Prevent an app from binding to a socket that is already bound

Question

How to prevent a Java app from binding to a socket that another process is already bound to on Windows?

I have an issue where I have a Java application that is listening on port 80. The application starts fine and reports no exceptions. I couldn't figure out why I couldn't connect on port 80. Other ports worked fine. I checked netstat for other processes listening on 80 and found Skype. I didn't think that was possible, but after some research, I'm guessing Skype is listening with the SO_REUSEADDR option. In this state the accepting application is indeterminate. I would like my Java application to fail in this instance with a bind exception (or other).

It looks like I could use SO_EXCLUSIVEADDRUSE if I had access to that option via Java but I don't think that is possible. There are lots of questions and answers around SO_REUSEADDR but none that I could find that answered my question. This isn't just about Skype (I can turn off the listening part), I want my program to be more robust in this situation.

This is snippet from netstat -abn on a Windows 7 box:

    Proto  Local Address          Foreign Address        State
    TCP    0.0.0.0:80             0.0.0.0:0              LISTENING [java.exe]
    TCP    0.0.0.0:80             0.0.0.0:0              LISTENING [Skype.exe]

This is why I'm assuming that Skype is using SO_REUSEADDR

The processes don't appear to be listening on different interfaces.

Here is a snippet of the code. The port is 80:

    myServerSocket = new ServerSocket(myTcpPort);
    while (true) {
        new HTTPSession( myServerSocket.accept(), threadPool );
    }

As further information I created a sample program to minimize any side effects or mishandled messages.

    import java.net.ServerSocket;

    public class PortTest
    {
        public static void main(String[] args)
        {
            System.out.println( "Hit Ctrl-C to stop.\n" );

            try {
                ServerSocket myServerSocket = new ServerSocket(80);
                System.out.println( "Before the accept() call.");
                myServerSocket.accept();
                System.out.println( "After the accept() call." );
            }
            catch (Exception ex ) {
                System.out.println("Error listening.");
                ex.printStackTrace();
            }
         }

    }

I still don't get an exception when running this sample program (PortTest) and when Skype is running and listening on port 80. And to further test, I executed a second instance PortTest and I do see the port in use message.

    Error listening.
    java.net.BindException: Address already in use: JVM_Bind
            at java.net.DualStackPlainSocketImpl.bind0(Native Method)
            at java.net.DualStackPlainSocketImpl.socketBind(Unknown Source)
            at java.net.AbstractPlainSocketImpl.bind(Unknown Source)
            at java.net.PlainSocketImpl.bind(Unknown Source)
            at java.net.ServerSocket.bind(Unknown Source)
            at java.net.ServerSocket.<init>(Unknown Source)
            at java.net.ServerSocket.<init>(Unknown Source)
            at PortTest.main(PortTest.java:10)

Answer 1

socket = new ServerSocket(0);

Will automatically select you a free port.

Furthermore, this code will tell you wether a port is avaliable:

boolean portAvaliable = true;
ServerSocket s = null;
try {
   s = new ServerSocket(yourPort);
}
catch (IOException e) {
   portAvaliable = false;
} 
finally {
   if (s != null)
      try {
        s.close();
      } 
      catch (IOException e) { 
         //handle the exception
      }
}

Check the boolean value from portAvaliable in order to identify the port status.