Ping a server in Java

Ping a server

It's not possible to really "ping" a machine to check if it's alive or not (it's a long story, but to keep it short I will just say that the Socket class is not low-level enough for that operation). But we can emulate a ping by talking the "echo port". On a server, the echo port is always port 7. We write a string to that port and the server will echo the string.

import java.io.*;
import java.net.*;

public class PseudoPing {
public static void main(String args[]) {
  try {
    Socket t = new Socket(args[0], 7);
    DataInputStream dis = new DataInputStream(t.getInputStream());
    PrintStream ps = new PrintStream(t.getOutputStream());
    ps.println("Hello");
    String str = dis.readLine();
    if (str.equals("Hello"))
      System.out.println("Alive!") ;
    else
      System.out.println("Dead or echo port not responding");            
    t.close();
  }
  catch (IOException e) {
    e.printStackTrace();}
  }
}
}

NOTE: To make this a more "complete PING", you may want to check this How-to to display the response time.

Since JDK1.5, java.net.InetAddress.isReachable(int) can be used to check if a server is reachable or not.

import java.io.*;
import java.net.*;

public class ReachableTest {
public static void main(String args[]) {
   try {
     InetAddress address = InetAddress.getByName("web.mit.edu");
     System.out.println("Name: " + address.getHostName());
     System.out.println("Addr: " + address.getHostAddress());
     System.out.println("Reach: " + address.isReachable(3000));
   }
   catch (UnknownHostException e) {
     System.err.println("Unable to lookup web.mit.edu");
   }
   catch (IOException e) {
     System.err.println("Unable to reach web.mit.edu");
   }
 }
}

isReachable() will use ICMP ECHO REQUESTs if the privilege can be obtained, otherwise it will try to establish a TCP connection on port 7 (Echo) of the destination host. But most Internet sites have disabled the service or blocked the requests (except some university web sites like the example above) .

String host = "172.16.0.2"
int timeOut = 3000; // I recommend 3 seconds at least
boolean status = InetAddress.getByName(host).isReachable(timeOut)

source
http://www.rgagnon.com/javadetails
http://blog.wu-boy.com/2008/12/11/620/
http://www.lslnet.com/linux/f/docs1/i25/big5216009.htm