java - 为什么我可以写入 "mnt/sdcard"而不是 "mnt/extsd"？

Question

我正在尝试将文件存储到外部存储设备 (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"/>
   ...

外置sd卡可用，我可以用其他app读写文件来证明。路径/mnt/extsd/是有效的，看下图。外置sd卡是可写的，因为我可以复制/粘贴文件到里面。

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;
}

我在 f.createNewFile(); 上收到错误:

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存储列表:

How can I get the list of mounted external storage of android device

并且，按照建议，我发布命令 adb shell cat/proc/mounts 的结果:

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

响应命令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

响应命令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

响应命令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

要写入 extsd，该进程需要成为 media_rw 组的一部分，但 WRITE_EXTERNAL_STORAGE 将其添加到 sdcard_rw团体。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中使用