Android4.4下MediaProvider无法向外置SD卡中文件写数据的解决方法

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。