I'm debugging my application on my Nexus 5 with Android M 6.0.1 and I'm getting problems with camera's permissions. In my application I need to open the camera when a button is clicked, but it throws me this exception:
04-06 14:03:25.213 15330-15330/clyky.cartracker W/System.err: java.lang.SecurityException: Permission Denial: starting Intent { act=android.media.action.IMAGE_CAPTURE flg=0x3 cmp=com.google.android.GoogleCamera/com.android.camera.activity.CaptureActivity clip={text/uri-list U:file:///storage/emulated/0/TesseractSample/imgs/ocr.jpg} (has extras) } from ProcessRecord{50bae23 15330:clyky.cartracker/u0a117} (pid=15330, uid=10117) with revoked permission android.permission.CAMERA
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.os.Parcel.readException(Parcel.java:1666)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.os.Parcel.readException(Parcel.java:1619)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.app.ActivityManagerProxy.startActivity(ActivityManagerNative.java:2658)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.app.Instrumentation.execStartActivity(Instrumentation.java:1507)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.app.Activity.startActivityForResult(Activity.java:3930)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.support.v4.app.BaseFragmentActivityJB.startActivityForResult(BaseFragmentActivityJB.java:50)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:79)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.app.Activity.startActivityForResult(Activity.java:3890)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:859)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at clyky.cartracker.activities.AddVehicleActivity.startCameraActivity(AddVehicleActivity.java:99)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at clyky.cartracker.activities.AddVehicleActivity.access$000(AddVehicleActivity.java:35)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at clyky.cartracker.activities.AddVehicleActivity$1.onClick(AddVehicleActivity.java:66)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.view.View.performClick(View.java:5204)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.view.View$PerformClick.run(View.java:21156)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.os.Handler.handleCallback(Handler.java:739)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:95)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.os.Looper.loop(Looper.java:148)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5458)
04-06 14:03:25.226 15330-15330/clyky.cartracker W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
04-06 14:03:25.227 15330-15330/clyky.cartracker W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
04-06 14:03:25.227 15330-15330/clyky.cartracker W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
I've already added permission of read/write external folder and camera in my manifest:
<uses-permission android:name="android.permission.CAMERA"/>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
This is the method where I try to start the camera:
private void startCameraActivity()
{
    try
    {
        Log.d("debug", "sto provando a lanciare la fotocamera");
        String IMGS_PATH = Environment.getExternalStorageDirectory().toString() + "/TesseractSample/imgs";
        prepareDirectory(IMGS_PATH);
        String img_path = IMGS_PATH + "/ocr.jpg";
        outputFileUri = Uri.fromFile(new File(img_path));
        final Intent takePictureIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
        takePictureIntent.putExtra(MediaStore.EXTRA_OUTPUT, outputFileUri);
        if (takePictureIntent.resolveActivity(getPackageManager()) != null) {
            startActivityForResult(takePictureIntent, PHOTO_REQUEST_CODE);
        }
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }
}
I think the problem is that the ask for camera's permissions is never shown. When my activity is launched it asks me for read/write external storage, but never for the camera. I've taken the code for the permissions on the web. It uses this class:
public class RequestPermissionsToolToolImpl implements RequestPermissionsTool
{
    private static final String CONFIRMATION_DIALOG = "ConfirmationDialog";
    private static final String TAG = RequestPermissionsToolToolImpl.class.getSimpleName();
    private Activity activity;
    @Override
    public void requestPermissions(Activity activity, String[] permissions)
    {
        Log.d("debug", "sto chiedendo i permessi.");
        int x = 0;
        Map<Integer, String> permissionsMap = new HashMap<>();
        this.activity = activity;
        for (int i = 0; i < permissions.length; i++) {
            permissionsMap.put(i, permissions[i]);
        }
        for (Map.Entry<Integer, String> permission : permissionsMap.entrySet())
        {
            if (!isPermissionGranted(activity, permission.getValue()))
            {
                if (ActivityCompat.shouldShowRequestPermissionRationale(activity, permission.getValue()))
                {
                    ConfirmationDialog.newInstance(permission.getKey(), permission.getValue()).show(activity.getFragmentManager(), CONFIRMATION_DIALOG);
                }
                else {
                    ActivityCompat.requestPermissions(activity, permissions,
                            permission.getKey());
                    return;
                }
            }
        }
    }
    @Override
    public boolean isPermissionsGranted(Context context, String[] permissions) {
        for (String permission : permissions)
        {
            if (!isPermissionGranted(context, permission)) {
                return false;
            }
        }
        return true;
    }
    @Override
    public void onPermissionDenied() {
        ErrorDialog.newInstance("Permission needs").
                show(activity.getFragmentManager(), CONFIRMATION_DIALOG);
    }
    private boolean isPermissionGranted(Context context, String permission)
    {
        return ActivityCompat.checkSelfPermission(context, permission) == PackageManager.PERMISSION_GRANTED;
    }
    /**
     * Shows OK/Cancel confirmation dialog about permission.
     */
    public static class ConfirmationDialog extends DialogFragment
    {
        private static final String ARG_PERMISSION = "permission";
        private static final String ARG_REQUEST_CODE = "request_code";
        public static ConfirmationDialog newInstance(int permissionKey, String permissionValue) {
            ConfirmationDialog dialog = new ConfirmationDialog();
            Bundle bundle = new Bundle();
            bundle.putString(ARG_PERMISSION, permissionValue);
            bundle.putInt(ARG_REQUEST_CODE, permissionKey);
            dialog.setArguments(bundle);
            return dialog;
        }
        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            return new AlertDialog.Builder(getActivity())
                    .setMessage("Please allow permission")
                    .setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            ActivityCompat.requestPermissions(getActivity(),
                                    new String[]{getArguments().getString(ARG_PERMISSION)},
                                    getArguments().getInt(ARG_REQUEST_CODE));
                        }
                    })
                    .setNegativeButton(android.R.string.cancel,
                            new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog, int which) {
                                    Toast.makeText(getActivity(), "Not available", Toast.LENGTH_SHORT).show();
                                }
                            })
                    .create();
        }
    }
    /**
     * Shows an error message dialog.
     */
    public static class ErrorDialog extends DialogFragment {
        private static final String ARG_MESSAGE = "message";
        public static ErrorDialog newInstance(String message) {
            ErrorDialog dialog = new ErrorDialog();
            Bundle args = new Bundle();
            args.putString(ARG_MESSAGE, message);
            dialog.setArguments(args);
            return dialog;
        }
        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            final Activity activity = getActivity();
            return new AlertDialog.Builder(activity)
                    .setMessage(getArguments().getString(ARG_MESSAGE))
                    .setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialogInterface, int i) {
//nothing
                        }
                    })
                    .create();
        }
    }
}
and these are methods in my activity which uses permissions:
private void requestPermissions()
    {
        String[] permissions = new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE};
        requestTool = new RequestPermissionsToolToolImpl();
        requestTool.requestPermissions(this, permissions);
    }
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        boolean grantedAllPermissions = true;
        for (int grantResult : grantResults) {
            if (grantResult != PackageManager.PERMISSION_GRANTED) {
                grantedAllPermissions = false;
            }
        }
        if (grantResults.length != permissions.length || (!grantedAllPermissions)) {
            requestTool.onPermissionDenied();
        } else {
            super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }
