CODEWITHSUNDEEP
javaandroid-ndkjava-native-interface
vitakot
vitakot

Reputation: 3844

Passing byte array from native code to the Java side

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);

Upvotes: 14

Views: 14231

Answers (3)

Shiv Buyya
Shiv Buyya

Reputation: 4130

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));
}

Upvotes: 3

Alexander Ushakov
Alexander Ushakov

Reputation: 5399

You can also use SetByteArrayRegion for setting jbyteArray content:

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

Upvotes: 0

Mārtiņš Možeiko
Mārtiņš Možeiko

Reputation: 12927

Try calling ReleasePrimitiveArrayCritical before you call CallVoidMethod.

Upvotes: 6

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