Using Sockets to send and receive data

Question

I am using sockets to connect my Android application  client  and a Java backend Server  From the client I would like to send two variables of data each time I communicate with the server   1  Some kind of message  Defined by the user through the interface   2  The language of the message  Defined by the user through the interface   How could I send these so that the server interprets each as a separate entity   Having read the data on the server side and made an appropriate conclusion  I would like to return a single message to the client   I think I will be okay with this   So my two questions are how can I establish that the two Strings being sent  client to server  are unique on the client side and how can I separate these two Strings on the server side   I was thinking an array of Strings  but could not establish if this was possible or appropriate    I was going to post some code but I m not sure how that would help

User · Accepted Answer

I assume you are using TCP sockets for the client-server interaction? One way to send different types of data to the server and have it be able to differentiate between the two is to dedicate the first byte (or more if you have more than 256 types of messages) as some kind of identifier. If the first byte is one, then it is message A, if its 2, then its message B. One easy way to send this over the socket is to use DataOutputStream/DataInputStream:

Client:

Socket socket = ...; // Create and connect the socket
DataOutputStream dOut = new DataOutputStream(socket.getOutputStream());

// Send first message
dOut.writeByte(1);
dOut.writeUTF("This is the first type of message.");
dOut.flush(); // Send off the data

// Send the second message
dOut.writeByte(2);
dOut.writeUTF("This is the second type of message.");
dOut.flush(); // Send off the data

// Send the third message
dOut.writeByte(3);
dOut.writeUTF("This is the third type of message (Part 1).");
dOut.writeUTF("This is the third type of message (Part 2).");
dOut.flush(); // Send off the data

// Send the exit message
dOut.writeByte(-1);
dOut.flush();

dOut.close();

Server:

Socket socket = ... // Set up receive socket
DataInputStream dIn = new DataInputStream(socket.getInputStream());

boolean done = false;
while(!done) {
  byte messageType = dIn.readByte();

  switch(messageType)
  {
  case 1: // Type A
    System.out.println("Message A: " + dIn.readUTF());
    break;
  case 2: // Type B
    System.out.println("Message B: " + dIn.readUTF());
    break;
  case 3: // Type C
    System.out.println("Message C [1]: " + dIn.readUTF());
    System.out.println("Message C [2]: " + dIn.readUTF());
    break;
  default:
    done = true;
  }
}

dIn.close();

Obviously, you can send all kinds of data, not just bytes and strings (UTF).

Note that writeUTF writes a modified UTF-8 format, preceded by a length indicator of an unsigned two byte encoded integer giving you 2^16 - 1 = 65535 bytes to send. This makes it possible for readUTF to find the end of the encoded string. If you decide on your own record structure then you should make sure that the end and type of the record is either known or detectable.

User · Answer

Client      import java io        import java net         public class Client           public static void main String   args             String hostname    localhost           int port   6789              declaration section             clientSocket  our client socket            os  output stream            is  input stream              Socket clientSocket   null                DataOutputStream os   null              BufferedReader is   null              Initialization section             Try to open a socket on the given port            Try to open input and output streams              try                   clientSocket   new Socket hostname  port                   os   new DataOutputStream clientSocket getOutputStream                     is   new BufferedReader new InputStreamReader clientSocket getInputStream                    catch  UnknownHostException e                    System err println  Don t know about host      hostname                 catch  IOException e                    System err println  Couldn t get I O for the connection to      hostname                             If everything has been initialized then we want to write some data            to the socket we have opened a connection to on the given port          if  clientSocket    null    os    null    is    null                System err println   Something is wrong  One variable is null                  return                     try               while   true                 System out print   Enter an integer  0 to stop connection  -1 to stop server                    BufferedReader br   new BufferedReader new InputStreamReader System in                String keyboardInput   br readLine                os writeBytes  keyboardInput     n                  int n   Integer parseInt  keyboardInput                if   n    0    n    -1                     break                             String responseLine   is readLine                System out println  Server returns its square as      responseLine                                 clean up                 close the output stream                close the input stream                close the socket              os close                is close                clientSocket close                 catch  UnknownHostException e                System err println  Trying to connect to unknown host      e             catch  IOException e                System err println  IOException       e                                               Server     import java io    import java net     public class Server1       public static void main String args          int port   6789      Server1 server   new Server1  port        server startServer                  declare a server socket and a client socket for the server      ServerSocket echoServer   null      Socket clientSocket   null      int port       public Server1  int port         this port   port             public void stopServer         System out println   Server cleaning up          System exit 0              public void startServer            Try to open a server socket on the given port        Note that we can t choose a port less than 1024 if we are not        privileged users  root           try           echoServer   new ServerSocket port                     catch  IOException e            System out println e                     System out println   Waiting for connections  Only one connection is allowed              Create a socket object from the ServerSocket to listen and accept connections         Use Server1Connection to process the connection       while   true             try           clientSocket   echoServer accept            Server1Connection oneconnection   new Server1Connection clientSocket  this           oneconnection run                         catch  IOException e            System out println e                            class Server1Connection       BufferedReader is      PrintStream os      Socket clientSocket      Server1 server       public Server1Connection Socket clientSocket  Server1 server        this clientSocket   clientSocket      this server   server      System out println   Connection established with      clientSocket        try           is   new BufferedReader new InputStreamReader clientSocket getInputStream              os   new PrintStream clientSocket getOutputStream           catch  IOException e            System out println e                    public void run             String line      try           boolean serverStop   false               while  true                    line   is readLine            System out println   Received     line                    int n   Integer parseInt line           if   n    -1                 serverStop   true              break                    if   n    0   break                  os println      n n                            System out println   Connection closed                  is close                os close                clientSocket close             if   serverStop   server stopServer          catch  IOException e            System out println e

User · Answer

the easiest way to do this is to wrap your sockets in ObjectInput OutputStreams and send serialized java objects   you can create classes which contain the relevant data  and then you don t need to worry about the nitty gritty details of handling binary protocols   just make sure that you flush your object streams after you write each object  message

[java] Using Sockets to send and receive data

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