mkdir()在内部闪存存储器内工作,但不是SD卡吗？

Question

我目前正在构建一个文件管理应用程序,允许用户浏览其设备的文件系统.用户/从其设备的根目录开始,但可以浏览到他们想要的任何位置,例如内部闪存或SD卡.

此应用程序的一个关键要求是允许用户在任何地方创建新文件夹.像这样的功能对应用程序非常有用.但是,该File#mkdir()方法在SD卡目录中根本不起作用.

我为清单文件添加了适当的权限.我还写了一个测试,看看哪些目录(我的Lollipop 5.0设备上都存在)允许创建一个新文件夹.根据我的观察,File#mkdir()只有在内部闪存目录中才有效.

注意:请不要Environment#getExternalStorageDirectory()与SD卡位置混淆,如本文所述.同样在Lollipop 5.0上,我相信/storage/emulated/0/并/storage/sdcard0/参考内部闪存存储,/storage/emulated/1/并/storage/sdcard1/参考SD卡(至少对我正在测试的设备是真的).

如何在非root用户Android设备上的外部存储路径之外的区域中创建新文件和文件夹？

---

表现:

...
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
...

测试:

...
public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final String NEW_FOLDER_NAME = "TestFolder";
        testPath(new File(Environment.getExternalStorageDirectory(), NEW_FOLDER_NAME));
        testPath(new File("/storage/emulated/0/", NEW_FOLDER_NAME));
        testPath(new File("/storage/emulated/1/", NEW_FOLDER_NAME));
        testPath(new File("/storage/sdcard0/Download/", NEW_FOLDER_NAME));
        testPath(new File("/storage/sdcard1/Pictures/", NEW_FOLDER_NAME));
    }

    private void testPath(File path) {
        String TAG = "Debug.MainActivity.java";
        String FOLDER_CREATION_SUCCESS = " mkdir() success: ";

        boolean success = path.mkdir();
        Log.d(TAG, path.getAbsolutePath() + FOLDER_CREATION_SUCCESS + success);
        path.delete();
    }
}

输出:

/storage/emulated/0/TestFolder mkdir() success: true
/storage/emulated/0/TestFolder mkdir() success: true
/storage/emulated/1/TestFolder mkdir() success: false
/storage/sdcard0/Download/TestFolder mkdir() success: true
/storage/sdcard1/Pictures/TestFolder mkdir() success: false

Answer 1

首先,您应该注意file.mkdir()并且如果目录已经存在则file.mkdirs()返回false.如果要在返回时知道目录是否存在,请使用(file.mkdir() || file.isDirectory())或只是忽略返回值并调用file.isDirectory()(请参阅文档).

也就是说,您真正的问题是您需要在Android 5.0+上的可移动存储上创建目录的权限.在Android上使用可移动SD卡是可怕的.

在Android 4.4(KitKat)上,Google限制访问SD卡(请参阅此处,此处和此处).如果您需要在Android 4.4(KitKat)上的可移动SD卡上创建目录,请参阅此StackOverflow答案,该答案会导致此XDA帖子.

在Android 5.0(Lollipop)上,谷歌推出了新的SD卡访问API.有关示例用法,请参阅此stackoverflow答案.

基本上,您需要使用DocumentFile#createDirectory(String displayName)来创建目录.在创建此目录之前,您需要要求用户向您的应用授予权限.

注意:这适用于可移动存储.File#mkdirs()如果您拥有该权限,则使用将用于内部存储(通常与Android上的外部存储混淆)android.permission.WRITE_EXTERNAL_STORAGE.

我将在下面发布一些示例代码:

检查您是否需要征得许可:

File sdcard = ... // the removable SD card
List<UriPermission> permissions = context.getContentResolver().getPersistedUriPermissions();
DocumentFile documentFile = null;
boolean needPermissions = true;

for (UriPermission permission : permissions) {
  if (permission.isWritePermission()) {
    documentFile = DocumentFile.fromTreeUri(context, permission.getUri());
    if (documentFile != null) {
      if (documentFile.lastModified() == sdcard.lastModified()) {
        needPermissions = false;
        break;
      }
    }
  }
}

接下来(如果needPermissions是true),您可以显示一个对话框,向用户解释他们需要选择"SD卡"以授予您的应用创建文件/目录的权限,然后启动以下活动:

if (needPermissions) {
  // show a dialog explaining that you need permission to create the directory
  // here, we will just launch to chooser (what you need to do after showing the dialog)
  startActivityForResult(new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE), STORAGE_REQUEST_CODE);
} else {
  // we already have permission to write to the removable SD card
  // use DocumentFile#createDirectory
}

您现在需要检查resultCode并requestCode在onActivityResult:

@Override protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  if (requestCode == STORAGE_REQUEST_CODE && resultCode == RESULT_OK) {
    File sdcard = ... // get the removable SD card

    boolean needPermissions = true;
    DocumentFile documentFile = DocumentFile.fromTreeUri(MainActivity.this, data.getData());
    if (documentFile != null) {
      if (documentFile.lastModified() == sdcard.lastModified()) {
        needPermissions = false;
      }
    }

    if (needPermissions) {
      // The user didn't select the "SD Card".
      // You should try the process over again or do something else.
    } else {
      // remember this permission grant so we don't need to ask again.
      getContentResolver().takePersistableUriPermission(data.getData(),
          Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
      // Now we can work with DocumentFile and create our directory
      DocumentFile doc = DocumentFile.fromTreeUri(this, data.getData());
      // do stuff...
    }
    return;
  }
  super.onActivityResult(requestCode, resultCode, data);
}

这应该会为您DocumentFile在Android 5.0+ 上使用和移动SD卡提供良好的开端.它可以是PITA.

此外,没有公共API来获取可移动SD卡的路径(如果存在的话).你不应该依赖硬编码"/storage/sdcard1"!StackOverflow上有很多关于它的帖子.许多解决方案都使用环境变量SECONDARY_STORAGE.以下是可用于查找可移动存储设备的两种方法:

public static List<File> getRemovabeStorages(Context context) throws Exception {
  List<File> storages = new ArrayList<>();

  Method getService = Class.forName("android.os.ServiceManager")
      .getDeclaredMethod("getService", String.class);
  if (!getService.isAccessible()) getService.setAccessible(true);
  IBinder service = (IBinder) getService.invoke(null, "mount");

  Method asInterface = Class.forName("android.os.storage.IMountService$Stub")
      .getDeclaredMethod("asInterface", IBinder.class);
  if (!asInterface.isAccessible()) asInterface.setAccessible(true);
  Object mountService = asInterface.invoke(null, service);

  Object[] storageVolumes;
  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
    String packageName = context.getPackageName();
    int uid = context.getPackageManager().getPackageInfo(packageName, 0).applicationInfo.uid;
    Method getVolumeList = mountService.getClass().getDeclaredMethod(
        "getVolumeList", int.class, String.class, int.class);
    if (!getVolumeList.isAccessible()) getVolumeList.setAccessible(true);
    storageVolumes = (Object[]) getVolumeList.invoke(mountService, uid, packageName, 0);
  } else {
    Method getVolumeList = mountService.getClass().getDeclaredMethod("getVolumeList");
    if (!getVolumeList.isAccessible()) getVolumeList.setAccessible(true);
    storageVolumes = (Object[]) getVolumeList.invoke(mountService, (Object[]) null);
  }

  for (Object storageVolume : storageVolumes) {
    Class<?> cls = storageVolume.getClass();
    Method isRemovable = cls.getDeclaredMethod("isRemovable");
    if (!isRemovable.isAccessible()) isRemovable.setAccessible(true);
    if ((boolean) isRemovable.invoke(storageVolume, (Object[]) null)) {
      Method getState = cls.getDeclaredMethod("getState");
      if (!getState.isAccessible()) getState.setAccessible(true);
      String state = (String) getState.invoke(storageVolume, (Object[]) null);
      if (state.equals("mounted")) {
        Method getPath = cls.getDeclaredMethod("getPath");
        if (!getPath.isAccessible()) getPath.setAccessible(true);
        String path = (String) getPath.invoke(storageVolume, (Object[]) null);
        storages.add(new File(path));
      }
    }
  }

  return storages;
}

public static File getRemovabeStorageDir(Context context) {
  try {
    List<File> storages = getRemovabeStorages(context);
    if (!storages.isEmpty()) {
      return storages.get(0);
    }
  } catch (Exception ignored) {
  }
  final String SECONDARY_STORAGE = System.getenv("SECONDARY_STORAGE");
  if (SECONDARY_STORAGE != null) {
    return new File(SECONDARY_STORAGE.split(":")[0]);
  }
  return null;
}