Use arrays

JNI provides special functions (relative to the type) to access Java arrays.

This example returns the maximum of an int array.
JNIEXPORT jint JNICALL Java_JavaHowTo_max(JNIEnv * env, jclass obj, jintArray arr) { int i; jsize len = env->GetArrayLength(arr, max = -1; jint *body = env->GetIntArrayElements(arr, 0); for (max = body[0], i=1; i < len; i++) if (max < body[i]) max = body[i]; env->ReleaseIntArrayElements(arr, body, 0); return max; }
The Java wrapper
class JavaHowTo { public static native int max(int [] t); static { System.loadLibrary("javahowto"); } }
The test program
class JNIJavaHowTo { public static void main(String[] args) { int [] myArray = {4, 7, 5, 9, 2, 0, 1}; System.out.println(JavaHowTo.max(myArray)); } }

This following how-to is currently broken...
Without passing the array directly to the JNI function, it's possible to fetch directly from the class the array.
JNIEXPORT jint JNICALL Java_JavaHowTo_printArray(JNIEnv * env, jclass obj) { jfieldID fid; jobject jtab; jsize len; int i; jclass cls = env->GetObjectClass(obj); fid = env->GetFieldID(cls, "myArray", "[I"); if (fid == 0) return; jtab = env->GetObjectField(obj, fid); tab = env->GetIntArrayElements(jtab, 0); len = env->GetArrayLength(jtab); printf("int [] : ["); for (i = 0; i < len ; i++) printf("%d ", tab[i]); printf("]\n"); }