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java socket - handling client disconnection [duplicate]

I am running into some issues with the Java socket API. I am trying to display the number of players currently connected to my game. It is easy to determine when a player has connected. However, it seems unnecessarily difficult to determine when a player has disconnected using the socket API.
Calling isConnected() on a socket that has been disconnected remotely always seems to return true. Similarly, calling isClosed() on a socket that has been closed remotely always seems to return false. I have read that to actually determine whether or not a socket has been closed, data must be written to the output stream and an exception must be caught. This seems like a really unclean way to handle this situation. We would just constantly have to spam a garbage message over the network to ever know when a socket had closed.
Is there any other solution?

There is no TCP API that will tell you the current state of the connection. isConnected() and isClosed() tell you the current state of your socket. Not the same thing.
isConnected() tells you whether you have connected this socket. You have, so it returns true.
isClosed() tells you whether you have closed this socket. Until you have, it returns false.
If the peer has closed the connection in an orderly way
read() returns -1
readLine() returns null
readXXX() throws EOFException for any other XXX.
A write will throw an IOException: 'connection reset by peer', eventually, subject to buffering delays.
If the connection has dropped for any other reason, a write will throw an IOException, eventually, as above, and a read may do the same thing.
If the peer is still connected but not using the connection, a read timeout can be used.
Contrary to what you may read elsewhere, ClosedChannelException doesn't tell you this. [Neither does SocketException: socket closed.] It only tells you that you closed the channel, and then continued to use it. In other words, a programming error on your part. It does not indicate a closed connection.
As a result of some experiments with Java 7 on Windows XP it also appears that if:
you're selecting on OP_READ
select() returns a value of greater than zero
the associated SelectionKey is already invalid (key.isValid() == false)
it means the peer has reset the connection. However this may be peculiar to either the JRE version or platform.

It is general practice in various messaging protocols to keep heartbeating each other (keep sending ping packets) the packet does not need to be very large. The probing mechanism will allow you to detect the disconnected client even before TCP figures it out in general (TCP timeout is far higher) Send a probe and wait for say 5 seconds for a reply, if you do not see reply for say 2-3 subsequent probes, your player is disconnected.
Also, related question

I see the other answer just posted, but I think you are interactive with clients playing your game, so I may pose another approach (while BufferedReader is definitely valid in some cases).
If you wanted to... you could delegate the "registration" responsibility to the client. I.e. you would have a collection of connected users with a timestamp on the last message received from each... if a client times out, you would force a re-registration of the client, but that leads to the quote and idea below.
I have read that to actually determine whether or not a socket has
been closed data must be written to the output stream and an exception
must be caught. This seems like a really unclean way to handle this
situation.
If your Java code did not close/disconnect the Socket, then how else would you be notified that the remote host closed your connection? Ultimately, your try/catch is doing roughly the same thing that a poller listening for events on the ACTUAL socket would be doing. Consider the following:
your local system could close your socket without notifying you... that is just the implementation of Socket (i.e. it doesn't poll the hardware/driver/firmware/whatever for state change).
new Socket(Proxy p)... there are multiple parties (6 endpoints really) that could be closing the connection on you...
I think one of the features of the abstracted languages is that you are abstracted from the minutia. Think of the using keyword in C# (try/finally) for SqlConnection s or whatever... it's just the cost of doing business... I think that try/catch/finally is the accepted and necesary pattern for Socket use.

I faced similar problem. In my case client must send data periodically. I hope you have same requirement. Then I set SO_TIMEOUT socket.setSoTimeout(1000 * 60 * 5); which is throw java.net.SocketTimeoutException when specified time is expired. Then I can detect dead client easily.

I think this is nature of tcp connections, in that standards it takes about 6 minutes of silence in transmission before we conclude that out connection is gone!
So I don`t think you can find an exact solution for this problem. Maybe the better way is to write some handy code to guess when server should suppose a user connection is closed.

As #user207421 say there is no way to know the current state of the connection because of the TCP/IP Protocol Architecture Model. So the server has to notice you before closing the connection or you check it by yourself.
This is a simple example that shows how to know the socket is closed by the server:
sockAdr = new InetSocketAddress(SERVER_HOSTNAME, SERVER_PORT);
socket = new Socket();
timeout = 5000;
socket.connect(sockAdr, timeout);
reader = new BufferedReader(new InputStreamReader(socket.getInputStream());
while ((data = reader.readLine())!=null)
log.e(TAG, "received -> " + data);
log.e(TAG, "Socket closed !");

