How do I check if an IP is alive in java?

Question

There are 5 devices in my network with different IP addresses. I wish to connect to these devices and get data from them over TCP/IP socket when they are available in my network. How can I check if they are available in java?

public void setUpConnection() {
    try {
        Socket client = new Socket(hostIp, hostPort);
        socketReader = client.getInputStream();
        socketWriter = new PrintWriter(client.getOutputStream());
    } catch (UnknownHostException e) {
        System.out.println("Error setting up socket connection: unknown host at " +   hostIp);
        System.out.println("host: " + hostIp + "port: " + hostPort);
    } catch (IOException e) {
        System.out.println("Error setting up socket connection: " + e);
        System.out.println("host: " + hostIp + "port:" + hostPort);
    }
}

Solution 1

InetAddress.getByName(host).isReachable(timeOut);

Further reference here

Solution 2

if you just want to check whether or not the host is up, you can use isReachable

Solution 3

For pure Java there are basically two methods:

1. Active: as your example, but looping thru all ports and all bound IP's on the host

2. Passive: running a small server on predefined port(s) and the other one will register themself when they get available.

isReachable may fail for a lot of reasons as stated in the docs.

Comments

[michdraft]

These are all my device's host and port: 1- 192.168.30.190:3001 2- 192.168.30.191:3001 3- 192.168.30.192:3001 4- 192.168.30.193:3001 5- 192.168.30.194:3001 Could you please give a sample of Passive way?

[michdraft]

Is this good? Can i relay on it? public boolean hostAvailabilityCheck(){ boolean available = true; try { (new Socket(host, port)).close(); } catch (UnknownHostException e) { // unknown host available = false; } catch (IOException e) { // io exception, service probably not running available = false; } return available; }

[PeterMmm]

If your devices config is a fix set, then write some thread that tries to connect with timeout to each of them as your example. For all other setups (host/ip/port detection) IMHO you need a DHCP server because pure Java don´t reach the network layers to do this in a clean way.