为什么我可以写"mnt/sdcard"而不是"mnt/extsd"？

Question

我正在尝试将文件存储到外部存储(Android 4.2.2).

我在manifest.xml中使用了正确的权限:

<manifest ...>
   ...
   <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
   <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
   ...

外部SD卡可用,我可以使用其他应用程序来读取和写入文件.路径/ mnt/extsd /有效,请看下面的图片.外部SD卡是可写的,因为我可以将文件复制/粘贴到其中.

代码:

try {
    String path = "/mnt/extsd/file.dat"
    File f = new File(path);
    f.createNewFile();
    return true;
} catch (Exception e) {
    e.printStackTrace();
    return false;
}

我收到错误f.createNewFile();:

W/System.err(2172): java.io.IOException: open failed: EACCES (Permission denied)
W/System.err(2172):     at java.io.File.createNewFile(File.java:948)
W/System.err(2172):     at solarapp.activities.settings.application.ActivityBackup$BackupTask.doInBackground(ActivityBackup.java:77)
W/System.err(2172):     at solarapp.activities.settings.application.ActivityBackup$BackupTask.doInBackground(ActivityBackup.java:1)
W/System.err(2172):     at android.os.AsyncTask$2.call(AsyncTask.java:287)
W/System.err(2172):     at java.util.concurrent.FutureTask.run(FutureTask.java:234)
W/System.err(2172):     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
W/System.err(2172):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)
W/System.err(2172):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)
W/System.err(2172):     at java.lang.Thread.run(Thread.java:856)
W/System.err(2172): Caused by: libcore.io.ErrnoException: open failed: EACCES (Permission denied)
W/System.err(2172):     at libcore.io.Posix.open(Native Method)
W/System.err(2172):     at libcore.io.BlockGuardOs.open(BlockGuardOs.java:110)
W/System.err(2172):     at java.io.File.createNewFile(File.java:941)

如果我使用String path = "/mnt/extsd/file.dat" 一切正常!那么,问题出在哪里？

编辑

我正在使用此解决方案来获取可用的存储列表:

如何获取Android设备的已安装外部存储列表

并且,正如所建议的,我发布命令的结果adb shell cat /proc/mounts:

rootfs / rootfs rw 0 0
tmpfs /dev tmpfs rw,nosuid,relatime,mode=755 0 0
devpts /dev/pts devpts rw,relatime,mode=600 0 0
proc /proc proc rw,relatime 0 0
sysfs /sys sysfs rw,relatime 0 0
none /acct cgroup rw,relatime,cpuacct 0 0
tmpfs /mnt/secure tmpfs rw,relatime,mode=700 0 0
tmpfs /mnt/asec tmpfs rw,relatime,mode=755,gid=1000 0 0
tmpfs /mnt/obb tmpfs rw,relatime,mode=755,gid=1000 0 0
none /dev/cpuctl cgroup rw,relatime,cpu 0 0
debugfs /sys/kernel/debug debugfs rw,relatime 0 0
/dev/block/by-name/system /system ext4 ro,relatime,data=ordered 0 0
/dev/block/by-name/cache /cache ext4 rw,nosuid,nodev,noatime,nomblk_io_submit,data=ordered 0 0
/dev/block/by-name/data /data ext4 rw,nosuid,nodev,noatime,nomblk_io_submit,noauto_da_alloc,data=ordered 0 0

/dev/block/vold/93:72 /mnt/sdcard vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro 0 0

/dev/block/vold/93:72 /mnt/secure/asec vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro 0 0

tmpfs /mnt/sdcard/.android_secure tmpfs ro,relatime,size=0k,mode=000 0 0

/dev/block/vold/179:1 /mnt/extsd vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1023,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro 0 0

对命令的响应 adb shell cat /proc/partitions

 major  minor    #blocks name
  93        0      32768 nanda
  93        8      16384 nandb
  93       16      16384 nandc
  93       24     786432 nandd
  93       32    1048576 nande
  93       40      16384 nandf
  93       48      32768 nandg
  93       56     524288 nandh
  93       64     131072 nandi
  93       72    4409344 nandj
 179        0    3849216 mmcblk0
 179        1    3845120 mmcblk0p1

对命令的响应 adb shell cat /proc/devices and adb shell ls -l /mnt

Character devices:
  1 mem
  4 /dev/vc/0
  4 tty
  5 /dev/tty
  5 /dev/console
  5 /dev/ptmx
  7 vcs
 10 misc
 13 input
 29 fb
 81 video4linux
100 telephony
108 ppp
116 alsa
128 ptm
136 pts
150 cedar_dev
166 ttyACM
180 usb
188 ttyUSB
189 usb_device
216 rfcomm
247 lcd
248 capture
249 disp
250 ttyGS
251 ttyS
252 sunxi_pwm
253 bsg
254 rtc

Block devices:
259 blkext
  7 loop
  8 sd
 65 sd
 66 sd
 67 sd
 68 sd
 69 sd
 70 sd
 71 sd
 93 nand
128 sd
129 sd
130 sd
131 sd
132 sd
133 sd
134 sd
135 sd
179 mmc
254 device-mapper
/system/bin/sh: cat: and: No such file or directory
/system/bin/sh: cat: adb: No such file or directory
/system/bin/sh: cat: shell: No such file or directory
/system/bin/sh: cat: ls: No such file or directory
/system/bin/sh: cat: -l: No such file or directory
/system/bin/sh: cat: /mnt: Is a directory

对命令的响应 adb shell ls -l /mnt

drwxr-xr-x root     system            2014-04-22 11:56 asec
d---rwxr-x system   media_rw          1970-01-01 01:00 extsd
drwxrwx--- media_rw media_rw          2014-04-22 11:56 media
drwxr-xr-x root     system            2014-04-22 11:56 obb
d---rwxr-x system   sdcard_rw          2014-04-22 12:26 sdcard
drwx------ root     root              2014-04-22 11:56 secure
drwx------ shell    shell             2014-04-22 11:56 shell
d--------- system   system            2014-04-22 11:56 usbhost1

Answer 1

Response to the command adb shell ls -l /mnt


d---rwxr-x system   media_rw          1970-01-01 01:00 extsd

编写extsd进程需要成为组的一部分media_rw，但WRITE_EXTERNAL_STORAGE将其添加到sdcard_rw组中。您需要的权限是WRITE_MEDIA_STORAGE，但不幸的是，它的保护级别是signatureOrSystem，所以您不能在您的应用程序中使用