Returning jbyteArray from native c in Android

Question

I use native c to read data from an audio file to jbyte pointer. Now i want to send it to java as an jbyteArray.

jbyteArray Java_com_app_audio_player_readData(JNIEnv * env, jobject jobj,jstring readPath)
{

FILE *fin;
const char *inFile= (*env)->GetStringUTFChars(env,readPath,0);
fin = fopen(inFile, "r");

fseek(fin, 0, SEEK_END); // seek to end of file
 int size = ftell(fin); // get current file pointer
fseek(fin, 0, SEEK_SET);

jbyte *data=(jbyte *)malloc(size*sizeof(jbyte));
int charCnt = 0;

charCnt=fread(data, 1, size, fin);


jbyteArray result=(*env)->NewByteArray(env, size);

     //-- I want to convert data to jbyteArray and return it to java    

fclose(fin);
 return result;

}

How it is done?

score 31 · Accepted Answer · edited May 23 '17 at 11:45

31

use SetByteArrayRegion

charCnt=fread(data, 1, size, fin);

jbyteArray result=(*env)->NewByteArray(env, size);

(*env)->SetByteArrayRegion(env, result, 0, size, data);

one could also use GetByteArrayElements eg:

jboolean isCopy;
jbyte* rawjBytes = (*env)->GetByteArrayElements(env, result, &isCopy);

//do stuff to raw bytes
memcpy(rawjBytes, data, size*sizeof(jbyte));

(*env)->ReleaseByteArrayElements(env, result, rawjBytes, 0);

see here for more details on SetByteArrayRegion, GetByteArrayElements and ReleaseByteArrayElements.

NB: this question is probably a special case of this question

edited May 23 '17 at 11:45

Community

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answered Apr 30 '12 at 13:34

violet313

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@Roman ~but the intention here *is* to use `memcpy` to copy `data` into `rawjBytes`?! & demonstrates an alternative to `SetByteArrayRegion` where lets say, large `data` needs to be constructed dynamically/incrementally: we can use `GetByteArrayElements` to gain access to the `jbyteArray` contents; then calling `ReleaseByteArrayElements` with zero-valued 3rd arg ensures data is copied back (whether-or-not the underlying memory is pinned). – violet313 Jul 24 '13 at 11:47
Ow... I'm sorry, I don't get it. Thank you very much anyway, it's helped me lot. – Roman Truba Jul 24 '13 at 11:48
@Roman ~ no problemo. & of course one could do what you're suggesting: gain access to the `rawjBytes` and `memcpy` them into some other memory location. so i appreciate the potential for confusion &&so it's good you pointed it out :) – violet313 Jul 24 '13 at 11:57
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In your SetByteArrayRegion() example, I think you're using "jbyteArray" in the args list when you mean to use "result", right? (ie: use the var name, not its type) – Trent Gray-Donald Jan 22 '14 at 02:41
@Trent Gray-Donald +1 ~blinking! imagine failing to notice that before.lol. ~fixed. – violet313 Mar 03 '14 at 00:35

Returning jbyteArray from native c in Android

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