Getting Valid JNIEnv pointer

Question

I have a C++ dll that I want to use in Unity by exporting the functions to C#. The Unity project runs on Android devices and the C++ code makes use of java. To initialize the C++ I need to call the following function first:

void api_initialize(JNIEnv* env, jobject* app_context, jobject* class_loader) {

    JavaVM* vm = nullptr;
    env->GetJavaVM(&vm);
    if (!vm) {
      return;
    }

    //Do other proprietary things
}

In Unity I have the following exported Dll function

    [DllImport (dllName)]
    private static extern void api_initialize (IntPtr java_env, IntPtr app_context, IntPtr class_loader);

My question is How do I get a JNIEnv pointer in my C# class to then pass as a parameter into this function?

I am not the creator of this API and don't have the access to modify it, so I need to get the JavaVM from the JNIEnv, not the other way around.

Answer 1

I guess there is no way you can do this (might be there but haven't seen it yet) because this type of NDK calls required java wrapping around so i would like to purpose an another solution that use Java as an intermediate for your C# calls and then you can redirect this call to NDK code from java.

using UnityEngine;
using System.Collections;
using System.IO;

#if UNITY_ANDROID
public class JavaCallPlugin {

  // Avoid invalid calls
  public static void initiateNDK() {
    if (Application.platform != RuntimePlatform.Android)
          return;

    var pluginClass = 
        new AndroidJavaClass("com.package.class.UnityJavaDelegate");
    AndroidJavaObject plugin = 
        pluginClass.CallStatic<AndroidJavaObject>("obj");
    return plugin.Call("javaCallToNDK");
  } 
}
#endif

And in your java file do this

package com.package.class;

import android.content.ContentValues;
import android.content.Intent;
import android.os.Environment;

public class UnityJavaDelegate{
  private static UnityJavaDelegate obj;

  // define function call to your NDK method with native keyword
  private native void api_initialize(); 

  static {
    System.loadLibrary("yourlibraryname"); // setup your ndk lib to use 
  }

  public static UnityJavaDelegate getObj() {
    if(m_instance == null)
       obj= new UnityJavaDelegate();
    return obj;
  }

  private UnityJavaDelegate(){
  }

  public void javaCallToNDK(){
    // this call may not work because i haven't passed class 
    // loader TO NDK before, if not then you should try links 
    // at the bottom of the post to know how to pass customize 
    // as parameter to NDK.

    // put your call to NDK function here 
    api_initialize(this.getClass().getClassLoader()); 
  }
}

When you declare native method the javah automatically generate a ndk call definition with javaEnv and jobject as parameter so i guess you only need to pass class loader here .You can try this link and this links in case for more clarification.

Good Luck.Hope This will help.