Can't Send More Than One Query To The Server (Using Jdbc And Ocsf)

hello,
i'm using JDBC and OCSF to make client-server connection including MySQL.
i'm now stuck at a very strange problem:
my server can receive either "select..." string, "insert..." string, "update..." string (those are treated as queries), or any other string, which is treated as a simple message to the server (i do nothing with in so far)

here is the code of the server:

Code :

01
public void handleMessageFromClient (Object msg1, ConnectionToClient client) {
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            String msg=((String)msg1).toLowerCase();
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            if ( !( msg.startsWith("select") || msg.startsWith("insert") ||
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                    msg.startsWith("update") ) ) {
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                System.out.println("Message received: " + msg + " from " + client);
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                this.sendToAllClients(msg);
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                return;
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            }
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            try {
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                Class.forName("com.mysql.jdbc.Driver").newInstance();
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            }
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            catch (Exception ex) {
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                System.out.println("Couldn't load MySQL driver...");
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            }
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            try {
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                  Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/test","root","Braude");
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                  System.out.println("Connected to the database");  // indication on server only
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                  Statement st = conn.createStatement();
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                  // user sent a "select" query
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                  if (msg.startsWith("select")){
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                      int isResultEmpty=1;  // for indicating if the query returned no results
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                      ResultSet rs=st.executeQuery(msg);
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                      ResultSetMetaData rsmd=rs.getMetaData();
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                      int numOfCols=rsmd.getColumnCount();
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                      Vector result=new Vector();
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                      while (rs.next()){
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                          String[] tuple=new String[numOfCols];
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                          for (int i=1;i<=numOfCols;i++)
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                              tuple[i-1]=rs.getString(i);
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                          result.addElement(tuple);
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                          isResultEmpty=0;  // indicating at least one tuple in the result
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                      }
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                      if (isResultEmpty==0)
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                          this.sendToAllClients(result);
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                      else
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                          this.sendToAllClients("No appropriate results!");
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                      rs.close();
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                  }  // end of select case
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                  // if user tries to only update the db (no resultset is returned)
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                  if (msg.startsWith("insert")  ||  msg.startsWith("update") ){
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                      st.executeUpdate(msg);
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                      this.sendToAllClients("Action successful!!");
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                  }  // end of update case
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                  st.close();
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                  conn.close();
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            }  // end of try
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            catch (SQLException ex) { // handle any errors
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                    this.sendToAllClients("SQLException: " + ex.getMessage());
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                    this.sendToAllClients("SQLState: " + ex.getSQLState());
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                    this.sendToAllClients("VendorError: " + ex.getErrorCode());
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            }
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  } // end of function

the problem is as follows:
if i send any series of simple messages (one or more), everything works fine.
when i send my first query (any of them, select / update / insert) , it executes.
but, after i send the first query, trying to send any message (query / simple one) ,
i get an error message, thrown by this code:

Code :

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public void handleMessageFromClientUI(String message)
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  {
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    try
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    {
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        sendToServer(message);
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    }
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    catch(IOException e)
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    {
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      clientUI.display
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        ("Could not send message to server.  Terminating client.");
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      quit();
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    }
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  }

the sendToServer() method is inherited from the OCSF, i didn't touch it.
For conclusion, the problem is:
after sending simple messages, all fine. after sending a query, nothing else can be sent.
any idea will be very appreciated, I don't have a clue about what's wrong here..