本文实例讲述了Android4.4下MediaProvider无法向外置SD卡中文件写数据的解决方法。分享给大家供大家参考，具体如下：

Android4.4平台限制应用对外置SD卡的读写权限。MediaProvider通过 checkAccess方法 限制对外置SD卡的读写。

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);  }}

从以上代码我们看出，如果sExternalPath 没有指向外置SD卡并且path 是外置SD卡的文件路径，那么该方法 就会抛出FileNotFoundException,sExternalPath 一般都是指向内部存储

在应用中 我们通常 通过contentresolver.openOutputStream(uri) 来打开存储卡上媒体文件的文件流，如果媒体文件在外置SD卡上，那么我们就无法打开对应的文件流，自然肯定无法向其中写数据。

为了解决该问题，我们只能改变Android4.4平台下Mediaprovider 对向SD卡写数据的限制，具体修改方式如下

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);  }},

对于满足isSecondaryExternalPath(path) 的文件路径，我们都可以进行读写，对于外置SD卡的文件而言 isSecondaryExternalPath(path) 肯定为true

希望本文所述对大家Android程序设计有所帮助。