//检查权限
 public void checkPermission() {
  if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
   ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
   //发现没有权限，调用requestPermissions方法向用户申请权限，requestPermissions接收三个参数，第一个是context，第二个是一个String数组，我们把要申请的权限
   //名放在数组中即可，第三个是请求码，只要是唯一值就行
  } else {
   openAlbum();//有权限就打开相册
  }
 }

 public void openAlbum() {
  //通过intent打开相册，使用startactivityForResult方法启动actvity，会返回到onActivityResult方法，所以我们还得复写onActivityResult方法
  Intent intent = new Intent("android.intent.action.GET_CONTENT");
  intent.setType("image/*");
  startActivityForResult(intent, CHOOSE_PHOTO);
 }
 //弹出窗口向用户申请权限


 @Override
 public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
  super.onRequestPermissionsResult(requestCode, permissions, grantResults);//弹出授权的窗口，这条语句也可以删除，没有影响
  //获得了用户的授权结果，保存在grantResults中，判断grantResult中的结果来决定接下来的操作
  switch (requestCode) {
   case 1:

    if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
     openAlbum();
    } else {
     Toast.makeText(this, "授权失败，无法操作", Toast.LENGTH_SHORT).show();
    }
    break;
   default:
    break;
  }
 }

 @Override
 protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  super.onActivityResult(requestCode, resultCode, data);
  switch (requestCode) {
   case CHOOSE_PHOTO:
    if (resultCode == RESULT_OK) {
     if (Build.VERSION.SDK_INT >= 19) {
      handleImageOnkitKat(data);//高于4.4版本使用此方法处理图片
     } else {
      handleImageBeforeKitKat(data);//低于4.4版本使用此方法处理图片
     }
    }
    break;
   default:
    break;
  }
 }

 @TargetApi(19)
 private void handleImageOnkitKat(Intent data) {
  String imagePath = null;
  Uri uri = data.getData();
  if (DocumentsContract.isDocumentUri(this, uri)) {
   //如果是document类型的uri，则通过document id处理
   String docId = DocumentsContract.getDocumentId(uri);
   if ("com.android.providers.media.documents".equals(uri.getAuthority())) {
    String id = docId.split(":")[1];//解析出数字格式的id
    String selection = MediaStore.Images.Media._ID + "=" + id;
    imagePath = getImagePath(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, selection);
   } else if ("com.android,providers.downloads.documents".equals(uri.getAuthority())) {
    Uri contentUri = ContentUris.withAppendedId(Uri.parse("content://downloads/public_downloads"), Long.valueOf(docId));
    imagePath = getImagePath(contentUri, null);
   }

  } else if ("content".equalsIgnoreCase(uri.getScheme())) {
   imagePath = getImagePath(uri, null);
  }
  displayImage(imagePath);
 }

 private void handleImageBeforeKitKat(Intent data) {
  Uri uri = data.getData();
  String imagePath = getImagePath(uri, null);
  displayImage(imagePath);
 }

 //获得图片路径
 public String getImagePath(Uri uri, String selection) {
  String path = null;
  Cursor cursor = getContentResolver().query(uri, null, selection, null, null); //内容提供器
  if (cursor != null) {
   if (cursor.moveToFirst()) {
    path = cursor.getString(cursor.getColumnIndex(MediaStore.Images.Media.DATA)); //获取路径
   }
  }
  cursor.close();
  return path;
 }
 //展示图片
 private void displayImage(String imagePath) {
  if (imagePath != null) {
   Bitmap bitImage = BitmapFactory.decodeFile(imagePath);//格式化图片

   mImage.setImageBitmap(bitImage);//为imageView设置图片

  } else {
   Toast.makeText(MainActivity.this, "获取图片失败", Toast.LENGTH_SHORT).show();
  }
 }