Array of array in JNI

Question

I have a problem that I've been stuck on for the last few days. I can get one array back from JNI side to java side, but I couldn't get two dimension array back from JNI to JAVA side
JNI side
JNIEXPORT jint JNICALL Java_JNITest_TestCall
(JNIEnv *env, jclass instance, jobject outJava)
OutStruct outObj; // local structure that the C function fills in
jintArray row_flags = (jintArray)(*env)->NewIntArray(env, 5);
jobjectArray arrayOfArrays_tmp = (jobjectArray)(*env)->NewObjectArray(env, 3, (*env)->GetObjectClass(env, row_flags), 0);
jintArray intArr_tmp = (*env)->NewIntArray(env, 5);
jclass objClass = (*env)->GetObjectClass(env,outJava);
jfieldID intNum = (*env)->GetFieldID(env, objClass, "javaIntNum", "I");
jfieldID intArr = (*env)->GetFieldID(env, objClass, "javaIntArr", "[I");
jfieldID arrayOfArrays = (*env)->GetFieldID(env,objClass,"javaArrayOfArrays","[Ljava.lang.reflect.Array");
// call C function to fill in outObj
(*env)->SetIntField(env, outJava,intNum,outObj.bag_roi->xmin);
for (j=0;j<=outObj.bulks->number_of_bulks;j++) {
(*env)->SetIntArrayRegion(env,intArr_tmp,j,1,&outObj.bulks[j].xmin); // this works
(*env)->SetIntArrayRegion(env,row_flags,0,5,&outObj.bulks[j].flags[0]); // this doesn't work
// I want an array of outObj.bulks[j].flags (which is an array of ints) in my java class
(*env)->SetObjectArrayElement(env,arrayOfArrays_tmp,j,row_flags); // this doesn't either
(*env)->SetObjectField(env,outJava,intArr,intArr_tmp);
Java class that corresponds with outJava is as follows:
import java.io.*;
import java.util.*;
public class JOutClass{
public int javaIntNum;
public int[] javaIntArr = new int[5];
public int[][] javaArrayOfArrays = new int[3][5];

Answer 1

Hi, maybe you can glean something from this test case I created when I was fighting with this. We seem to have taken slightly different approaches, but see if it helps at all. The trouble might be that you did not release the array(s) back to the java memory space with ReleaseIntArrayElements.
Also, because the hardcoded array lengths seem unavoidable ...remember that a public final static int field in the class that you compile against jni will also be available in the .h file (not shown in this example). It's still hardcode ...but at least you can alter the constant in the Java class without having to recompile the native code. Anyway, here you go. I have a 3D array example as well ...but that was just plain sick, so I will spare you the nausea.
File TestRaster2DImp.c
#include <jni.h>
#include "TestRaster2D.h"
JNIEXPORT void JNICALL
Java_TestRaster2D_nextGeneration(JNIEnv *env, jobject obj, jobjectArray arr)
  int i, j;
  jint *body[2];
  jintArray oneDim[2];
  jsize len1;
  jsize len2;
  oneDim[0] = (*env)->GetObjectArrayElement(env, arr, 0);
  len1 = (*env)->GetArrayLength(env, arr);
  len2 = (*env)->GetArrayLength(env, oneDim[0]);
  for(i=0; i<len1; i++) {
    oneDim[i] = (*env)->GetObjectArrayElement(env, arr, i);
    body[i] = (*env)->GetIntArrayElements(env, oneDim, 0);
for(j=0; j<len2; j++) {
body[i][j]++;
printf("%d\n", body[i][j] );
printf("\nOut: %d\n", body[0][2] );
printf("Out: %d\n", body[1][2] );
(*env)->ReleaseIntArrayElements(env, oneDim[0], body[0], 0);
(*env)->ReleaseIntArrayElements(env, oneDim[1], body[1], 0);
File TestRaster2D.java
public class TestRaster2D {
  int[][] ra = {
    { 1, 2, 3, 4, 5 },
    { 11, 12, 13, 14, 15 }
  public TestRaster2D() {
    System.out.println( " is " + ra[0][2] );
    System.out.println( " is " + ra[1][2] );
    System.out.print( "\n" );
    nextGeneration( ra );
    System.out.print( "\n" );
    System.out.println( " is " + ra[0][2] );
    System.out.println( " is " + ra[1][2] );
  public native void nextGeneration( int[][] theArray );
  static {
    System.loadLibrary("raster2D");
  public static void main(String[] args) {
    new TestRaster2D();
}Good luck.