I want to create a UDP OUTPUT=UPLOAD stream using java. I will get my INPUT=SOURCE data from a named pipe or contiguous file opened as a file input stream.
My problem is, ALL of the UDP examples i can find on the internet, only demonstrate console sessions. Echo servers and such.
I'm trying to create a way to stream continuous content such as audio/video, and i don't care what gets lost, i'm leaving that up to my user to be concerned with, however my code does need to allow setting the buffer size, and creating a UDP connection.
Ideally a GOOD example would show how to do upload and download mode connections (client mode and server mode)
Can you provide some code to do this, or show me a link on the internets? The fact that I cannot find a UDP stream client/server example is ridiculous. Using UDP to do console sessions tests the limits of a person's sanity! That should never even be considered optional, let alone useful. The client/server code i need must be compatible with GNU netcat. (to ensure correct performance)
I have tried this with a client:
byte[] buffer = new byte[udpPacketSize]; // 4096
int len;
while ((len = standardInput.read(buffer)) != -1) {
udpSocket.send(new DatagramPacket(buffer, len, host, port));
}
But when I stop sending data, and disconnect, I cannot reconnect to send more data. I'm not sure if that is what is supposed to happen, because 1) I am completely out of my element here and 2) when I disconnect after sending the data, the remote instance of GNU netcat does not exit like it does in TCP mode.
HELP, I need a real network systems engineer, to show me how to implement UDP for practical applications!
[and somebody to remove all of that garbage from the internet, but let's keep it simple]
[further: please do not respond with libraries, packages, or shell commands as a solution. i must be able to execute on any embedded device which may not have the programs, and libraries are not teaching me or anyone else how to do anything on their own.]
But when I stop sending data, and disconnect, I cannot reconnect to send more data.
That's the way GNU Netcat UDP mode works, as far as netcat to netcat on a single machine goes...
Your client needs to read a response from the server before disconnecting. So, while acting as "middle-man", you should not be concerned with this, as long as you can connect your network-client's local-client, to the server's response mechanism.
In other words, you need to provide a bi-directional-half-duplex-connection (1:1 communications), since you are not managing the protocol.
Alternatively, you can use a different UDP server than Gnu Netcat. I have tested this one, and it works without the MUST-READ-REPLY-BUG, effectively meaning, there is nothing wrong with my example code. The must-read-reply feature has nothing to do with a correct UDP server implementation (unless you must connect with a Gnu Netcat 0.7.1 compatible server).
It is worth nothing that it isn't very useful to use Gnu Netcat UDP server mode without a driver (script/program) behind it, especially if you want a continuous process, as you could lock your remote client out of access until the process is respawned.
Related
I am completely new to creating a network connection in java so I apologize if this is a stupid question.
I am trying to create a D&D companion in java that will allow a player to create their character and then send it to the DM so that they can view it and make changes and send it back to the player. I want to be able to make it so that any time a field is changed on one computer it will also be changed on the other computer.
After a bunch of research online I have been able to create a socket connection between the DM(server) and the player(client) and pass a message between the two but I am not sure how a socket connection works after this initial connection is made. My research has not been very clear on this. I have found many resources that have said that java closes the socket after a message has been passed and many that say that the socket stays open.
If java closes the socket then my problem is easy enough to solve because then I will just have to open a new socket every time I need to pass data making sure that I pass the IP address of the client to the server the first time I make a connection.
My real questions come in when a socket stays open.
If the socket stays open and multiple clients connect to the server, will the server just shout over the network whenever it transmits a message so that all clients receive the message? (If this is the case then I know I can just attach a username to the front of the message so that the client can determine if the server is talking to it.)
If the server does not shout then how do I specify which client I want the server to talk to?
Will I have to add a loop to my receive methods so that the client/server is constantly listening for a transmission from the server/client or will java automatically do so after I run the method the first time?
I have found many resources that have said that java closes the socket after a message has been passed
You found them where?
and many that say that the socket stays open.
All those are correct. Java never closes connections. The application closes connections.
If java closes the socket then my problem is easy enough to solve because then I will just have to open a new socket every time I need to pass data making sure that I pass the IP address of the client to the server the first time I make a connection.
It doesn't.
My real questions come in when a socket stays open.
If the socket stays open and multiple clients connect to the server, will the server just shout over the network whenever it transmits a message so that all clients receive the message?
No. It will respond via the socket that is connected to the corresponding client.
(If this is the case then I know I can just attach a username to the front of the message so that the client can determine if the server is talking to it.)
Unnecessary.
If the server does not shout then how do I specify which client I want the server to talk to?
The server responds via the same socket it read the request from.
Will I have to add a loop to my receive methods so that the client/server is constantly listening for a transmission from the server/client
No, you will have to add a thread per accepted socket, that loops reading requests until end of stream.
or will java automatically do so after I run the method the first time?
No.
You seem to have been reading some truly appalling drivel. Take a look at the Custom Networking section of the Java Tutorial.
Adding to EJP's wise answer, it might be worth clarifying:
Sounds like you (wisely) use TCP, so your Socket represents a connection between 1 server and 1 client. No "shouting". In examples such as this , when connection is established (namely, client obtains a Socket by calling "new Socket" and server obtains a Socket by calling "accept"), those Sockets are dedicated to those 2 specific endpoints. So if 10 clients connect to 1 server, the server will keep 10 Sockets and won't mix them up. A bit like a poor secretary that has 10 phones on his desk and answers them all - despite the mess, each earpiece is clearly connected to 1 customer.
The connection can hold for a while & serve several messages. It will terminate when either one of the sides calls 'socket.close', or it can be terminated by underlying 3rd parties (operating system, proxies, firewalls).
For your first version, or for simple business requirements, it's probably enough to converse over this 1 simple connection. However, for commercial critical data that requires 'assurance of delivery', you might need to invest some careful thought & possibly tools such as RabbitMQ.
Good luck:)
my task is to receive UDP packets on an Google Compute Engine. Given is the port, which is 300 and I already have a simple Java program which can handle these UDP packets.
My problem now. Get the UDP packets to my Java program.
For that I don't know how to do this.
What I have tried so far ist to let the Java program direct listen to the port, which didn't work. I suspect the traffic from outside the GCE must be routed to the inside?
First make sure you've added a firewall rule on your GCE VM instance's network which allows incoming traffic for UDP protocol on port 300.
You can do this by going to the Developers Console, select your project, then Compute-> Compute Engine-> Networks, click on the right network and verify firewall rules (and tags as well if you used them). If the rule is not added, then add a rule for the traffic.
"gcloud" is a command-line tool which you can use it to list and verify your firewall rules as well [1]:
$ gcloud compute firewall-rules list
If the firewall rules are good, then use a simple troubleshooting tool like netcat to test if traffic is being forwarded to your VM instance.
1. Listing Google Compute Engine firewall rules
I recommend not using Java to process UDP. I experienced inexplicable short periods where all UDP traffic arriving was being lost. This was caused by garbage collection.
The architecture that works for me is to use a GCE VM with a C++ app receiving UDP data, then using libCURL, convert the data to HTTP and dispatch it to Google App Engine for processing. To make the UDP receiver scalable and tolerant, use network balancing and multiple VMs listening for UDP.
There is an existing service that i would like to write a dummy service (using Netty) for. It will be used for testing purposes.
The existing client code fragment for the service looks like:
Socket socket = new java.net.Socket();
socket.connect(new InetSocketAddress("localhost", 8080), 10000);
socket.setSoTimeout(20000); // set a timeout of 20 seconds
InputStreamReader ir = new InputStreamReader(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
// write some string to the server and wait for answer
out.println("SomeCommand");
// server has written some answer, read it
char[] c = new char[2];
ir.read(c, 0, 2);
String cs = new String(c);
if ("OK".equals(cs.toString())) {
// write some more string's to the server
}
// we're done, close the connection
out.println("BYE");
out.close();
socket.close();
Is Netty the right framework to create a server for java.net.Socket connections? (If not, which framework should be used, if any?)
I am trying to find a way to start with Netty using the QuoteOfTheMoment example. The QuoteOfTheMomentServerHandler does basically what i want, upon the incoming message, return some answer so that the above snippet can read the answer using the inputstream but the above socket cannot make a connection to the QuoteOfTheMomentServer. The error is "connection refused".
[EDIT] More clarification:
The problem (i think) is not connecting or the port i use. Let me try to better ask the question:
I just started with netty (no nio experience) and am not familiar with the different types of channels, pipelines and what not.
The server should, like a servlet request/response (like, not http or trying to rebuild a http servlet impl), react on a inputString written to the output-stream as in the code fragment and write back some string/bytes to the input-stream as in the code fragment, so the client only then moves on. So the connection should stay open but also be synchronous, the client waits for answer from the server. If i use the example "Writing the Server Side of a Socket" in the java tutorial i am able to get it working for the client. But i want to utilize the thread handling etc. from netty.
The QuoteOfTheMomentServerHandler seems as server side implementation what i want but can that setup handle the given client code ?
So the question is which kind of pipeline, channel or something like that should be used given the way the client works ?
Again, the client and server are existing. I want to build a dummy server implementation to work with the existing client.
Netty is a TCP/IP framework. So yes if you are developing a TCP/IP server this toolkit is good to use.
I assume you are getting a error when trying to connect the client to the server. Also the server should also be running.
When getting a the connection refused error there are a couple of thine to check. First one is the firewall(if any) on the server allowing connections to port 8080? Secondly from your client machine try open a telnet session to the server something like:
Telnet yourserverip 8080
This opens a socket connection to the server. If you get a error message Google it.
The last one is that you might be running a server like tomcat, glassfish, IIS which uses port 8080 already. Try a non standard port like 10810 for example.
UPDATES:
If you are new to netty please read the users guide found here http://netty.io/docs/stable/guide/html/.
I had a look at the Quote of the moment service and I do believe I found part of the problem. The Quote of the moment service is a broadcast UDP/IP client and server. UDP is a much more lightweight "version" of TCP IP. It does not guarantee delivery to the client or server and it is broadcast. UDP is sort of like a radio broadcast as it is generally not targeted to a specific IP but broadcast over the entire network. Thus you normal TCP IP connection will not be able to work on the UDP server.
See this link on how to write a UDP Client http://systembash.com/content/a-simple-java-udp-server-and-udp-client/.
I would suggest that you convert the Quote of the moment server from UDP to TCP/IP server as this will give you some practise in creating a TCP/IP server without getting into too much detail. Once you are comfortable with that you should be able to start once from scratch.
Just remember that Netty handles the NIO part for you. It is a higher level framework based on NIO thus hiding a lot of the detail from you. You dont need to know NIO that well to use netty but you need to understand the Netty concepts well.
given a flash application that opens a socket connection to a webserver, is it possible to reads packets exchanged with a java application, without redirect all the flash traffic ( that is, without programming a socket proxy)?
What you are trying to do requires lower level network analysis than sockets. Namely libpcap and its Java bindings , jNetPcap. This will let you capture packets much in the same way wireshark does, but from Java. The other options are analysing Wireshark logs after an experiment and that can get clunky quite quickly. You may also consider writing a custom wireshark dissector.
The only way I can think is to modify the hosts file to list your Java server address as if it was the destination address.
BTW if your are only interested in examine the network traffic for that app, you could also use fiddler
I try to develop a file transfer application in Java, with an applet as client, and a standalone java app as server (on a dedicated machine hosted in a datacenter).
I use DataOutputStream/DataInputStream to transfers the data on both sides.
When I send big volumes of data, the bandwith is very variable : all is okay first, then the tcp stream is freezed during 40-50 seconds while nothing is transferring, and then it starts again.
When I look at the tcp stream with Ethereal, I see duplicate acks, fast retransmits, and tcp retransmits.
But I don't think that the problem is originating from Java : I have the same problem with FTP transfers in FileZilla.
But ... when I try to transfer data using netcat (netcat client + netcat server), all is fine, the bandwith is stable, the tcp lost packets seems to be retransmitted immediately without any pause, no matter of the volume transferred.
It's like if Java was not as talented as netcat to play with tcp streams ...
I tried to play with Socket.setSendBufferSize(), but I didn't see any difference.
Any idea ?
Thanks !
And sorry for my bad english ...
Mr amischiefr is right !
It's the same problem that on the other thread.
My problem was solved by replacing DataXXXputStream by BufferedXXXputstream.
The write(byte[], off, len) methods are the same, and the doc doesn't talk about such different behavior. DataOutputStream is buffered, BufferedOutputStream too, but the second one does it much better.
Thanks !
Sounds more like your network is bogged down and you are seeing TCP windowing (I believe that's the correct term) basically limiting your bandwidth.