Here you are another general solution for any data type.
int offset = 0;
byte[] buffer = new byte[8192];
try {
do {
int b = inputStream.read();
if (b == -1)
break;
buffer[offset++] = (byte) b;
//check offset with buffer length and reallocate array if needed
} while (inputStream.available() > 0);
} catch (SocketException e) {
//connection was lost
}
//process buffer

Thats how I handle it
while(true) {
if((receiveMessage = receiveRead.readLine()) != null ) {
System.out.println("first message same :"+receiveMessage);
System.out.println(receiveMessage);
}
else if(receiveRead.readLine()==null)
{
System.out.println("Client has disconected: "+sock.isClosed());
System.exit(1);
} }
if the result.code == null

On Linux when write()ing into a socket which the other side, unknown to you, closed will provoke a SIGPIPE signal/exception however you want to call it. However if you don't want to be caught out by the SIGPIPE you can use send() with the flag MSG_NOSIGNAL. The send() call will return with -1 and in this case you can check errno which will tell you that you tried to write a broken pipe (in this case a socket) with the value EPIPE which according to errno.h is equivalent to 32. As a reaction to the EPIPE you could double back and try to reopen the socket and try to send your information again.

Java SSLSocket what to do after read returns -1?

I'm using a java server to connect to a browser with secure websockets. All works fine with the connect, but many times i get an unexpected -1 result from socket.in.read(buffer,off,len), this happens also in the middle of a frame. Normally i close a socket directly upon reception of -1, since it is end of stream. However i noted that it can also happen on a connection reset. I have come over many cases where in my tests the socket whould return valuable data after read returned -1. I even have the feeling this is more often than not. My problem arrises when sometimes i just get some scrambled data out of the socket after such a case. Another problem is that the other side is not notified when a frame cannot be delivered... So what good is TCP/SSL than? if you need to consider it an unreliable connection for transporting websocket frames in java?
I have some schemes to use that are used to deal with unreliable connections for making shure a packet arrives. But i hope that somebody knows what to do after read returns -1.
Sorry for the somewhat vague description in this one... i'm getting tired with solving this issue.
Just an example of some rubbish comming in (only text frames are submitted containing JSON data):
16-06-13 22:43:13.918;WebSocket;7: Read frame from websocket: 377, opcode:UNKNOWN
data: null
16-06-13 22:43:13.918;WebSocket;7: Read frame from websocket: 377, opcode:PONG_FRAME
data: null
16-06-13 22:43:13.918;WebSocket;7: Read frame from websocket: 377, opcode:TEXT_FRAME
data: =,6GiGGV7C6_TfPHg\~\c
Here another example of a received frame that is just a bit malformed!? how is this possible with a TCP/TLS connection???:
17-06-13 09:42:37.510;WebSocket;7: Read frame from websocket: 15, opcode:TEXT_FRAME
data: "kep-aiveY:"d613Nb2-N24eV463K-808-fJb30I9e3M02
It is supposed to read {"keep-alive":"[UUID]"}
Meanwhilst i have done some more testing and found that 9 out of 10 times it works if you continue reading after reception of -1. So even if you are reading halfway the frame and receive a -1 then you should test somehow if the socket is closed or not, i now use: socket.isInputShutdown(). if this is not the case then just continue filling up the buffer. To do so i now use the following code where socket is the SSLSocket:
public static int readFully(Socket socket, InputStream is, byte[] buffer, int off, int len) throws IOException
{
int read = 0;
while(read < len)
{
int b = is.read();
if(b < 0)
{
Logger.log(TAG, "readFully read returned: " + b + " testing if connection is reset or closed.", Logger.WARNING);
if(socket.isInputShutdown())
{
throw new IOException("InputStream closed before data could be fully read! (readFully read returned -1 and socket.isInputShutdown() is true");
}
}
else
{
buffer[off + (read++)] = (byte) b;
}
}
return read;
}
It is still not a hundred % correct but at leas i get more reliable results then before.

