Passing byte array from native code to the Java side

问题

My goal is passing a byte array of variable length from native code to Java side. The Java class instance method takes bytearray as its argument:

private void writeBuffer(final byte[] buffer)
{
}

I am able to find a method ID in the native code:

jclass cls = (*env)->FindClass(env,"class_path");
jmethodID writeBufferMethodID = (*env)->GetMethodID(env, cls, "writeBuffer", "([B)V");

But still can't figure out how to pass a byte array properly. I have tried:

jbyteArray retArray = (*env)->NewByteArray(env, data_size);
void *temp = (*env)->GetPrimitiveArrayCritical(env, (jarray)retArray, 0);
memcpy(temp, decoded_frame->data[0], data_size);
(*env)->CallVoidMethod(env, obj, writeBufferMethodID, retArray);
(*env)->ReleasePrimitiveArrayCritical(env, retArray, temp, 0);

and also:

retArray = (*env)->NewByteArray(env, data_size);
(*env)->SetByteArrayRegion(env, retArray, 0, data_size, (jbyte *)decoded_frame->data[0]);
(*env)->CallVoidMethod(env, obj, writeBufferMethodID, retArray);

The Java method gets called, but after a while the application crashes. Moreover, all values in Java buffers I receive equal to zero so it seems the content is not copied at all.

I verified the buffers content (decoded_frame->data[0]) on the native side by writing them into the binary file and there is no problem, the file contains exactly what I expect.

I call that method periodically; the size of array can vary in each call.

What is the correct and most effective way? Allocating a new array during each call is obviously a silly idea, but I do not know, how to avoid that if the array size varies.

EDIT:

I have rewritten my code this way and it seems to be OK now.

called in a while loop:

...do some decoding...

if(!retArray)
retArray = (*env)->NewByteArray(env, data_size);

if((*env)->GetArrayLength(env, retArray) != data_size)
{
(*env)->DeleteLocalRef(env, retArray);
retArray = (*env)->NewByteArray(env, data_size);
}

void *temp = (*env)->GetPrimitiveArrayCritical(env, (jarray)retArray, 0);
memcpy(temp, decoded_frame->data[0], data_size);
(*env)->CallVoidMethod(env, obj, writeBufferMethodID, retArray);
(*env)->ReleasePrimitiveArrayCritical(env, retArray, temp, 0);

called at loop exit:

(*env)->DeleteLocalRef(env, retArray);

回答1:

Try calling ReleasePrimitiveArrayCritical before you call CallVoidMethod.

回答2:

Below example works for passing char[] from C code to Java byte[].

void JNICALL Java_com_example_testapplication_MainActivity_getJNIByteArrayArg(JNIEnv    *jenv, jobject jobj)
{
jclass clazz = (*jenv)->FindClass(jenv, "com/example/testapplication/MainActivity"); // class path
jmethodID mid = (*jenv)->GetMethodID(jenv, clazz, "addData", "([B)V");// function name

jbyteArray retArray;
char data[] = {'a','b',3,4,5};
int data_size = 5;
if(!retArray)
retArray = (*jenv)->NewByteArray(jenv, data_size);

if((*jenv)->GetArrayLength(jenv, retArray) != data_size)
{
    (*jenv)->DeleteLocalRef(jenv, retArray);
    retArray = (*jenv)->NewByteArray(jenv, data_size);
}

void *temp = (*jenv)->GetPrimitiveArrayCritical(jenv, (jarray)retArray, 0);
memcpy(temp, data, data_size);
(*jenv)->ReleasePrimitiveArrayCritical(jenv, retArray, temp, 0);

(*jenv)->CallVoidMethod(jenv, jobj, mid, retArray);
}
public void addData(byte[] data) {
System.out.println("Buyya: From C: " + new String(data));
}

回答3:

You can also use SetByteArrayRegion for setting jbyteArray content:

(*env)->SetByteArrayRegion(env, retArray, 0, data_size, decoded_frame->data[0]);

来源：https://stackoverflow.com/questions/9518855/passing-byte-array-from-native-code-to-the-java-side