Java socket API How to tell if a connection has been closed

Question

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

User · Answer

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

User · Answer

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    gt  0     catch  SocketException e          connection was lost      process buffer

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 - gt      data   log e TAG   Socket closed

User · Answer

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

[java] Java socket API: How to tell if a connection has been closed?

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