File.canRead（）返回false

Question

I have a server client program, with server running on Raspberry Pi (running Linux) and client being a Android application. I am writing a in application file browser, which simply sends a path to the server and in turn server returns the list of files using file.listFiles() (server is written in Java).

I run the server / Java program as sudo , hoping that this would give access to all the files across system, but to my surprise, this user does not have read permissions on /home directory of Pi.

So here is what happens in sequence:

I request file.listFiles(new File("/")); which is sent over socket from Pi to Android app. In the app, before making the next request, I check if I can read the file using file.canRead(); and for /home directory, this returns false.

File permissions on Pi:

pi@raspberrypi ~ $ ls -l / | grep home
drwxr-xr-x   3 root root  4096 Dec 31  1969 home

How I run my Java server:

pi@raspberrypi ~ $ sudo java -cp "/usr/local/lib/bluecovelib/bluecove/target/bluecove-2.1.1-SNAPSHOT.jar:/usr/local/lib/bluecovelib/bluecove-gpl-2.1.1-SNAPSHOT/target/bluecove-gpl-2.1.1-SNAPSHOT.jar:/home/pi/severinteractionutils.jar:." AppConnect

Why does sudo user does not have access to /home directory from the Java program?

EDIT:

I ran a simple program on Pi to test if it is because the underlying OS has changed. Apparently it does have an effect.

public class TestFileBrowser {
    public static void main(String[] args) { 
        System.out.println("Hello, World");
        File file = new File("/");
        File [] files = file.listFiles();
        for(int i=0; i<files.length; ++i) {
            if(files[i].getAbsolutePath().equals("/home")) {
                File homeDir = files[i];
                if(homeDir.canRead()) {
                    System.out.println("Can Read");
                    if (homeDir.isDirectory()) {
                        System.out.println("Is directory");
                    }
                }
            }
        }
    }
}

and here is the console snapshot:

pi@raspberrypi ~ $ sudo javac TestFileBrowser.java 
pi@raspberrypi ~ $ sudo java TestFileBrowser
Hello, World
Can Read
Is directory

Answer 1

and for /home directory, this returns false.

Correct, because you can't read from a directory using typical File Stream IO. You can call File.isDirectory() which (per the Javadoc)

Tests whether the file denoted by this abstract pathname is a directory.

Where it is required to distinguish an I/O exception from the case that the file is not a directory, or where several attributes of the same file are required at the same time, then the Files.readAttributes method may be used.

And you can check permission to read with the attributes.