Why does Java hide their getLocalHostName() implementation?

Question

Asking how to find the current machine's host name is one of the more frequent Java questions here and in other programming forums, and the common answer - InetAddress.getLocalHost().getHostName() is not that good (as explained here).

But apparently Java already does the correct thing - as is documented for the getLocalHost() method mentioned above:

Returns the address of the local host. This is achieved by retrieving the name of the host from the system, then resolving that name into an InetAddress.

So if the code example above is to be used, what we essentially do is:

1. retrieve the name of the host from the system
2. resolve the name into an internet address
3. resolve the name back to the host name (likely using the local /etc/hosts file)

Which seems both a bit redundant and error prone (step 2 is likely to fail if you are not on a well configured server, and step 3 may also fail), if all you wanted is step 1.

What getLocalHost() does internally is to call getLocalHostName() on an implementation of java.net.InetAddressImpl that it instantiates statically to an IPv4 or an IPv6 implementation depending on some system parameters (it doesn't even care about platform support - I assume all the platform-specific coding is handled in the native implemnetation for InetAddressImpl).

So why can't we just use this directly? Well, we "can't" because all the entry points to get to getLocalHostName() are "package private", which really is no protection - just drop this implementation somewhere in your class path and get direct access:

package java.net;
public class HostNameHelper {
    public static String getLocalHostName() throws UnknownHostException {
        return java.net.InetAddress.impl.getLocalHostName();
    }
}

So why bother at all? Why didn't Sun/Oracle give Java developers what they are so desperate for?

Answer 1

It fails in java 8 with : Caused by: java.lang.SecurityException: Prohibited package name: java.net

It fails in java 9 with : Caused by: java.lang.ClassNotFoundException: java.net.HostNameHelper