How to build JNI App in Android

1 - Open Android Studio
2 - Click "Start a new Android Studio Project"
3 - Name your app whatever you want

4 - Check "Include C++ Support"
5- Click "Next"
6 - "Phone and Tablet" helps you to set minimum Android Version, you can change it but leave the default value and click "Next"
7 - Choose Empty Activity and click "Next"
8 - Name your activity class or leave the default name and click "Finish"
9 - Wait until finishes
10 - Under project directory go to app > res > layout > activity_main.xml and write:


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


<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">


<TextView
android:id="@+id/txtViewID"
android:layout_width="match_parent"
android:layout_height="30dp"
android:gravity="center"
android:text="........"/>


<EditText
android:id="@+id/txtInputID"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:hint="type.."/>


<Button
android:id="@+id/enterBtnID"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Enter"/>

</LinearLayout>

</LinearLayout>

11 - Under project directory go to app > java> com.android.app > MainActivity and write:

package com.android.jniapp;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends Activity implements View.OnClickListener {

// Used to load the 'native-lib' library on application startup.
static {
System.loadLibrary("native-lib");
}

private TextView txtView;
private EditText txtInput;
private Button enterBtn;

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


txtView = findViewById(R.id.txtViewID);
txtInput = findViewById(R.id.txtInputID);
enterBtn = findViewById(R.id.enterBtnID);
enterBtn.setOnClickListener(this);


}

public native int getTxtLen(String txt);

private void enterText(){
String txt = txtInput.getText().toString();
int txtLen = getTxtLen(txt);
txtView.setText("Text Length: " + txtLen);
}


@Override
public void onClick(View view) {
if (view == enterBtn){
enterText();
}
}
}


12 - Under project directory go to app > cpp> native-lib.cpp and write:


#include <jni.h>
#include <string>
#include <string.h>

extern "C"
JNIEXPORT jint JNICALL
Java_com_android_jniapp_MainActivity_getTxtLen(JNIEnv *env, jobject instance, jstring txt_) {
const char *txt = env->GetStringUTFChars(txt_, 0);
int len = strlen(txt);


return len;
}

13 - Click "Run" button
14 - Under "Connected Devices" choose your device and click "OK"
15 - Wait until Android Studio builds and installs the app into your phone
16 - Congratulations, you build your first Android App


Full Video: