AppletTalk.com Java discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Sockets and Selectors -- Is Data One Way?          AppletTalk.com Forum Index -> Java Help View previous topic :: View next topic   Author Message Hal Vaughan Guest Posted: Sun Mar 19, 2006 10:12 pm    Post subject: Sockets and Selectors -- Is Data One Way? I'm using some sockets and selectors. I read from one and write to the other. Here's where I'm confused: when I write to the second, I would have expected that data to be outgoing, as in an OutputStream, but once I write data to that socket (using SocketChannel.wrtie(ByteBuffer), the Selector keeps returning a key to read from that socket and when I try to read, there is nothing to read. I've also had this happen when a ServerSocket (or actually a ServerSocketChannel) creates a new Socket (or SocketChannel) from SocketChannel.accept(). When I first send test data through, it works, but after it has been idle for a while, the Selector keeps providing a key to read from the socket, but there's nothing to read. I solved it with the SocketChannel created by ServerSocketChannel.accept() by closing it once there was no more data so ServerSocketChannel had to accept() again if more data came in. (Isn't that time consuming and likely to lead to lost data -- forcing it to open a new socket after every pause in data?) I can post source code, but on this topic, I've found when I've been posting the source (which is long), I'm not getting responses and when I post the questions, I am more likely to get help. (I have no idea why!) Thanks for any help on this. Hal Back to top Guest Posted: Tue Mar 21, 2006 11:12 pm    Post subject: Re: Sockets and Selectors -- Is Data One Way? My guess is that the socket is getting closed unexpectedly. I think when the socket is shut down or closed, the selector will select it with the OP_READ bit ready. Attempting to read from it will return -1, and you are expected to close the socket at that point. It's only a guess, though. I could try to look at code if you wanted to send any. Back to top Hal Vaughan Guest Posted: Thu Mar 23, 2006 8:12 pm    Post subject: Re: Sockets and Selectors -- Is Data One Way? Thanks for the reply, and sorry it took me, as the one asking for help, to get back here.  I've been trying a lot of different things, including other programs and such.  It is very hard to find good info on this, either in Java, or in other languages.  I'm beginning to think the number of programmers that actually work with with networking, sockets, and communication, must be rather small.  I'm sure there are people who open sockets for communication, but not for anything more than just retrieving files or e-mail or something where the protocols are all laid out. mandelin (AT) cs (DOT) berkeley.edu wrote: Quote: My guess is that the socket is getting closed unexpectedly. I think when the socket is shut down or closed, the selector will select it with the OP_READ bit ready. Attempting to read from it will return -1, and you are expected to close the socket at that point. It's only a guess, though. I could try to look at code if you wanted to send any. I'm not getting any -1 lengths, all 0.  I also added in debugging lines to see if the socket was half closed in either direction, and didn't get anything.  I've seen one or two books that include some info, including one with a port forwarder, but they don't get too detailed and generally present an example or two and that's it.  The one with the port forwarder does not use Selector.select() and, instead, used a loop to wait for data, which I figure is VERY time consuming on the CPU and want to avoid. Am I right that when I write data to a socket, it is outbound, and should not be seen at all by my program after it has been sent out? And here's another point that confuses me.  I thought once a connection was opened, the socket should stay open until it was no longer needed.  From what I've seen, at least with server sockets, you open with accept(), handled the data that is there, and when there's no stream, close the socket and wait for a new one to be created when there's more incoming data.  But that doesn't tell me what to do in that case when there's been no incoming data, and you need to send data back to the client you were connected to a second ago. Hal Back to top Dave Mandelin Guest Posted: Mon Mar 27, 2006 7:12 pm    Post subject: Re: Sockets and Selectors -- Is Data One Way? Hal Vaughan wrote: Quote: Thanks for the reply, and sorry it took me, as the one asking for help, to get back here. I've been trying a lot of different things, including other programs and such. It is very hard to find good info on this, either in Java, or in other languages. I'm beginning to think the number of programmers that actually work with with networking, sockets, and communication, must be rather small. I'm sure there are people who open sockets for communication, but not for anything more than just retrieving files or e-mail or something where the protocols are all laid out. That may be. I am not an expert on networking myself, but I have successfully programmed network applications before, including a client/server VoIP application just this week, so I am confident we can get your program working. One general point to keep in mind is that minor variations in drivers and network stack implementations can have suprising effects, so that may be what you are seeing. But your application seems fairly standard, so I think it should not be affected by such things. Quote: mandelin (AT) cs (DOT) berkeley.edu wrote: My guess is that the socket is getting closed unexpectedly. I think when the socket is shut down or closed, the selector will select it with the OP_READ bit ready. Attempting to read from it will return -1, and you are expected to close the socket at that point. It's only a guess, though. I could try to look at code if you wanted to send any. I'm not getting any -1 lengths, all 0. I also added in debugging lines to see if the socket was half closed in either direction, and didn't get anything. I've seen one or two books that include some info, including one with a port forwarder, but they don't get too detailed and generally present an example or two and that's it. The one with the port forwarder does not use Selector.select() and, instead, used a loop to wait for data, which I figure is VERY time consuming on the CPU and want to avoid. Yes. I believe the reasonable alternatives for a Java server are multithreading and select. I think multithreading is simpler to program, but select may be higher performance. I'm not really sure about the performance, though. If the length is 0, the socket is not actually ready for reading. My guess is that there is a bug in the code causing the read code to be run even though select, by its proper semantics, has not actually indicated that data is ready. If you send code I will read it for such errors. Quote: Am I right that when I write data to a socket, it is outbound, and should not be seen at all by my program after it has been sent out? Yes. Quote: And here's another point that confuses me. I thought once a connection was opened, the socket should stay open until it was no longer needed. From what I've seen, at least with server sockets, you open with accept(), handled the data that is there, Sounds fine so far. Quote: and when there's no stream, close the socket and wait for a new one to be created when there's more incoming data. This I don't quite understand. What does "when there's no stream" mean? Quote: But that doesn't tell me what to do in that case when there's been no incoming data, and you need to send data back to the client you were connected to a second ago. I don't quite understand the question, but in a typical TCP client/server setup, you keep the sockets connected as long as the server may need to send information back to the client. -- Need to get from a Foo object to a Bar object in Java? Ask Prospector: http://snobol.cs.berkeley.edu Want to play tabletop RPGs over the internet? 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