先贴代码，和参考的文章，后续再补充说明
参考的文章[https://blog.csdn.net/renweiyi1487/article/details/100083783]

package com.example.camera2;

import androidx.annotation.NonNull;
import androidx.annotation.RequiresApi;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.graphics.ImageFormat;
import android.hardware.Camera;
import android.hardware.camera2.CameraAccessException;
import android.hardware.camera2.CameraCaptureSession;
import android.hardware.camera2.CameraCharacteristics;
import android.hardware.camera2.CameraDevice;
import android.hardware.camera2.CameraManager;
import android.hardware.camera2.CaptureRequest;
import android.media.ImageReader;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.Looper;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.widget.Toast;

import java.util.Arrays;

@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
public class MainActivity extends AppCompatActivity {
    public static final int REQUEST_CAMERA_CODE = 100;
    private CaptureRequest.Builder mPreviewRequestBuilder;
    private CameraCaptureSession cameraCaptureSession;
    private Handler mainHandler,childHandler;
    private SurfaceView mSurfaceView;
    private  CameraManager cameraManager;
    private  String mCameraID;
    private ImageReader imageReader;
    private CameraDevice mCameraDevice;
    private SurfaceHolder mSurfaceHolder;







    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();


    }

    private CameraDevice.StateCallback stateCallback=new CameraDevice.StateCallback() {
        @Override
        public void onOpened(@NonNull CameraDevice camera) {
            mCameraDevice=camera;
            try {
                preview();
            } catch (CameraAccessException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onDisconnected(@NonNull CameraDevice camera) {
            if(mCameraDevice!=null){
                mCameraDevice.close();

            }
        }

        @Override
        public void onError(@NonNull CameraDevice camera, int error) {

        }
    };
    private void initView(){
        mSurfaceView=findViewById(R.id.surfaceView);
        mSurfaceHolder=mSurfaceView.getHolder();
        mSurfaceHolder.setKeepScreenOn(true);
        mSurfaceHolder.addCallback(new SurfaceHolder.Callback() {
            @RequiresApi(api = Build.VERSION_CODES.M)
            @Override
            public void surfaceCreated(SurfaceHolder holder) {
                initcamera();
            }

            @Override
            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

            }

            @Override
            public void surfaceDestroyed(SurfaceHolder holder) {
                if(mCameraDevice!=null){
                    mCameraDevice.close();;

                }
            }
        });
    }

    @RequiresApi(api = Build.VERSION_CODES.M)
    private void initcamera(){
        HandlerThread handlerThread=new HandlerThread("Camera2");
        handlerThread.start();
        childHandler=new Handler(handlerThread.getLooper());//???
        mainHandler=new Handler(Looper.getMainLooper());//???
        mCameraID=""+CameraCharacteristics.LENS_FACING_FRONT;
        imageReader=ImageReader.newInstance(1080,1920, ImageFormat.JPEG,1);


        cameraManager = (CameraManager)MainActivity.this.getSystemService(Context.CAMERA_SERVICE);
        try{
            if(ActivityCompat.checkSelfPermission(MainActivity.this, Manifest.permission.CAMERA)!= PackageManager.PERMISSION_GRANTED){
                requestPermissions(new String[]{Manifest.permission.CAMERA},REQUEST_CAMERA_CODE);
            }else{
                cameraManager.openCamera(mCameraID,stateCallback,mainHandler);
            }

        } catch (CameraAccessException e) {
            e.printStackTrace();
        }

    }

    private void preview() throws CameraAccessException {
        final CaptureRequest.Builder previewRequestBuilder=mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        previewRequestBuilder.addTarget(mSurfaceHolder.getSurface());
        mCameraDevice.createCaptureSession(Arrays.asList(mSurfaceHolder.getSurface(), imageReader.getSurface()), new CameraCaptureSession.StateCallback() {
            @Override
            public void onConfigured(@NonNull CameraCaptureSession session) {
                if( mCameraDevice==null)return;
                cameraCaptureSession=session;
                try{
                    previewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE,CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
                    previewRequestBuilder.set(CaptureRequest.CONTROL_AE_MODE,CaptureRequest.CONTROL_AE_MODE_ON_AUTO_FLASH);
                    CaptureRequest previewRequest=previewRequestBuilder.build();
                    cameraCaptureSession.setRepeatingRequest(previewRequest,null,childHandler);
                } catch (CameraAccessException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onConfigureFailed(@NonNull CameraCaptureSession session) {
                Toast.makeText(MainActivity.this,"配置失败",Toast.LENGTH_SHORT).show();
            }
        },childHandler);
    }



}