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home / developersection / forums / how would i choose in java that which port is free to use for serversocket or socket?
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How would I choose in java that which port is free to use for ServerSocket or Socket?

Anonymous User 1507 01 Dec 2015
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?
java java
Updated 01 Dec 2015
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1 Answers

Anonymous User
Anonymous User 01 Dec 2015 Accepted Answer
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You wouldn't need to check. You would specify port 0, which causes the system to give you a free port.
Yes , It is given the IP address that the peer connected to, and the same port as the listening socket.
No we won't need to specify the port and IP address.

You can use the java.net.ServerSocket constructor with port 0 which tells ServerSocket to find a free port.
Example :
int port = -1;
try {
    ServerSocket socket = new ServerSocket(0);
    // here's your free port
    port = socket.getLocalPort();
    socket.close();
} 
catch (IOException ioe) {}


Also, we can use 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
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