Security and Permissions安全与权限（二）

User IDs and File Access
Each Android package (.apk) file installed on the device is given its own unique Linux user ID, creating a sandbox for it and preventing it from touching other applications (or other applications from touching it). This user ID is assigned to it when the application is installed on the device, and remains constant for the duration of its life on that device.
用户IDs和文件存取
每一个Android应用程序（.apk文件）都会在安装时就分配一个独有的Linux用户ID，这就为它建立了一个沙盒，使其不能与其他应用程序进行接触（也不会让其它应用程序接触它）。这个用户ID会在安装时分配给它，并在该设备上一直保持同一个数值。

Because security enforcement happens at the process level, the code of any two packages can not normally run in the same process, since they need to run as different Linux users. You can use the sharedUserId attribute in the AndroidManifest.xml's manifest tag of each package to have them assigned the same user ID. By doing this, for purposes of security the two packages are then treated as being the same application, with the same user ID and file permissions. Note that in order to retain security, only two applications signed with the same signature (and requesting the same sharedUserId) will be given the same user ID.
由于安全性限制措施是发生进程级，所以两个package中的代码不会运行在同一个进程当中，他们要作为不同的Linux用户出现。我们可以通过使用AndroidManifest.xml文件中的manifest标签中的sharedUserId属性，来使不同的package共用同一个用户ID。通过这种方式，这两个package就会被认为是同一个应用程序，拥有同一个用户ID（实际不一定），并且拥有同样的文件存取权限。注意：为了保持安全，只有当两个应用程序被同一个签名签署的时候（并且请求了同一个sharedUserId）才会被分配同样的用户ID.

Any data stored by an application will be assigned that application's user ID, and not normally accessible to other packages. When creating a new file with getSharedPreferences(String, int), openFileOutput(String, int), or openOrCreateDatabase(String, int, SQLiteDatabase.CursorFactory), you can use the MODE_WORLD_READABLE and/or MODE_WORLD_WRITEABLE flags to allow any other package to read/write the file. When setting these flags, the file is still owned by your application, but its global read and/or write permissions have been set appropriately so any other application can see it.
所有存储在应用程序中的数据都会赋予一个属性-该应用程序的用户ID,这使得其他package无法访问这些数据。当通过这些方法getSharedPreferences(String, int), openFileOutput(String, int), or openOrCreateDatabase(String, int, SQLiteDatabase.CursorFactory)来创建一个新文件时，你可以通过使用MODE_WORLD_READABLE and/or MODE_WORLD_WRITEABLE标志位来设置是否允许其他package来访问读写这个文件。当设置这些标志位时，该文件仍然属于该应用程序，但是它的global read and/or write权限已经被设置，使得它对于其他任何应用程序都是可见的。