Convert jobjectarray into vector<uint8_t>

Question

I'm using JNI to pass a jobjectarray from java (which is in fact a byte[][]).

I'd to convert this into a usable "byte array" in the form of uint8_t* or vector (ideally the latter) so that it is compatible with all of my existing code which uses it in those forms.

Any advice on how I can best do this? Open to using jbytearray instead if that's cleaner...

Answer 1

I'm not sure if vector<uint8_t*> would be a good idea here. A vector<vector<uint8_t>> might be better.

jsize numRows = env->GetArrayLength(objArray);
vector<vector<uint8_t>> v(numRows);

Write a loop where you get the n:th row from the jobjectArray :

jbyteArray bytes = (jbyteArray) env->GetObjectArrayElement(objArray, n);

Then get a pointer to the actual bytes in that row, and insert those bytes into your vector:

jsize numBytes = env->GetArrayLength(bytes);
uint8_t *data = (uint8_t*) env->GetByteArrayElements(bytes, NULL);
std::copy(data, data + numBytes, std::back_inserter(v[n]));
env->ReleaseByteArrayElements(bytes, JNI_ABORT);

Immediately deleting the local reference to bytes afterwards may be a good idea, to avoid overflowing the local reference table if you've got a large number of rows:

env->DeleteLocalRef(bytes);

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Although a more efficient approach than a byte[][] might be to use a java.nio.ByteBuffer obtained with allocateDirect .