JNI accessing array of array

Question

I am trying to write a benchmark for different sorting algorithms in java, for that I wanted to explore the world of JNI. I am trying to sort a set of int arrays which are contained in an array. This is my Java header:

public static native void sort(int[][] c);

I used javah to generate this C header for it:

JNIEXPORT void JNICALL Java_org_jku_ssw_prsw2_ue6_jni_JNISorter_sort (JNIEnv *env, jclass cls, jobjectArray objArr)

Then I try to get the length of the array with

jsize len = (*env)->GetArrayLength(env, objArr);

but from here I'm kinda stuck, I can get a jobject element from this array using GetObjectArrayElement but how do I procceed from there to get a jint array which I can then finally sort?

Jorn Vernee · Accepted Answer · 2018-06-15 19:22:45Z

4

Since you know the jobject you retrieve is actually an int[], you can safely cast it to a jintArray and then go from there e.g.:

JNIEXPORT void JNICALL Java_Main_sort(JNIEnv *env, jclass _, jobjectArray objArr) {
    jsize len = (*env)->GetArrayLength(env, objArr);

    for(int i = 0; i < len; i++) {
        jintArray arr = (jintArray) (*env)->GetObjectArrayElement(env, objArr, i);
        jsize innerLen = (*env)->GetArrayLength(env, arr);
        jint* vals = (*env)->GetIntArrayElements(env, arr, NULL);            

        /* make changes to 'vals' */

        (*env)->ReleaseIntArrayElements(env, arr, vals, JNI_COMMIT);
        (*env)->DeleteLocalRef(env, arr);
    }
}

edited Jun 15, 2018 at 19:22

answered Jun 14, 2018 at 19:36

Jorn Vernee

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3 Comments

user207421 Over a year ago

Tha'ts not a cast, it is a function call.

Jorn Vernee Over a year ago

@EJP What do you mean? The jintArray arr = (jintArray) ... uses a cast.

Andrew Henle Over a year ago

You probably should add a ( *env )->DeleteLocalRef( env, arr ); call at the end of your for loop. With a large object array, you might fill the local reference table. See stackoverflow.com/questions/24289724/…

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