Returning jint array from c to java through jni

Question

I have created an integer array in java and passed the array to a cpp programme through jni My Code is:

import java.util.*;

class SendArray {
  //Native method declaration
  native int[] loadFile(int[] name);
  //Load the library
  static {
    System.loadLibrary("nativelib");
  }

  public static void main(String args[]) {
    int arr[] = {1,2,3,4,5,6,7,8,9,10};
    //Create class instance
    SendArray mappedFile=new SendArray();
    //Call native method to load SendArray.java
    int[] buf = mappedFile.loadFile(arr);
    //Print contents of SendArray.java
    for(int i=0;i<buf.length;i++) {
      System.out.print(buf[i]);
    }
  }
}

In cpp programme I am reversing the array and returning the array back to java programee My Code is::

#include <iostream>

using namespace std;

JNIEXPORT jintArray JNICALL Java_SendArray_loadFile
  (JNIEnv * env, jobject jobj, jintArray array) {
          cout<<"Orignal Array is:"<<endl; 
          int i;
          jboolean j;
          int ar[100];
          // for(i = 0; i < 10; i++){
          int * p= env->GetIntArrayElements(array, &j);
          //jint *array=env->GetIntArrayElements(one, 0);
          //ar[i] = array[i];
          //}

          for(i = 0 ; i < 10 ; i++){
            cout << p[i];
          }

          for(i = 10 ; i > 0 ; i--){
            ar[10-i] = p[i];
          }
          jintArray ret = env->NewIntArray(10);

          for(i = 0; i >10 ; i++){
            ret[i]=ar[i];
          }
          return ret;
}

error I am gettin is:

error: no match for 'operator=' in '*(ret +((long unsigned int)((long unsigned int)i))) = ar[i]'

What should I do to return the array back to java programme???? please help!!!!!

maba · Accepted Answer · 2012-08-09 07:58:12Z

19

Change your native code to this instead:

JNIEXPORT jintArray JNICALL Java_SendArray_loadFile(JNIEnv *env, jobject obj, jintArray oldArray) {
    const jsize length = env->GetArrayLength(oldArray);
    jintArray newArray = env->NewIntArray(length);
    jint *oarr = env->GetIntArrayElements(oldArray, NULL);
    jint *narr = env->GetIntArrayElements(newArray, NULL);

    for (int o = 0, n = length - 1; o < length; o++, n--) {
        narr[n] = oarr[o];
    }

    env->ReleaseIntArrayElements(newArray, narr, NULL);
    env->ReleaseIntArrayElements(oldArray, oarr, NULL);

    return newArray;
}

Your main problem was that you tried to manipulate the ret object directly and that is not possible. You have to use JNI functions to manipulate a jintArray object.

And you also have to make sure you release your objects when done with them.

answered Aug 9, 2012 at 7:58

maba

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3 Comments

Fynn Over a year ago

This doesn't work on (at least) java 8, should be like (*env)->GetIntArrayElements(env, oldArray, 0), others methods should me modified accordingly.

maba Over a year ago

@Fynn No it will work perfectly well with a C++ compiler. Your notation is used for Ansi C.

Newbie Over a year ago

Hi, I am executing the below code snippet vector<int> vec_data = {1,2,3,4,5}; jintArray jarray = env->NewIntArray(vec_data.size()); jint* elements = env->GetIntArrayElements(jarray, 0); for(int i=0; i<vec_data.size(); i++){ elements[i] = vec_data[i]; cout<<elements[i]<<endl; cout<<jarray[i]<<endl; } But i could see that jarray is always printing 0 or some grabage value, am i missing something here? Basically i want to return jintArray from the vector.

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