Re: Bind several times to the same port in Mac OS X

• Subject: Re: Bind several times to the same port in Mac OS X
• From: Dan Creswell <email@hidden>
• Date: Tue, 14 Dec 2004 19:56:25 +0000
• Delivered-to: email@hidden
• Delivered-to: email@hidden
• User-agent: Mozilla Thunderbird 1.0RC1 (Macintosh/20041201)

Hmmm, something like this came up recently under the subject:
ServerSocket on Mac OS X.  Here was the conclusion of the conversation:

Hello again,

Thank you very much for all your help. The problem was indeed that java
used ipv6 instead of ipv4. To solve it, it sufficed to add the line:

System.setProperty("java.net.preferIPv4Stack","true");

Thanks
Frank

And here's the original question:

All I want to do at the moment is figuring out how to determine whether a
port is in use. Knowing java from different systems / programming languages
this should be pretty straightforward. The snippet:

boolean bInUse;
try{
	ServerSocket oSocket = new ServerSocket();
	oSocket.bind(new InetSocketAddress("host",nPort));
	bInUse = false;
	oSocket.close();
}
catch(BindException oBind)
{
	bInUse = true;
}

should give me the answer right away. The problem though is that I have a
C++ application sitting on the port first. And still the snippet tells me
that this port would not be in use.

Intrigued by that I wrote another small example accepting a connection to
this Java program. Here the outcome:

1. start C++ application on port X	--> success
2. start Java application on port X --> success
3. telnet to localhost port X --> java responds (and shuts down)
4. telnet to localhost port X --> C++ responds

Now I'm completely clueless. I mean the C++ application will not bind to the
port if Java is already on the port, so what is going on here.

My PowerBook lets me bind to the same port twice using NIO. The following code throws an IOException under Windows and Linux, but not
under Mac OS X:

*public* *void* bindTwice(*int* port) *throws* IOException { InetSocketAddress listenAddress = *new* InetSocketAddress(port);

ServerSocketChannel firstChannel = ServerSocketChannel.open(); ServerSocket firstSocket = firstChannel.socket(); firstSocket.bind(listenAddress);

ServerSocketChannel secondChannel = ServerSocketChannel.open(); ServerSocket secondSocket = secondChannel.socket(); secondSocket.bind(listenAddress); }

It wouldn't be too big a deal but I want my application to fail if it
 can't bind! One horrible problem that has come up in my tests is
this: 1. Bind to port X as server. 2. Connect to self on port X as
client. Now the client has fully connected but server has not
received anything! Another server process that has binded to the same
port is receiving the data instead.

Is this a bug in the Mac JVM?

Any information on this problem will be appreciated, thanks!

Cheers, Jesse Wilson

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