How To Use Java Arrays In C?

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The JNI equivalent of an array is jarray. To access an array from a Java program in a C program, you must copy the array into a buffer defined in the C program. The JNI provides two methods that enable you to access arrays from a Java program in C:

Get<type>ArrayRegion: Copies the contents of an array into a buffer.

Set<type>ArrayRegion: Copies the contents of a buffer into an array.

For example, to copy the contents of an integer array, n, with 10 elements into a buffer, buf, you need to include the following in the C code:

Code:

(*exeenv)->GetIntArrayRegion(exeenv, n, 0, 10, buf)

Accessing An Integer Array In C

Code:

class ArrayDemo 
{
	private native void disArray(int[] n);
	public static void main(String[] args) 
	{
		ArrayDemo dem = new ArrayDemo();
		//Initializing the array
		int n[] = {45,78,35,34,65,32,98,11,3,67};
		//Invoking the native method
		dem.disArray(n);
	}
	static
	{
		//Loading the library
		System.loadLibrary("ArrayDemo_lib");
	}
}

This code shows how an array is initialized. The array is then passed as a parameter to a native method. The implementation of the native method is provided by the ArrayDemo_lib library.

Following code shows how the header file that corresponds to the ArrayDemo class file is generated.

Code:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class ArrayDemo */
#ifndef _Included_ArrayDemo
#define _Included_ArrayDemo
#ifdef __cplusplus
extern "C" 
{
	#endif
	/*
	 * Class:     ArrayDemo
	 * Method:    disArray
	 * Signature: ([I)V
	 */
	JNIEXPORT void JNICALL Java_ArrayDemo_disArray (JNIEnv *, jobject, jintArray);
	#ifdef __cplusplus
}
#endif
#endif

This code shows how to generate the header file for the ArrayDemo class file.

Implementation of the disArray Method

Code:

#include <stdio.h>
#include <jni.h>
#include "ArrayDemo.h"
JNIEXPORT void JNICALL 
Java_ArrayDemo_disArray(JNIEnv *exeenv, jobject javaobj, jintArray n)
{
     jint *buf;
     jint i;
	 printf("Printing the values of the array in C\n");
	 //Storing the elements of the array in a buffer
     buf = (*exeenv)->GetIntArrayElements(exeenv, n, NULL);
     if (buf == NULL) 
	 {
         return; /* exception occurred */
     	 }
     for (i=0; i<10; i++) 
	 {
         //Printing the array elements
         printf("%d\n",buf[i]);
     	 }
     (*exeenv)->ReleaseIntArrayElements(exeenv, n, buf, 0);
     
 }

The above code shows how an array of integers from Java code are accepted and stored in a buffer defined in C. The array is then depicted in the C program.

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