1. 用openFileOutput方法改变文件的属性，可是该方法有局限性，他创建的文件仅仅能位于该程序的私有文件夹下，即/data/data/app-package/files/。
FileOutputStream fos;
fos = openFileOutput("filename", MODE_WORLD_READABLE);
FileOutputStream android.content.ContextWrapper.openFileOutput(String name, int mode) throws FileNotFoundException
Open a private file associated with this Context's application package for writing. Creates the file if it doesn't already exist.
可用的mode 參数例如以下：
/**
* File creation mode: the default mode, where the created file can only
* be accessed by the calling application (or all applications sharing the
* same user ID).
* @see #MODE_WORLD_READABLE
* @see #MODE_WORLD_WRITEABLE
*/
public static final int MODE_PRIVATE = 0x0000;
/**
* File creation mode: allow all other applications to have read access
* to the created file.
* @see #MODE_PRIVATE
* @see #MODE_WORLD_WRITEABLE
*/
public static final int MODE_WORLD_READABLE = 0x0001;
/**
* File creation mode: allow all other applications to have write access
* to the created file.
* @see #MODE_PRIVATE
* @see #MODE_WORLD_READABLE
*/
public static final int MODE_WORLD_WRITEABLE = 0x0002;
/**
* File creation mode: for use with {@link #openFileOutput}, if the file
* already exists then write data to the end of the existing file
* instead of erasing it.
* @see #openFileOutput
*/
public static final int MODE_APPEND = 0x8000;
终于的实现方式剖析：
终于还是调用了系统的chmod来实现的改变文件权限的功能。
Runtime.getRuntime().exec("chmod 644 " + filename);
2. 用Runtime.getRuntime().exec()调用系统命令chmod来改变文件的权限，为了能推断命令的返回值，最好写成：
Process p = Runtime.getRuntime().exec("chmod 644 " + filename);
int status = p.waitFor();
if (status == 0) {
//chmod succeed
} else {
//chmod failed
}
