Android 4.4系统中SD存储卡将成“废品”？

俞帅明

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);
}
}

复制代码

原文链接：http://www.iteye.com/news/28768-Android-KitKat

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Android 4.4系统中SD存储卡将成“废品”？

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