i get an unexpected -1 result from socket.in.read(buffer,off,len)
You have already reached EOS (end of stream) before you called this method.
this happens also in the middle of a frame.
There is no such thing as a 'frame' in TCP. If you mean it happens in the middle of an application message, you have an application protocol error.
Normally i close a socket directly upon reception of -1, since it is end of stream.
Correct.
However i noted that it can also happen on a connection reset
No it doesn't. If it did, you could not possibly have detected the reset. The statement is self-contradictory.
I have come over many cases where in my tests the socket whould return valuable data after read returned -1.
No you haven't. A socket can't return anything but -1 after it first does so. You can't be getting any data at all, let alone 'valuable' data, unless you are ignoring the -1 somewhere.
My problem arrises when sometimes i just get some scrambled data out of the socket after such a case.
Only if you ignore the -1, as you are doing.
Another problem is that the other side is not notified when a frame cannot be delivered.
Of course it isn't. If you could deliver a notification to the other side, you could deliver the packet. This doesn't make sense either. If you mean that the other side doesn't get notified when it couldn't deliver the packet, you are up against the fact that TCP sends are asyncrhonous, so you won't normally get a send error on the send that caused it. You will get it on a later send. If you need per-send acknowledgements, you need to build them into your application protocol.
So what good is TCP/SSL then?
TCP is a reliable data-stream protocol, and SSL is a secure reliable data-stream protocol. That's what use they are.
if you need to consider it an unreliable connection for transporting websocket frames in java?
Neither of them is unreliable.
I hope that somebody knows what to do after read returns -1.
Close the socket.
Meanwhilst i have done some more testing and found that 9 out of 10 times it works if you continue reading after reception of -1.
No it doesn't. 1000 times of 1000 it continues to return -1. All you are seeing here is the effect of other bugs in your code.
So even if you are reading halfway the frame and receive a -1 then you should test somehow if the socket is closed or not
You can't. The socket isn't closed. Proof: you just read from it without getting an exception. You can't test whether the connection is closed either, other than by read() returning -1.
I now use: socket.isInputShutdown().
Pointless. That tells you whether you have called Socket.shutdownInput() on your own socket. It doesn't tell you diddly-squat about the state of the connection. There is no TCP API that can do that, other than reading or writing.
if this is not the case then just continue filling up the buffer.
I.e. reading gargabe by ignoring the -1 that read() is returning.
To do so i now use the following code where socket is the SSLSocket:
Why? DataInputStream.readFully() already exists. Re-implementing it won't help.
if(b < 0)
{
Logger.log(TAG, "readFully read returned: " + b + " testing if connection is reset or closed.", Logger.WARNING);
if(socket.isInputShutdown())
At this point it is 100% irrelevant whether your Socket is shutdown for input. read() has returned -1, which means the peer has closed the connection. Period.
{
throw new IOException("InputStream closed before data could be fully read! (readFully read returned -1 and socket.isInputShutdown() is true");
}
This is all nonsense.
}
else
{
buffer[off + (read++)] = (byte) b;
}
Here you are adding the low byte of -1, which is 0xff, to the buffer. This is also nonsense.

Setting timeout on blocking I/O ops with ServerSocketChannel doesn't work as expected

I'm working on a small group conversation server in Java and I'm currently hacking network code, but it seems like I cannot set right timeout on blocking I/O ops: chances are I've been bitten by some Java weirdness (or, simply, I misinterpret javadoc).
So, this is the pertinent code from ConversationServer class (with all security checks and logging stripped for simplicity):
class ConversationServer {
// ...
public int setup() throws IOException {
ServerSocketChannel server = ServerSocketChannel.open();
server.bind(new InetSocketAddress(port), Settings.MAX_NUMBER_OF_PLAYERS + 1);
server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS);
int numberOfPlayers;
for (numberOfPlayers = 0; numberOfPlayers < Settings.MAX_NUMBER_OF_PLAYERS; ++numberOfPlayers) {
SocketChannel clientSocket;
try {
clientSocket = server.accept();
} catch (SocketTimeoutException timeout) {
break;
}
clients.add(messageStreamFactory.create(clientSocket));
}
return numberOfPlayers;
}
// ...
}
The expected behaviour is to let connect Settings.MAX_NUMBER_OF_PLAYERS clients at most, or terminate setup anyway after Settings.AWAIT_PLAYER_MS milliseconds (currently, 30000L).
What happens, is that if I connect Settings.MAX_NUMBER_OF_PLAYERS clients, everything is fine (exit because of for condition), but if I don't, the SocketTimeoutException I'd expect is never thrown and the server hangs forever.
If I understand right, server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS); should be sufficient, but it doesn't give the expected behaviour.
So, can anyone spot the error here?

It looks like the timeout works if you change from
server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS);
server.accept();
to
server.socket().setSoTimeout((int) Settings.AWAIT_PLAYERS_MS);
server.socket().accept();
I.e. call accept() on the same object on which you set the SO timeout. I don't know enough about NIO sockets to say exactly what results from doing this. Maybe somebody else can shed some light.

