SYNOPSIS
--------
Concurrent Java 6 and Java 7 processes can open ServerSockets on the same port

OPERATING SYSTEM
----------------
Windows 7 specifically
Does not appear using XP.

FULL JDK VERSION
----------------
Java 7 combined with any other JDK version.
Tested with 1.7.0_01 and 1.6.0_30.

PROBLEM DESCRIPTION
-------------------
On Windows 7 two java processes can open sockets on the same port. This is only possible if one of the processes is Java 7 and the other is another version of Java.

If this is tried with Java 6 in both processes, or Java 7 in both processes, the second attempt to open a socket will always fail with a Bind exception, which is the expected behaviour.

This problem only occurs on Windows 7.

Also,if only IPV4 addresses are used in both the process by specifying these two options:

    -Djava.net.preferIPv4Stack=true
    -Djava.net.preferIPv6Address=false

a BindException is thrown as expected.

REPRODUCTION INSTRUCTIONS
-------------------------
The testcase below demonstrates the errant behaviour. If the testcase is invoked with any parameter it will open a ServerSocket on port 1979 and then wait. If it is invoked with no parameters it will just try and open a ServerSocket on port 1979, then close the socket.

1. Using Java 6:

      java BPWinWSocket wait

   This will open a ServerSocket on port 1979 and then wait with the
   socket open.

2. In a separate command window, using Java 7:

      java BPWinSocket

Observed result:
The second attempt to open the socket apparently succeeds - no Exception is thrown.

Expected result:
The second attempt to open the ServerSocket should fail with an BindException.

TESTCASE SOURCE
---------------
import java.io.IOException;
import java.net.*;

public class BPWinSocket {
    private int port = 1979;
    private ServerSocket s1 = null;

    public BPWinSocket(boolean wait) {
        open(s1, 1);
        if (wait) {
            synchronized (this) {
                try {
                    System.err.println("====> " + "Waiting ...");
                    this.wait();
                } catch (InterruptedException e) {
                }
            }
        }
        printStatus();
        close();
    }

    public static void main(String[] args) {
        if (args.length == 0) {
            new BPWinSocket(false);
        } else {
            new BPWinSocket(true);
        }
    }

    private void open(ServerSocket sock, int num) {
        try {
            sock = new ServerSocket(port);
            System.err.println("====> " + "ServerSocket " +sock + " created on port "
                               + port);
        } catch (IOException e) {
            System.err.println("====> " + "ServerSocket s" + num + " failed on port "
                               + port);
            e.printStackTrace();
        }
    }

    private void close() {
        if (s1 != null) {
            try {
                s1.close();
            } catch (IOException e) {
            }
        }
    }

    /**
     * Just to make sure nothing odd happens with the sockets like premature GC
     */
    private void printStatus() {
        if (s1 != null) {
            System.err.println("====> " + s1.toString());
        }
    }
}