JNI pass interface as function to CPP code

I have a function in CPP code which is defined as

using Data = std::vector<byte>;


void sign(const std::function<Data(Data)> externalSigner) {
   ...
}

And I need to pass java interface as a parameter to this function.

public interface ByteArrayProvider {
    byte[] provide(byte[] data);
}

I'm able to find my interface, call and get result. But I can't figure out how to pass it to C++ function.

First you have to create a new native library inside your project under File > New > New Module... > Android Native Library. I selected Java as language.

NativeLib.java:

package com.example.nativelib;

public class NativeLib {
  static { System.loadLibrary("nativelib"); }
  public static native void sign(ByteArrayProvider bap);
}

nativelib.cpp:

#include <jni.h>
#include <vector>

Data myFunc(JNIEnv* env, jobject bap, Data data) {
  jbyteArray a = env->NewByteArray(data.size());
  env->SetByteArrayRegion(a, 0, data.size(), (const jbyte*)data.data());

  jclass cls = env->GetObjectClass(bap);
  jmethodID provide = env->GetMethodID(cls, "provide", "([B)[B");
  jbyteArray aret = (jbyteArray)env->CallObjectMethod(bap, provide, a);

  jbyte* bytes = env->GetByteArrayElements(aret , 0);
  std::vector<char> ret(bytes, bytes + env->GetArrayLength(aret));
  env->ReleaseByteArrayElements(aret, bytes, JNI_ABORT);
  return ret;
}

extern "C"
JNIEXPORT void JNICALL
Java_com_example_nativelib_NativeLib_sign(JNIEnv* env, jclass clazz, jobject data) {
    sign(std::bind(myFunc, env, data, std::placeholders::_1));
}

Then you can simply call (bap is your ByteArrayProvider):

NativeLib.sign(bap);