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.

I believe that you could find an answer here-> http://stackoverflow.com/questions/21951711/how-to-pass-a-jni-c-sharp-class-into-java-or-handle-this- or create your custom java call using the Oracle documentation -> situationhttp://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/design.html#wp715 Anyway is a very good question. — Cabrra, Aug 12 '16 at 17:13

score 5 · Answer 1 · edited May 23 '17 at 12:08

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.

This helped, and this works! I confirmed with implementation. — Maestro, Oct 18 '17 at 19:28

Vyacheslav Gerasimov · Answer 2 · 2016-08-17T22:14:51.487

4

I think you can create another native plugin which will give you JNIEnv.

Plugins for Android

Summarizing this article you should try something like this (untested pseudo code):

JavaVM* g_JVM; // JavaVM is valid for all threads, so just save it globally 

extern "C" {
    JNIEnv* GetJniEnv();
}

// The VM calls JNI_OnLoad when the native library is loaded
jint JNI_OnLoad(JavaVM* vm, void* reserved) {
    g_JVM = vm;
    return JNI_VERSION_1_6;
}

// The JNI interface pointer (JNIEnv) is valid only in the current thread.
JNIEnv* GetJniEnv() {
    JNIEnv* jni_env = 0;
    g_JVM->AttachCurrentThread(&jni_env, 0);
    return jni_env;
}

Then in C#

[DllImport ("PluginName")]
private static extern IntPtr GetJniEnv();

edited Aug 17 '16 at 22:14

answered Aug 17 '16 at 21:59

Vyacheslav Gerasimov

1,334
9
10

1

I think JNI_OnLoad is only invoked if a Java module explicitly requests loading the library (System.loadLibrary). – Maestro Oct 18 '17 at 19:27
Using `GetJniEnv` as the function name is confusing here as the pointer is needed in the C++ world. However I have just tested this solution from a Unity3d C# script `[DllImport("mynative")] private static extern float helloWorld();` and it works perfectly as of Unity 2021.1.25f. – Crog Feb 14 '22 at 16:34

Mark Bamford · Answer 3 · 2018-04-20T19:50:25.897

Expanding on Vyacheslav Gerasimov's answer, This code is tested and drop in without any additional work required:

JNIEnv* jni_env = NULL;
JavaVM* JVM;

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved)
{
    JVM = vm;
    if (vm->GetEnv((void**)&jni_env, JNI_VERSION_1_6) != JNI_OK)
        __android_log_print(ANDROID_LOG_ERROR, "Unity", "ERROR: GetEnv failed")

    return JNI_VERSION_1_6;
}

JNI_OnLoad is called automatically by Unity/Java

No C# is required as you can pass your jni_env or JVM variable in c++.

Getting Valid JNIEnv pointer

3 Answers3