Camera won't turn on/off flash for camera2

I need help with turning on/off flash for camera2. The code is not working and it won't turn on the flash when taking a picture. I have tried a lot of different methods but have come up short. Here is my code. Please help!

MainActivity.java

public class MainActivity extends AppCompatActivity {
    private CameraCaptureSession mSession;
    private CaptureRequest.Builder mBuilder;
    private CameraDevice mCameraDevice;
    private CameraManager mCameraManager;
    Boolean isOn = false;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (null == savedInstanceState) {
            getFragmentManager().beginTransaction()
                    .replace(R.id.container, Camera2BasicFragment.newInstance())
                    .commit();
        }


        final Button flashOn = (Button) findViewById(R.id.flah_off);
        flashOn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(isOn) {
                    flashOn.setBackgroundResource(R.drawable.flash_on);
                    turnOnFlashLight();
                }else{
                    flashOn.setBackgroundResource(R.drawable.flah_off);
                    turnOffFlashLight();
                }
                isOn = !isOn;
            }
        });

    }

    public void turnOnFlashLight() {
        try {
            mBuilder.set(CaptureRequest.FLASH_MODE, CameraMetadata.FLASH_MODE_TORCH);
            mSession.setRepeatingRequest(mBuilder.build(), null, null);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void turnOffFlashLight() {
        try {
            mBuilder.set(CaptureRequest.FLASH_MODE, CameraMetadata.FLASH_MODE_OFF);
            mSession.setRepeatingRequest(mBuilder.build(), null, null);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }