private void checkAccess(Uri uri, File file, int modeBits) throws FileNotFoundException {
  final boolean isWrite = (modeBits & MODE_WRITE_ONLY) != 0;
  final String path;
  try {
   path = file.getCanonicalPath();
  } catch (IOException e) {
   throw new IllegalArgumentException("Unable to resolve canonical path for " + file, e);
  }
  Context c = getContext();
  boolean readGranted =
    (c.checkCallingOrSelfUriPermission(uri, Intent.FLAG_GRANT_READ_URI_PERMISSION)
    == PackageManager.PERMISSION_GRANTED);
  if (path.startsWith(sExternalPath) || path.startsWith(sLegacyPath)) {
   if (!readGranted) {
    c.enforceCallingOrSelfPermission(
      READ_EXTERNAL_STORAGE, "External path: " + path);
   }
   if (isWrite) {
    if (c.checkCallingOrSelfUriPermission(uri, Intent.FLAG_GRANT_WRITE_URI_PERMISSION)
      != PackageManager.PERMISSION_GRANTED) {
     c.enforceCallingOrSelfPermission(
       WRITE_EXTERNAL_STORAGE, "External path: " + path);
    }
   }
  } else if (path.startsWith(sCachePath)) {
   if (!readGranted) {
    c.enforceCallingOrSelfPermission(
      ACCESS_CACHE_FILESYSTEM, "Cache path: " + path);
   }
  //外置SD卡，isWrite = true
  } else if (isWrite) {
   // don't write to non-cache, non-sdcard files.
   throw new FileNotFoundException("Can't access " + file);
  } else if (isSecondaryExternalPath(path)) {
   // read access is OK with the appropriate permission
   if (!readGranted) {
    c.enforceCallingOrSelfPermission(
      READ_EXTERNAL_STORAGE, "External path: " + path);
   }
  } else {
   checkWorldReadAccess(path);
  }
}

從以上代碼我們看出，如果sExternalPath 沒有指向外置SD卡并且path 是外置SD卡的文件路徑，那么該方法就會(huì)拋出FileNotFoundException,sExternalPath 一般都是指向內(nèi)部存儲(chǔ)

在應(yīng)用中我們通常通過contentresolver.openOutputStream(uri) 來打開存儲(chǔ)卡上媒體文件的文件流，如果媒體文件在外置SD卡上，那么我們就無法打開對(duì)應(yīng)的文件流，自然肯定無法向其中寫數(shù)據(jù)。

為了解決該問題，我們只能改變Android4.4平臺(tái)下Mediaprovider 對(duì)向SD卡寫數(shù)據(jù)的限制，具體修改方式如下

private void checkAccess(Uri uri, File file, int modeBits) throws FileNotFoundException {
  final boolean isWrite = (modeBits & MODE_WRITE_ONLY) != 0;
  final String path;
  try {
   path = file.getCanonicalPath();
  } catch (IOException e) {
   throw new IllegalArgumentException("Unable to resolve canonical path for " + file, e);
  }
  Context c = getContext();
  boolean readGranted =
    (c.checkCallingOrSelfUriPermission(uri, Intent.FLAG_GRANT_READ_URI_PERMISSION)
    == PackageManager.PERMISSION_GRANTED);
  if (path.startsWith(sExternalPath) || path.startsWith(sLegacyPath) || isSecondaryExternalPath(path)) {
   if (!readGranted) {
    c.enforceCallingOrSelfPermission(
      READ_EXTERNAL_STORAGE, "External path: " + path);
   }
   if (isWrite) {
    if (c.checkCallingOrSelfUriPermission(uri, Intent.FLAG_GRANT_WRITE_URI_PERMISSION)
      != PackageManager.PERMISSION_GRANTED) {
     c.enforceCallingOrSelfPermission(
       WRITE_EXTERNAL_STORAGE, "External path: " + path);
    }
   }
  } else if (path.startsWith(sCachePath)) {
   if (!readGranted) {
    c.enforceCallingOrSelfPermission(
      ACCESS_CACHE_FILESYSTEM, "Cache path: " + path);
   }
  //外置SD卡，isWrite = true
  } else if (isWrite) {
   // don't write to non-cache, non-sdcard files.
   throw new FileNotFoundException("Can't access " + file);
  } else {
   checkWorldReadAccess(path);
  }
},

對(duì)于滿足isSecondaryExternalPath(path) 的文件路徑，我們都可以進(jìn)行讀寫，對(duì)于外置SD卡的文件而言 isSecondaryExternalPath(path) 肯定為true

希望本文所述對(duì)大家Android程序設(shè)計(jì)有所幫助。

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