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javanetwork-programmingpingunknown-host

While trying to ping, java.net.UnknownHostException gets thrown. I do not understand the reason

I wrote the following code to try a ping. But as I run it, the following exception gets thrown :

java.net.UnknownHostException: http://localhost:8084/server/index.jsp
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
    at java.net.InetAddress$1.lookupAllHostAddr(Unknown Source)
    at java.net.InetAddress.getAddressFromNameService(Unknown Source)
    at java.net.InetAddress.getAllByName0(Unknown Source)
    at java.net.InetAddress.getAllByName0(Unknown Source)
    at java.net.InetAddress.getAllByName(Unknown Source)
    at java.net.InetAddress.getByName(Unknown Source)
    at Tester.main(Tester.java:10)

import java.net.InetAddress;

class Tester {
public static void main(String args[]) {
    try {
      InetAddress address = InetAddress.getByName("http://localhost:8084/server/index.jsp");
      boolean isReachable = address.isReachable(2000);
      if(isReachable)
        System.out.println("The address is reachable");
      else
        System.out.println("The address is not reachable");

    } catch(Exception exc) {
       exc.printStackTrace();
      }
}
}

Why is it so ? The server is running and the page is opening fine in the web-browser.

Solution

  • The problem is in this line:

    InetAddress address = InetAddress.getByName(
        "http://localhost:8084/server/index.jsp");

    The InetAddress.getByName(String) method requires a hostname. You've given it a URL string. The hostname component of that address is "localhost".

    If you want to "ping" the host associated with a URL, then you need to parse the URL and extract the hostname component something like this:

    String hostname = new URL(str).getHost();

    But you need to deal with the cases where the URL is malformed, or where it doesn't have a host name component.

    I imagine that you are actually trying to test some other hostname, because sending an ICMP_PING request to "localhost" (typically 127.0.0.1) is kind of pointless.