEnv *env, jclass clazz, jdoubleArray a)
{//...

Create the array on the Java side, cache it in an instance variable or something, and pass it to JNI to be filled whenever needed. Make sure the assumptions about array size are the same on the Java side and on the C side.