Expanding my Java program to send a alert message to other computers

I've written a java intake program that send an PDF-formatted intake to a shared folder so that other people in the network can read it. However, there is not a way for the other people to know that an intake was sent unless someone tells them, so I want the program to send an alert message to the other computers telling them that an intake has been sent.
Now I've done some research into this and figured that TCP is the way to go since it's reliable. I also know that this is a one-to-many sending going on, so I assume that my Intake program will act as the server an the other computers will be the client, or should it be the other way around?
Now I assume that I have to create a client program that listens to the server and waits for it to send a message.
With that in mind, how do I:
Create a client program that listens for the message continuously until the program is closed. I assume that I'll be using "while (true)" and sleep. If so, how long do I put the program to sleep?
Make it as part of Windows service so that can load up when Windows start.
On the server end, how do I:
Send messages to more than one computer, since TCP is not capable of multicasting or broadcasting. I assume an array/vector will play a part here.
Oh, this is a one-way communication. The client doesn't have to respond back to the server.

First of all, UDP is quite reliable (in fact, as reliable as the IP protocol itself). TCP simply ensures that the data was received which involved quite a lot of magic in the back end. Unless you absolutely need to be sure that other machines got the message, you could do it with UDP. Mind that I'm not saying “Don't use TCP”, I just want to make it straight that you should take UDP into consideration as well.
Anyway, yes, you can create a simple listening program. Here is an example of a client in Java that reads messages from the server. It overrides the run method of a Thread class:
public void run() {
try {
String messageFromServer = reader.readLine();
while (messageFromServer != null) {
// Do things with messageFromServer here
// processor.processFromServer(messageFromServer);
messageFromServer = reader.readLine(); // Blocks the loop, waits for message
}
}
catch (IOException e) {
// Handle your exception
}
}
Amongst other things, my thread was set up as such:
public CommunicationThread(String hostname, int port, int timeout) throws IOException, SocketTimeoutException {
InetSocketAddress address = new InetSocketAddress(hostname, port);
socket = new Socket();
socket.connect(address, 2000); // 2000ms time out
// You can use the writer to write messages back out to the server
writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
}
Now, regards to server-side you can do something as follows:
Write a program to allow clients to contact, given that they know your address.
Accept the connections, and store the sockets in a list.
When you need to send out a message, traverse the list and send the data to everyone on it.
You can start listening on your server with
this.socket = new ServerSocket(port);
You could (or even should(?)) make it threaded so that you can accept clients while serving others. You can accept new clients with:
socket.accept(); // Blocks, waiting for someone to connect, returns open socket
Feel free to pass that to a whole new class which can deal with BufferedWriter (and maybe even BufferedReader if you want to read from clients as well). That class is where you would implement things such as writeToClient(message)
Consider the situation where you have a ClientConnection class that has writeToClient(String s) method and (Server server, Socket socket) and initialized ArrayList conList.
Here is how you would follow:
In a separate thread in Server, accept connections with
ClientConnection con = new ClientConnection(this, socket.accept());
conList.add(con);
Then, when you want to write to clients:
for (ClientConnection c : conList) {
c.writeToClient("I'm sending you a message!");
}
I hope you get a vague idea of what you need to do. Read the Socket documentation, it's very useful. Also, as always with threaded applications, make sure you aren't doing things such as modifying a list while traversing it and avoid race conditions.
Good luck!

