Question

I am new in the Android programming world and I have already made a full search on the Internet about the JNI implementation in Android. By the way, there are lots of ways to do it, and I don’t find too much documentation about DSP in C/C++ for Android.

I have already got the audio data from a recording (PCM to float), and I would like to send the data to a C/C++ function to process this data. After the transformation I would like to return the array to the Java activity to playback the result. Is there any simple example or documentation about sending and receiving arrays with the JNI?

I am using Windows 7, Eclipse and Cygwin.

Thank you in advance

Answer 1

Should be quite simple,

To transfer a byteArray from Java to C++ you can consider the following example :

Step 1: in the java class which will call the native method declare the method (in my example a simple zip)

private native byte[] jniZipString(byte[] unpackedData, int datasize);

Step 2: in c++ generate the header and implementation using javah.exe
[note] call javah from the bin/classes foler (since your coding for android

javah -jni com.example.JavaClassWithNativeMethods

Step 3: in C++ receive the byte array and covert to C++ bytearray

jbyte* tempPointer = env->GetByteArrayElements(jInput, 0);
const char* cinput = (const char*) tempPointer;
int dataSize = (int)env->GetArrayLength(jInput)

Step 4: in process your data

whatever you do to process the bytearray

Step 5: return the resulting byte array to java

jbyteArray result = (env)->NewByteArray(newDataSize);
(env)->SetByteArrayRegion(result, 0, newDataSize, (jbyte *) processedCharArray); 
return result;