Sample working JNI code to get address of given object, to get object at given address and pin/unpin given object to memory for not to be collected by GC

gistfile1.txt

/**
 * @author SERKAN OZAL
 *         
 *         E-Mail: <a href="mailto:[email protected]">[email protected]</a>
 *         GitHub: <a>https://github.com/serkan-ozal</a>
 */

#include <jni.h>
#include "tr_com_serkanozal_jvm_playground_MemoryUtil.h"

void *p32 = NULL;
void *p64 = NULL;

JNIEXPORT jobject JNICALL Java_tr_com_serkanozal_jvm_playground_MemoryUtil_pinObject(
		JNIEnv *env, jclass ownerClass, jobject object) {
	return (*env)->NewGlobalRef(env, object);
}

JNIEXPORT void JNICALL Java_tr_com_serkanozal_jvm_playground_MemoryUtil_unpinObject(
		JNIEnv *env, jclass ownerClass, jobject object) {
	return (*env)->DeleteGlobalRef(env, object);
}

JNIEXPORT jlong JNICALL Java_tr_com_serkanozal_jvm_playground_MemoryUtil_getAddress(
		JNIEnv *env, jclass ownerClass, jobject object, jbyte addressSize) {
	jlong address = NULL;
	if (addressSize == SIZE_4_BYTE || addressSize == SIZE_32_BIT) {
		memcpy(&address, object, SIZE_4_BYTE); 
	}
	else if (addressSize == SIZE_8_BYTE || addressSize == SIZE_64_BIT) {
		memcpy(&address, object, SIZE_8_BYTE);
	}
	return address;
}

JNIEXPORT jobject JNICALL Java_tr_com_serkanozal_jvm_playground_MemoryUtil_getObject(
		JNIEnv *env, jclass ownerClass, jlong address, jbyte addressSize) {
	void *p = NULL;
	if (addressSize == SIZE_4_BYTE || addressSize == SIZE_32_BIT) {
		if (p32 == NULL) {
			p32 = malloc(SIZE_4_BYTE);
		}
		memcpy(p32, &address, SIZE_4_BYTE);
		p = p32;
	}
	else if (addressSize == SIZE_8_BYTE || addressSize == SIZE_64_BIT) {
		if (p64 == NULL) {
			p64 = malloc(SIZE_8_BYTE);
		}
		memcpy(p64, &address, SIZE_8_BYTE);
		p = p64;
	}	
	return (jobject)p;
}