Java -> C++ TCP Socket does not send until close

I am facing another TCP Socket issue.
I've read through a huge bunch of questions an answers to similar issues, but my problem is somehow different.
I have a Java Client and C++ Server. Everything goes as expected until I'm using different machines (equal to other issues so far)
The messages from the client seem to getting stuck in den TCP Buffer. When I finally close the socket, everything is sent to the server. But these single messages are controlmessages so I need them to be send immediatly. As far as I read this is expected behavior, but how do I send reliable control messages.
Is there a way to force the messages to be sent. (I can leave the socket open for a couple of minutes with nothing is sent.)
Is there something wrong? (see the following code)
Do I have to close the socket each time to perform a REAL flush?
Should I use UDP instead, with an additional amount of protocol work?
Javacode:
mSocketSend = new Socket();
mSocketSend.connect(new InetSocketAddress(mServerIp, mSocketPortSend), mTimeOut);
PrintWriter pw = new PrintWriter(mSocketSend.getOutputStream(), true);
pw.println(data);
C++ Code:
opening socket...(i leave that)
char* buffer = new char[1024];
int rc = recv(mConnectedSocket, buf, 1024, 0);
If you want more of it. Write it. I left almost everything out. ^^ I dont think its relevant. The Communication wents great usually.. No errors at all. So its just this TCPBuffer thingi.
I know there should be some delimiter or message length stuff. But in fact: A message length, which is not sent, does not help. ^^
Thanks for your help.
EDIT #01 The whole bunch of code:
mSocket->createSocketServer(22);
char* buffer = new char[1024];
while(true){
int numberBytes = mSocket->receiveChars(buffer, 1024);
if (numberBytes > 0){
uninterestingHandlingFunction(buffer);
}else{
mSocket->createSocketServer(22);
}
}
bool Socket::createSocketServer(u_short port)
{
if (mConnectedSocket != INVALID_SOCKET)
{
closesocket(mConnectedSocket);
}
if (s == INVALID_SOCKET)
{
WSADATA wsa;
if (WSAStartup(MAKEWORD(2,0), &wsa) != 0)
return 0;
s = socket(AF_INET, SOCK_STREAM, 0);
if (s == INVALID_SOCKET)
return 0;
SOCKADDR_IN addr;
memset(&addr, 0, sizeof(SOCKADDR_IN));
addr.sin_family=AF_INET;
addr.sin_port=htons(port);
addr.sin_addr.s_addr=ADDR_ANY;
if (bind(s, (SOCKADDR*)&addr, sizeof(SOCKADDR_IN)) == SOCKET_ERROR)
{
s = INVALID_SOCKET;
} else if (listen(s, 10) == SOCKET_ERROR)
{
s = INVALID_SOCKET;
}
if (s == INVALID_SOCKET)
{
closesocket(s);
return 0;
}
}
mConnectedSocket = accept(s, NULL, NULL);
if (mConnectedSocket == INVALID_SOCKET)
{
closesocket(s);
return 0;
}
return 1;
}
int Socket::receiveChars(char* buf, unsigned maxSize)
{
if (mConnectedSocket == INVALID_SOCKET)
return -1;
int rc = recv(mConnectedSocket, buf, maxSize, 0);
if (rc == SOCKET_ERROR)
{
std::cout << "Socket: error " << WSAGetLastError() << std::endl;
}
return rc;
}
You wanted it....
EDIT #2 Give it one more try
There are few more things I tried out.
At first: This problem does not occure on a device connected over real network everytime. -> Full Reboot Client&Server -> Problem does not occure -> Full Reboot Client&Server -> Problem occures
Sadly, I don't know what to take from this habit.
Another thing I stumbled over is the bind and listen socket (in Code SOCKET s). This socket listens for connections and if the working thread needs a new connection (on startup or if the previous closes) the socket s gives the next queued connection to mConnectedSocket for recv, other connections are backlogged while one is processed.
From the Java view: a Socket is connected (Device A). next socket (Device B) tries to connect. -> Connection success (its properly controlled in code if this is happens indeed) -> followed by sending data in natural matter. (The socket is still in the backlog on c++ side)
Well, this is hard to transform to the habit I experienced. I'll try to express my thoughts.
Javaside: PrintWriter is created. Feeded with data and is flushed. Because the connection is not fully established (No additional connectedSocket on C++ side). The flush doesn't work. And onClose the socket finally flushes its content.
Please tell me to shut up, if you think so. I dont really know what the "Connection is backlogged" ACTUALLY mean in implementation" ^^
I know, I should open a new thread for each connection, but I can't at the moment. So stick with this server code.

You need to do a flush(), that pushes the data out.

PrintWriter.flush();
Or use a writer with automatic flushing.
You should also make sure that the server reads a line (until \n) and not the full 1024 chars, but I don't know what recv() does so I don't know about that.

Fixed it. Kind of embarassing...
The backlog, i noticed in my edit was indeed the problem.
If two at clients at a time connect to the server, the second is backlogged and his messages would be processed, when the first disconnects.
Additionally (here comes the clue)
As mentioned before its an android java client. There is another thread on java side to receive data from the C++ server. This socket connects on another port.
BUT i set the ports and ip addresses to connect to in a settings activity and there was a bad port for the other socket as default value (same as for the issuesocket, wrong variable taken)
So this socket connects first and the issuesocket connects into the backlog.
This default value is only taken if I enter the settings to set another IPAddress (For example, when I connect to a remote host instead of localhost)
Incredible circumstances... i didnt even wrote the settings...
WIRESHARK would have fixed this.

PrintWriter pw = new PrintWriter(mSocketSend.getOutputStream(), true);
pw.println(data);
pw.flush();

Actually your problem could be on the receiving end, your recv needs to be in a loop (you can google for examples of this). There is no guarantee not much each call to recv will get. If you know you are flushing the data on the Java site, that's probably your problem.

As you are using auto flush and even tried using an explicit flush():
Could be because you don't open the inputStream. Try and do a getInputStream() as well.
Otherwise, Have you tried:
any diff you don't use connect but just give the parameters directly in the Socket constructor?
setTcpNoDelayon the socket (shouldn't cause minutes of delay though!!)?

Your loop is incorrectly coded. Every new recv() will overwrite the previous one. You should advance the offset parameter.