Get Unix hostid into Java

Question

How can I get the unix hostid into Java through some sort of call?

http://linux.about.com/library/cmd/blcmdl1_hostid.htm

Answer 1

If it has been set by a previous call to sethostid(long int id) it will reside in the HOSTIDFILE, typically /etc/hostid.

If it is not there, you fetch the machine's hostname. You pull out the address for the hostname, and if that's IPv4, it is the IPv4 address formatted from dotted decimal to binary with the top 16 bits and the lower 16 bits swapped.

InetAddress addr = InetAddress.getLocalHost();
byte[] ipaddr = addr.getAddress();
if (ipaddr.length == 4) {
  int hostid = 0 | ipaddr[1] << 24 | ipaddr[0] << 16 | ipaddr[3] << 8 | ipaddr[2];
  StringBuilder sb = new StringBuilder();
  Formatter formatter = new Formatter(sb, Locale.US);
  formatter.format("%08x", hostid);
  System.out.println(sb.toString());
} else {
  throw new Exception("hostid for IPv6 addresses not implemented yet");
}

Answer 2

You're going to have to write JNI (or JNA), I'm afraid.

Answer 3

Perhaps the following article will help.

http://www.devdaily.com/java/java-exec-processbuilder-process-1

Answer 4

Call Runtime.exec(String) where the argument is the path to the "hostid" executable then, drain both streams of the resulting Process object and take the contents of the standard output stream as your string value.

This simple class demonstrates how you could implement this strategy (but needs improvement for error handling [e.g. stderr, exceptions] and OOP best-practices [e.g. returning an object with bean properties, etc.]):

public class RunCommand {
  public static String exec(String command) throws Exception {
    Process p = Runtime.getRuntime().exec(command);
    String stdout = drain(p.getInputStream());
    String stderr = drain(p.getErrorStream());
    return stdout; // TODO: return stderr also...
  }
  private static String drain(InputStream in) throws IOException {
    int b = -1;
    StringBuilder buf = new StringBuilder();
    while ((b=in.read()) != -1) buf.append((char) b);
    return buf.toString();
  }
}

Your program could use it as such:

String myHostId = RunCommand.exec("/usr/bin/hostid").trim();

Note that using a ProcessBuilder to create the Process might be more appropriate than Runtime.exec() if your command needs arguments or an environment, etc.

Answer 5

Try this (wrapped in some class, of course):

import java.net.InetAddress;
import java.net.UnknownHostException;

public static String getLocalHostIP()
    throws UnknownHostException
{
    InetAddress  ip;

    ip = InetAddress.getLocalHost();
    return ip.getHostAddress();
}

The method returns a string of the form "xxx.xxx.xxx.xxx".

UPDATE

An improved method is:

// Determine the IP address of the local host
public static String getLocalHostIP()
{
    try
    {
        return InetAddress.getLocalHost().getHostAddress();
    }
    catch (UnknownHostException ex)
    {
        return null;
    }
}