How would I choose in java that which port is free to use for ServerSocket or Socket?

Question

I'm working on java. How would I check that a port is free to use for a ServerSocket? Moreover when a Socket is returned by the accept() function is it given a port and IP address by default or I would have to specify that too?

[This](http://stackoverflow.com/questions/434718/sockets-discover-port-availability-using-java) might help. — Bubletan, Jul 07 '15 at 19:49
Note you should limit the ports you choose from to the [dynamic / private port range](https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers#Dynamic.2C_private_or_ephemeral_ports), so 49152–65535. — dimo414, Jul 08 '15 at 03:51

score 9 · Answer 1 · answered Dec 01 '15 at 08:29

You can use the java.net.ServerSocket constructor with port 0 which tells ServerSocket to find a free port.

documentation:

port - the port number, or 0 to use a port number that is automatically allocated.

Example:

int port = -1;
try {
    ServerSocket socket = new ServerSocket(0);
    // here's your free port
    port = socket.getLocalPort();
    socket.close();
} 
catch (IOException ioe) {}

waders group · Accepted Answer · 2015-07-08T03:20:17.100

2

Use Try catch to find a free port

private static int port=9000;
public static int detectPort(int prt)
{
try{
//connect to port
}
catch(Exception e)
{
return detectPort(prt+1);
}
return prt;
}

// Write detectPort(port); inside main

edited Jul 08 '15 at 03:20

answered Jul 07 '15 at 20:05

waders group

538
3
7

`detectPort(prt++);` will result in an infinite loop, right? Using `prt+1` would be better, I think. – JackWhiteIII Jul 07 '15 at 20:46
I did a mistake .. thnx fr letting me know – waders group Jul 08 '15 at 03:21
1

Recursion risks a stack overflow here, simpler to just use a loop. You shouldn't be grabbing any ports lower than 49152, as well. – dimo414 Jul 08 '15 at 04:37
1

Complete waste of time and space. Specifying zero will give you a free port without any retrying at all. – user207421 Dec 01 '15 at 08:38

score 1 · Answer 3 · answered Dec 29 '16 at 18:20

I have released a tiny library for doing just that with tests in mind. Maven dependency is:

<dependency>
    <groupId>me.alexpanov</groupId>
    <artifactId>free-port-finder</artifactId>
    <version>1.0</version>
</dependency>

Once installed, free port numbers can be obtained via:

int port = FreePortFinder.findFreeLocalPort();

Bian Jiaping · Answer 4 · 2017-10-14T15:20:21.450

We can refer to how Spring's SocketUtils works.

The key idea is to generate a random port, check whether it's available, and repeat until an available port is found.

private enum SocketType {

    TCP {
        @Override
        protected boolean isPortAvailable(int port) {
            try {
                ServerSocket serverSocket = ServerSocketFactory.getDefault().createServerSocket(
                        port, 1, InetAddress.getByName("localhost"));
                serverSocket.close();
                return true;
            }
            catch (Exception ex) {
                return false;
            }
        }
    },

    UDP {
        @Override
        protected boolean isPortAvailable(int port) {
            try {
                DatagramSocket socket = new DatagramSocket(port, InetAddress.getByName("localhost"));
                socket.close();
                return true;
            }
            catch (Exception ex) {
                return false;
            }
        }

};

Nice race condition if another process on the system does the same. — weberjn, Nov 24 '20 at 14:38

score 0 · Answer 5 · answered Dec 01 '15 at 08:37

How would I check that a port is free to use for a ServerSocket?

You wouldn't. You would specify port 0, which causes the system to give you a free port.

Moreover when a Socket is returned by the accept() function is it given a port and IP address by default

Yes. It is given the IP address that the peer connected to, and the same port as the listening socket.

or I would have to specify that too?

No.

How would I choose in java that which port is free to use for ServerSocket or Socket?

5 Answers5