安卓4.4中应用无法读取修改sd卡的问题——程序员解决方案

Google去年11月正式发布了Android 4.4，代号为KitKat（奇巧，雀巢的一款巧克力品牌）， 该系统带来了诸多新的特性 。 

但需要注意的是，该系统可能会让你之前一直正常使用的SD卡变为无用的“摆设”，因为 根据新版本的API改进，应用程序将不能再往SD卡中写入文件。  

来看Android开发者网站的 “外部存储技术信息”文档 中的描述： 

引用

WRITE_EXTERNAL_STORAGE只为设备上的主要外部存储授予写权限，应用程序无法将数据写入二级外部存储设备，除非指定了应用程序允许访问的特定的目录。

这目前只影响双存储设备， 如果你的设备有内部存储空间，即通常所说的机身存储（这就是指主要外部存储），那么你的SD卡就是一个二级外部存储设备。 

在Android 4.4中，如果你同时使用了机身存储和SD卡，那么应用程序将无法在SD卡中创建、修改、删除数据。比如，你无法使用文件管理器通过无线网络从电脑往SD卡中复制文件了。但是应用程序仍然可以往主存储的任意目录中写入数据，不受任何限制。 

Google表示， 这样做的目的是，通过这种方式进行限制，系统可以在应用程序被卸载后清除遗留文件。  

目前三星已经通过OTA向部分手机发送了Android 4.4的更新，已经有Note3用户抱怨FX文件管理器现在不能往SD卡中复制内容了。 

解决办法  

获得系统的ROOT权限是一个解决方法。 

很显然，这是针对用户的解决办法，但是并不是所有的用户都愿意进行ROOT，那么需要SD卡写入权限的开发者该如何做呢？ 

XDA论坛已经有大神给出了解决方案——在应用中嵌入一段代码 ，这段代码作用是在Android 4.4+设备上，如果其他方式写入失败，则将数据写入二级存储设备。 

详细方案： http://forum.xda-developers.com/showthread.php?p=50008987  

 /*
  * Copyright (C) 2014 NextApp, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  * http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS"
  * BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language
  * governing permissions and limitations under the License.
  */

 package nextapp.mediafile;

 import java.io.File;
 import java.io.IOException;
 import java.io.OutputStream;

 import android.content.ContentResolver;
 import android.content.ContentValues;
 import android.net.Uri;
 import android.provider.MediaStore;

 /**
  * Wrapper for manipulating files via the Android Media Content Provider. As of Android 4.4 KitKat, applications can no longer write
  * to the "secondary storage" of a device. Write operations using the java.io.File API will thus fail. This class restores access to
  * those write operations by way of the Media Content Provider.
  *
  * Note that this class relies on the internal operational characteristics of the media content provider API, and as such is not
  * guaranteed to be future-proof. Then again, we did all think the java.io.File API was going to be future-proof for media card
  * access, so all bets are off.
  *
  * If you're forced to use this class, it's because Google/AOSP made a very poor API decision in Android 4.4 KitKat.
  * Read more at https://plus.google.com/+TodLiebeck/posts/gjnmuaDM8sn
  *
  * Your application must declare the permission "android.permission.WRITE_EXTERNAL_STORAGE".
  */
 public class MediaFile {

     private final File file;
     private final ContentResolver contentResolver;
     private final Uri filesUri;
     private final Uri imagesUri;

     public MediaFile(ContentResolver contentResolver, File file) {
         this.file = file;
         this.contentResolver = contentResolver;
         filesUri = MediaStore.Files.getContentUri("external");
         imagesUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
     }

     /**
      * Deletes the file. Returns true if the file has been successfully deleted or otherwise does not exist. This operation is not
      * recursive.
      */
     public boolean delete()
             throws IOException {
         if (!file.exists()) {
             return true;
         }

         boolean directory = file.isDirectory();
         if (directory) {
             // Verify directory does not contain any files/directories within it.
             String[] files = file.list();
             if (files != null && files.length > 0) {
                 return false;
             }
         }

         String where = MediaStore.MediaColumns.DATA + "=?";
         String[] selectionArgs = new String[] { file.getAbsolutePath() };

         // Delete the entry from the media database. This will actually delete media files (images, audio, and video).
         contentResolver.delete(filesUri, where, selectionArgs);

         if (file.exists()) {
             // If the file is not a media file, create a new entry suggesting that this location is an image, even
             // though it is not.
             ContentValues values = new ContentValues();
             values.put(MediaStore.Files.FileColumns.DATA, file.getAbsolutePath());
             contentResolver.insert(imagesUri, values);

             // Delete the created entry, such that content provider will delete the file.
             contentResolver.delete(filesUri, where, selectionArgs);
         }

         return !file.exists();
     }

     public File getFile() {
         return file;
     }

     /**
      * Creates a new directory. Returns true if the directory was successfully created or exists.
      */
     public boolean mkdir()
             throws IOException {
         if (file.exists()) {
             return file.isDirectory();
         }

         ContentValues values;
         Uri uri;

         // Create a media database entry for the directory. This step will not actually cause the directory to be created.
         values = new ContentValues();
         values.put(MediaStore.Files.FileColumns.DATA, file.getAbsolutePath());
         contentResolver.insert(filesUri, values);

         // Create an entry for a temporary image file within the created directory.
         // This step actually causes the creation of the directory.
         values = new ContentValues();
         values.put(MediaStore.Files.FileColumns.DATA, file.getAbsolutePath() + "/temp.jpg");
         uri = contentResolver.insert(imagesUri, values);

         // Delete the temporary entry.
         contentResolver.delete(uri, null, null);

         return file.exists();
     }

     /**
      * Returns an OutputStream to write to the file. The file will be truncated immediately.
      */
     public OutputStream write()
             throws IOException {
         if (file.exists() && file.isDirectory()) {
             throw new IOException("File exists and is a directory.");
         }

         // Delete any existing entry from the media database.
         // This may also delete the file (for media types), but that is irrelevant as it will be truncated momentarily in any case.
         String where = MediaStore.MediaColumns.DATA + "=?";
         String[] selectionArgs = new String[] { file.getAbsolutePath() };
         contentResolver.delete(filesUri, where, selectionArgs);

         ContentValues values = new ContentValues();
         values.put(MediaStore.Files.FileColumns.DATA, file.getAbsolutePath());
         Uri uri = contentResolver.insert(filesUri, values);

         if (uri == null) {
             // Should not occur.
             throw new IOException("Internal error.");
         }

         return contentResolver.openOutputStream(uri);
     }
 }