activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/activity_frame_layout"
    tools:context="com.example.myapplication.MainActivity">
    <org.opencv.android.JavaCamera2View
        android:id="@+id/CameraView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="visible" />
</FrameLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapplication">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.MyApplication">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

    <!--Allow to use a camera-->
    <uses-permission android:name="android.permission.CAMERA"/>
    <uses-feature android:name="android.hardware.camera" android:required="false"/>
    <uses-feature android:name="android.hardware.camera.autofocus" android:required="false"/>
    <uses-feature android:name="android.hardware.camera.front" android:required="false"/>
    <uses-feature android:name="android.hardware.camera.front.autofocus" android:required="false"/>
</manifest>

MainActivity.java

//package com.example.myapplication;
//
//import androidx.appcompat.app.AppCompatActivity;
//
//import android.graphics.Bitmap;
//import android.graphics.ImageFormat;
//import android.hardware.camera2.CameraManager;
//import android.media.Image;
//import android.media.ImageReader;
//import android.os.Bundle;
//import android.os.Handler;
//import android.os.HandlerThread;
//import android.util.Log;
//
//import org.opencv.core.Core;
//import org.opencv.core.CvType;
//import org.opencv.core.Mat;
//import org.opencv.core.MatOfByte;
//import org.opencv.imgcodecs.Imgcodecs;
//import org.opencv.imgproc.Imgproc;
//
//import java.nio.ByteBuffer;
//import java.util.ArrayList;
//
//public class MainActivity extends AppCompatActivity {
//
//    private ImageReader mImageReader;
//    private HandlerThread mCameraThread;
//    private Handler mCameraHandler;
//    private Camera mCamera;
//
//    @Override
//    public void onResume() {
//        super.onResume();
//        System.loadLibrary("opencv_java4");
////        mOpenCvCameraView.enableView();
//    }
//
//    @Override
//    protected void onCreate(Bundle savedInstanceState) {
//        super.onCreate(savedInstanceState);
//        setContentView(R.layout.activity_main);
//
//        mCameraThread = new HandlerThread("CameraBackground");
//        mCameraThread.start();
//        mCameraHandler = new Handler(mCameraThread.getLooper());
//
//        mCamera = Camera.getInstance();
//        mCamera.initializeCamera(this, mCameraHandler, mOnImageAvailableListener);
//        try {
//            Thread.sleep(1000);
//        } catch (InterruptedException e) {
//            e.printStackTrace();
//        }
//        mCamera.takePicture();
//    }
//
//    private ImageReader.OnImageAvailableListener mOnImageAvailableListener =
//            new ImageReader.OnImageAvailableListener() {
//        @Override
//        public void onImageAvailable(ImageReader reader) {
//            Log.e("MYTAG", "IMAGE");
//            Image image = reader.acquireLatestImage();
//            // get image bytes
//            ByteBuffer imageBuf = image.getPlanes()[0].getBuffer();
//            Mat buf = new Mat(1, imageBuf.remaining(), CvType.CV_8UC1, imageBuf);
//            image.close();
//            Mat img = Imgcodecs.imdecode(buf, Imgcodecs.IMREAD_COLOR);
//
//            Log.e("MYTAG", "" + img.cols() + " " + img.rows() + " " + img.channels());
//            Log.e("MYTAG", "" + android.os.Build.VERSION.SDK_INT);
//
//            // Drawing
//            Mat rgba = new Mat();
//            Imgproc.cvtColor(img, rgba, Imgproc.COLOR_RGB2RGBA, 4);
//            Bitmap outputBmp = Bitmap.createBitmap(rgba.cols(), rgba.rows(), Bitmap.Config.ARGB_8888);
//        }
//    };
//}

package com.example.myapplication;

import android.Manifest;
import android.content.pm.PackageManager;
import android.graphics.Camera;
import android.os.Bundle;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

import androidx.core.content.ContextCompat;

import org.opencv.android.CameraActivity;
import org.opencv.android.CameraBridgeViewBase;
import org.opencv.android.CameraBridgeViewBase.CvCameraViewFrame;
import org.opencv.android.CameraBridgeViewBase.CvCameraViewListener2;

import org.opencv.core.Mat;
import org.opencv.imgproc.Imgproc;

import java.security.Policy;
import java.util.Collections;
import java.util.List;

public class MainActivity extends CameraActivity implements CvCameraViewListener2 {

    @Override
    public void onResume() {
        super.onResume();
        System.loadLibrary("opencv_java4");
        mOpenCvCameraView.enableView();
    }

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

        // Set up camera listener.
        mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.CameraView);
        mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE);
        mOpenCvCameraView.setCameraPermissionGranted();
        mOpenCvCameraView.setCvCameraViewListener(this);
    }

    @Override
    protected List<? extends CameraBridgeViewBase> getCameraViewList() {
        return Collections.singletonList(mOpenCvCameraView);
    }

    @Override
    public void onCameraViewStarted(int width, int height) {
    }

    @Override
    public void onCameraViewStopped() {
    }

    public Mat onCameraFrame(CvCameraViewFrame inputFrame) {
        Mat frame = inputFrame.rgba();
        return frame;
    }

    private CameraBridgeViewBase mOpenCvCameraView;
}