JNI - Arguments Passing: Object Arrays

• Accessing Object Arrays
1. (example) ObjectArrayTest.java: calls a native method to create a two-dimensional array of int and then prints the content of the array.

class ObjectArrayTest {
    private static native int[][] initInt2DArray(int size);
    public static void main(String[] args) {
        int len = 5;
        int[][] i2arr = initInt2DArray(len);
        for (int i = 0; i < len; i++) {
            for (int j = 0; j < len; j++) {
                System.out.print(" " + i2arr[i][j]);
            }
            System.out.println();
        }
    }
    static {
        System.loadLibrary("ObjectArrayTest");
    }
}

2. Generate C/C++ header file
ObjectArrayTest.h

/* DO NOT EDIT THIS FILE - it is machine generated */                           
#include <jni.h>
/* Header for class ObjectArrayTest */

#ifndef _Included_ObjectArrayTest
#define _Included_ObjectArrayTest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     ObjectArrayTest
 * Method:    initInt2DArray
 * Signature: (I)[[I
 */
JNIEXPORT jobjectArray JNICALL Java_ObjectArrayTest_initInt2DArray
  (JNIEnv *, jclass, jint);

#ifdef __cplusplus
}
#endif
#endif

3. Implementation of ObjectArrayTest.c
ObjectArrayTest.c
• The static native method initInt2DArray creates a two-dimensional array of the given size.

JNIEXPORT jobjectArray JNICALL
 Java_ObjectArrayTest_initInt2DArray(JNIEnv *env, jclass cls, int size)
 {
     jobjectArray result;
     int i;
     jclass intArrCls = (*env)->FindClass(env, "[I");
     if (intArrCls == NULL) {
         return NULL; /* exception thrown */
     }
     result = (*env)->NewObjectArray(env, size, intArrCls,
                                     NULL);
     if (result == NULL) {
         return NULL; /* out of memory error thrown */
     }
     for (i = 0; i < size; i++) {
         jint tmp[256];  /* make sure it is large enough! */
         int j;
         jintArray iarr = (*env)->NewIntArray(env, size);
         if (iarr == NULL) {
             return NULL; /* out of memory error thrown */
         }
         for (j = 0; j < size; j++) {
             tmp[j] = i + j;
         }
         (*env)->SetIntArrayRegion(env, iarr, 0, size, tmp);
         (*env)->SetObjectArrayElement(env, result, i, iarr);
         (*env)->DeleteLocalRef(env, iarr);
     }
     return result;
 }

• FindClass(): to obtain a reference of the element class of the two-dimensional int array.
• FindClass() returns NULL and throws an exception if class loading fails. (for example, a missing class file or an out-of-memory condition)
• The "[I" argument to FindClass is the JNI class descriptor that corresponds to the int[ ] type in the Java programming language.
• NewObjectArray(): allocates an array whose element type is denoted by the intArrCls class reference.
• NewIntArray(): allocates the individual array elements.
• SetIntArrayRegion(): copies the contents of the tmp[ ] buffer into the newly allocated one-dimensional arrays.
• SetObjectArrayElement(): sets the jth element of the ith one-dimensional array has value i+j.
• DeleteLocalRef(): ensures that the virtual machine does not run out of the memory used to hold JNI references such as iarr.
4. Generate shared library file - libObjectArrayTest.so

cc -shared -I/usr/local/jdk1.6.20_21/include -I/usr/local/jdk1.6.20_21/include/linux ObjectArrayTest.c -o libObjectArrayTest.so

5. Run ObjectArrayTest output (size=5):

0 1 2 3 4
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
4 5 6 7 8