Android 中的Storage state

查看源码可知，android中的storage state 共有以下几种状态

/*** Unknown storage state, such as when a path isn't backed by known storage* media.** @see #getExternalStorageState(File)*/public static final String MEDIA_UNKNOWN = "unknown";    未知/*** Storage state if the media is not present.** @see #getExternalStorageState(File)*/public static final String MEDIA_REMOVED = "removed";/*** Storage state if the media is present but not mounted.** @see #getExternalStorageState(File)*/public static final String MEDIA_UNMOUNTED = "unmounted";/*** Storage state if the media is present and being disk-checked.** @see #getExternalStorageState(File)*/public static final String MEDIA_CHECKING = "checking";/*** Storage state if the media is present but is blank or is using an* unsupported filesystem.** @see #getExternalStorageState(File)*/public static final String MEDIA_NOFS = "nofs";/*** Storage state if the media is present and mounted at its mount point with* read/write access.** @see #getExternalStorageState(File)*/public static final String MEDIA_MOUNTED = "mounted";/*** Storage state if the media is present and mounted at its mount point with* read-only access.** @see #getExternalStorageState(File)*/public static final String MEDIA_MOUNTED_READ_ONLY = "mounted_ro";/*** Storage state if the media is present not mounted, and shared via USB* mass storage.** @see #getExternalStorageState(File)*/public static final String MEDIA_SHARED = "shared";/*** Storage state if the media was removed before it was unmounted.** @see #getExternalStorageState(File)*/public static final String MEDIA_BAD_REMOVAL = "bad_removal";/*** Storage state if the media is present but cannot be mounted. Typically* this happens if the file system on the media is corrupted.** @see #getExternalStorageState(File)*/public static final String MEDIA_UNMOUNTABLE = "unmountable";/*** Storage state if the media is in the process of being ejected.** @see #getExternalStorageState(File)*/public static final String MEDIA_EJECTING = "ejecting";  

以上这些状态中

/*** Storage state if the media is present and mounted at its mount point with* read/write access.** @see #getExternalStorageState(File)*/public static final String MEDIA_MOUNTED = "mounted";

以上的这些状态中只有mounted 状态是同时可读可写的

而

/*** Storage state if the media is present and mounted at its mount point with* read-only access.** @see #getExternalStorageState(File)*/public static final String MEDIA_MOUNTED_READ_ONLY = "mounted_ro";

mounted_ro 有读的权限，无写的权限。

结论：

因此，在文件读写的时候，通常会这么判断

if (

Environment.MEDIA_MOUNTED

.equals(

Environment.getExternalStorageState()

)){

}

else{

return null;

}