查了很多资料，其他博客大多都是介绍以前的的方案，基于9.0系统的基本没有找到，要么就不全，特别是获取手电筒状态的资料，下面就大致介绍一下：

打开和关闭的方法是：toggleLight
获取手电筒状态的通过回调方法：mTorchCallback（这就是为什么进入页面就需要注册该回调）

    private CameraManager mCameraManager;
    public void initData() {
    //ivFlashlight是界面上的imageview控件
    ivFlashlight.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                toggleLight(!isFlashLightOn);
            }
        });
        //获取CameraManager
        if(mCameraManager == null){
            mCameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
        }
        mCameraManager.registerTorchCallback(mTorchCallback, null);
    }
    //callback成员变量初始化
    private CameraManager.TorchCallback mTorchCallback = new CameraManager.TorchCallback() {
        @Override
        public void onTorchModeChanged(@NonNull String cameraId, boolean enabled) {
            super.onTorchModeChanged(cameraId, enabled);
            Log.e(TAG, "onTorchModeChanged cameraId=" + cameraId + ";enabled=" + enabled);
            isFlashLightOn = enabled;
            ivFlashlight.setIcon(enabled ? MaterialDrawableBuilder.IconValue.FLASHLIGHT : MaterialDrawableBuilder.IconValue.FLASHLIGHT_OFF);
        }
    };
    public void toggleLight(boolean OPEN) {
        try {
            //获取当前手机所有摄像头设备ID
            String[] ids = mCameraManager.getCameraIdList();
            for (String id : ids) {
                CameraCharacteristics c = mCameraManager.getCameraCharacteristics(id);
                //查询该摄像头组件是否包含闪光灯
                Boolean flashAvailable = c.get(CameraCharacteristics.FLASH_INFO_AVAILABLE);

                /*
                 * 获取相机面对的方向
                 * CameraCharacteristics.LENS_FACING_FRONT 前置摄像头
                 * CameraCharacteristics.LENS_FACING_BACK 后只摄像头
                 * CameraCharacteristics.LENS_FACING_EXTERNAL 外部的摄像头
                 */
                Integer lensFacing = c.get(CameraCharacteristics.LENS_FACING);
                if (flashAvailable != null && flashAvailable
                        && lensFacing != null && lensFacing == CameraCharacteristics.LENS_FACING_BACK) {
                    //打开或关闭手电筒
                    mCameraManager.setTorchMode(id, OPEN);
                    break;
                }
            }
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        if(mCameraManager != null && mTorchCallback != null){
            mCameraManager.unregisterTorchCallback(mTorchCallback);
        }
    }