Cannot resolve corresponding JNI function Java_com_mozilla_greetings_RustGreetings_greeting

Question

I'm trying to replicate the this tutorial for building and using Rust lib in Android app, I build the library successful, and uploaded the generated libs here

The function required to be called by Android using the Java_<Package>_Class_function theme is:

Java_com_mozilla_greetings_RustGreetings_greeting

My android app structure is as below:

I'm getting the below error at my JNI wrapper:

Cannot resolve corresponding JNI function Java_com_mozilla_greetings_RustGreetings_greeting

The JNI wrapper is:

package com.mozilla.greetings;

public class RustGreetings {

    private static native String greeting(final String pattern);

    public String sayHello(String to) {
        return greeting(to);
    }
}

And the main class is:

package com.mozilla.greetings;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class GreetingsActivity extends AppCompatActivity {

    static {
        System.loadLibrary("greetings");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_greetings);

        RustGreetings g = new RustGreetings();
        String r = g.sayHello("world");
        ((TextView) findViewById(R.id.greetingField)).setText(r);
    }
}

Answer 1

I solved it using JNA whcih I think is slower than JNI, I'll write my solution below using JNA hoping someone provide the required fix using JNI

My app structure is as below, using kotlin:

• I added the libjnidispatch.so to each library build folder, , this can be obtained by extracted the requiredandroid architecture from here, download the required jar, then extract it to get the libjnidispatch.so
• I created interbface for jna

JNA.kt

package com.mozilla.greetings

import com.sun.jna.Library

   interface JNA : Library {
      fun rust_greeting(pattern: String): String
   }

3. I created wrapper for jna, RustGreetings.kt:

   package com.mozilla.greetings
   
   import com.sun.jna.Native
   
   class RustGreetings {
   
   fun sayHello(to: String): String =
           Native.loadLibrary<JNA>("greetings", JNA::class.java).rust_greeting(to)
   }
   

4. Main activity, GreetingsActivity.kt:

   package com.mozilla.greetings
   
   import android.support.v7.app.AppCompatActivity
   import android.os.Bundle
   import kotlinx.android.synthetic.main.activity_greetings.*
   
   
   class GreetingsActivity : AppCompatActivity() {
   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       setContentView(R.layout.activity_greetings)
   
       val g = RustGreetings()
       val r = g.sayHello("Rust")
       greetingField.text = r
   }
   
   companion object {
       init {
           System.loadLibrary("greetings")
       }
   }
   }
   

Note: In order to avoid using findViewById I used kotlin extension, as explained here and addeding the below to the build.gradle (module):

apply plugin: 'kotlin-android-extensions'

UPDATE

It looks it is an IDE issue nothing with the code, the app had been executed. I re-wrote it using Kotlin, and it was executed smoothly as well, below my kotlin code:

GreetingsActivity.kt

package com.mozilla.greetings

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_greetings.*

class GreetingsActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_greetings)

        val g = RustGreetings()
        val r = g.sayHello("My Rust")
        greetingField.text = r
    }

    companion object {

        init {
            System.loadLibrary("greetings")
        }
    }
}

RustGreetings.kt

package com.mozilla.greetings

class RustGreetings {

    private external fun greeting(pattern: String): String

    fun sayHello(to: String): String = greeting(to)
}

build.gradle

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android-extensions'

apply plugin: 'kotlin-android'

apply plugin: 'kotlin-android-extensions'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.mozilla.greetings"
        minSdkVersion 28
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation"org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

activity_greetings.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".GreetingsActivity">

    <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello World!"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" android:id="@+id/greetingField"/>

</android.support.constraint.ConstraintLayout>

Below structure and execution, , the apk is here: