Java Multithreading,Sockets

Question

I am trying to implement a Thread which pings LAN connections continously. I would like to do that by creating new Socket for each IP, while handling exceptions if it fails to connect. However, the execution of the sequence time outs at creating the socket (i signed it with a comment in the code).

How could i solve this Problem?

class Ping implements Runnable
{

    private int actPort = 1024; 

    public void run()
    {           
        Socket s;
        int[] ip = {192,168,0,0};

        while(true){

            try {

                for(int i = 0;i<256;i++)
                {
                    ip[2] = i;

                    for(int j = 0;j<256;j++)
                    {

                        ip[3] = j;
                        String address = ip[0]+"."+ip[1]+"."+ip[2]+"."+ip[3];
                        s = new Socket(address,actPort); // EXECUTION STOPS
                        System.out.println(address);                            
                    }
                }                   

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

        }

    }
}

Thanks for your time

so why do you think it would not time out, if the ip address does not exist? — Scary Wombat, Apr 04 '16 at 08:00
The timeout could be as high as three minutes. This could be a long process to hit 65536 IP addresses. You could use multiple threads however an asynchronous connection with your own timeout might be more practical. — Peter Lawrey, Apr 04 '16 at 08:06

score 1 · Answer 1 · answered Apr 04 '16 at 08:26

You are creating Socket, which is a TCP/IP Stream and creates statefull connection between you (client) and given IP, PORT. I assume, this program stop while creating first Socket.

If you look at the API of Socket class:

 * If UDP socket is used, TCP/IP related socket options will not apply.
 *
 * @param      host     the IP address.
 * @param      port      the port number.
 * @param      stream    if {@code true}, create a stream socket;
 *                       otherwise, create a datagram socket.
@Deprecated
public Socket(InetAddress host, int port, boolean stream) throws IOException {
    this(host != null ? new InetSocketAddress(host, port) : null,
         new InetSocketAddress(0), stream);
}

This is @Deprecated version of creating UDP Socket, but what you REALLY need is DatagramPacket from java.net, which creates stateless connection, and may be timed out if packet didn't come to the end.

Look here for some example code of writing DatagramPacket:

https://github.com/awadalaa/Socket-Programming-Java/blob/master/UDP-Pinger/PingClient.java

score 1 · Answer 2 · edited May 23 '17 at 11:50

Your are trying to do both an ip scanner( loop on all 192.168) and a port scanner. So you could search solutions for both: java code to ping an IP address Sockets: Discover port availability using Java

For the multithreading part, it makes 65536 value to test, it would make too many Threads, so you can use a ThreadPool with executors, and cancel it if it takes too long.

see the method to use (i let you do the algorithm

tp = Executors.newWorkStealingPool(512); // to create up to 512 Threads
//todo the creation loop...
future = tp.submit(()->{/* your port scan */ return true;});
//todo the waiting processing
future.cancel(true);// to cancel your future

Java Multithreading,Sockets

2 Answers2

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