Java server would accept only one entry from my client

Question

I've made a multithreaded server that connects to a MySQL database and accepts requests from a client. The server should always be listening from client commands but this doesn't happen. It just accepts one command sends it to the database and gets whatever I need, then it's done. It doesn't accept any more commands. I provide you with the client and the server.

 import java.io.BufferedReader; import java.io.DataInputStream; import java.io.DataOutputStream; import java.io.IOException; import java.io.InputStreamReader; import java.io.PrintWriter; import java.net.ServerSocket; import java.net.Socket; import java.rmi.ConnectException; import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Statement; import java.util.Scanner; public class Airlines { public static void main(String[] args) { ServerSocket serverSocket = null; Socket connection = null; int port = 1234; try{ serverSocket = new ServerSocket(port); while ((connection = serverSocket.accept())!=null){ System.out.println("Client connected!"); Thread client = new Thread (new AirlinesThread(connection)); client.start(); } }catch (IOException e){ System.out.println("Binding unsuccesful..."); } } } class AirlinesThread implements Runnable{ Socket connection = null; public AirlinesThread (Socket connection){ this.connection = connection; } private static Connection connect(String url, String user, String password){ Connection result = null; try{ result = DriverManager.getConnection(url, user, password); System.out.println("Database connection successful!"); } catch(SQLException e){ System.out.println("Could not connect to the database!"); } return result; } String url = "jdbc:mysql://localhost:3306/Airlines"; String user = "root"; String pass = "123456"; Connection link = AirlinesThread.connect(url, user, pass); Statement stmt = null; ResultSet resultSet = null; public void run() { PrintWriter socketOut = null; DataInputStream socketIn = null; try{ socketOut = new PrintWriter(this.connection.getOutputStream(),true); socketIn = new DataInputStream(this.connection.getInputStream()); int command; boolean exists = false; socketOut.flush(); loop:do{ socketOut.flush(); command = socketIn.readInt(); switch (command){ case 1: try{ stmt = link.createStatement(); resultSet = stmt.executeQuery("select Flight.id, Flight.Date, Flight.Time, Flight.Total_Flight_Time, Airports.Departure, Airports.Arrivals FROM Flight, Airports WHERE Flight.id = Airports.Flight"); socketOut.flush(); socketOut.println("FlightID\\tDate\\t\\tTime\\t\\tTotal Time\\tDeparture\\tArrivals"); while (resultSet.next()) { socketOut.println(resultSet.getString("Flight.id")+"\\t\\t"+resultSet.getDate("Flight.Date")+"\\t"+resultSet.getString("Flight.Time")+"\\t"+resultSet.getString("Flight.Total_Flight_Time")+"\\t\\t"+resultSet.getString("Airports.Departure")+"\\t"+resultSet.getString("Airports.Arrivals")); } } catch(SQLException e){ System.out.println("Something went wrong at 1"); } break; case 2: try{ stmt = link.createStatement(); resultSet = stmt.executeQuery("select Flight.id, Flight.Date, Flight.Time, Pilots.First_Name, Pilots.Last_Name from Flight RIGHT JOIN Pilots ON Flight.id = Pilots.FlightID;"); socketOut.flush(); socketOut.println("FlightID\\tDate\\t\\tTime\\t\\tFirst Name\\tLast Name"); exists = resultSet.next(); if(exists == false){ socketOut.flush(); socketOut.println("Wrong request!"); System.out.println("Wrong query at 2"); } while (resultSet.next()) { socketOut.flush(); socketOut.println(resultSet.getString("Flight.id")+"\\t\\t"+resultSet.getDate("Flight.Date")+"\\t"+resultSet.getString("Flight.Time")+"\\t"+resultSet.getString("Pilots.First_Name")+"\\t\\t"+resultSet.getString("Pilots.Last_Name")); } } catch(SQLException e){ System.out.println("Something went wrong at 2"); } break; case 3: try{ stmt = link.createStatement(); resultSet = stmt.executeQuery("select Food.Breakfast, Airplanes.Plane_Model FROM Food, Airplanes Where Food.FlightID=Airplanes.Plane_Model;"); exists = resultSet.next(); if(exists == false){ socketOut.flush(); socketOut.println("Wrong request!"); System.out.println("Wrong query at 3"); } while (resultSet.next()) { socketOut.flush(); socketOut.println(resultSet.getString("Food.Breakfast")+"\\t\\t"+resultSet.getString("Airplanes.Plane_Model")); } } catch(SQLException e){ System.out.println("Something went wrong at 3"); } break; case 0 : socketOut.flush(); socketOut.println("Exitting..."); break loop; default: System.out.println("Unknown command!"); socketOut.println("Unknown command!"); break; } }while(command!=0); System.out.println("Closing connection to the client!"); }catch (IOException e){ e.printStackTrace(); }finally{ try{ if (socketIn!=null) socketIn.close(); if (socketOut!=null) socketOut.close(); if (connection!=null) connection.close(); System.out.println("Connection to server closed!"); } catch(IOException e){ System.err.println("Could not close connection!"); } } try{ if(stmt != null) stmt.close(); if(resultSet != null) resultSet.close(); if(link != null) link.close(); System.out.println("Database connection closed successfully!"); }catch(SQLException ex){ System.out.println("Could not close connection to the database!"); } } } 

The client:

 import java.io.BufferedReader; import java.io.DataInputStream; import java.io.DataOutputStream; import java.io.IOException; import java.io.InputStreamReader; import java.io.PrintWriter; import java.net.ConnectException; import java.net.Socket; import java.util.Scanner; public class AirlinesClient { public static void main(String[] args) { Socket connection = null; Scanner socketIn = null; DataOutputStream socketOut = null; int port = 1234; String host = "localhost"; Scanner keyIn = new Scanner(System.in); try{ try{ connection = new Socket(host,port); socketIn = new Scanner(new BufferedReader(new InputStreamReader(connection.getInputStream()))); socketOut = new DataOutputStream(connection.getOutputStream()); }catch(ConnectException e){ System.out.println("Could not connect to the host!"); return; } System.out.println("Successfully connected to the server!"); System.out.println("Select from the options below:\\n1: Check schedules\\n2: Check pilots shifts\\n3: Check corresponding airplanes and food offered\\n4: Possible pilot shift changes"); loop:do{ int command; socketOut.flush(); command = keyIn.nextInt(); socketOut.flush(); socketOut.writeInt(command); while (socketIn.hasNext()){ System.out.println(socketIn.nextLine()); } }while(keyIn.nextInt()!=0); System.out.println("Closing connection to server!"); }catch(IOException e){ e.printStackTrace(); } finally{ try{ if(socketIn!=null) socketIn.close(); if(socketOut!=null) socketOut.close(); if(connection!=null) connection.close(); } catch(IOException e){ System.err.println("Socket could not be closed!"); } } } } 

Answer 1

The problem is that you're using Scanner in your client. Execution hangs on socketIn.hasNext() , because

public boolean hasNext()

Returns true if this scanner has another token in its input. This method may block while waiting for input to scan. The scanner does not advance past any input.

The code will proceed in case there's new input or underlying stream is closed (and we don't want that). You have to have a way of knowing end of transmission without closing the stream. Look below for very simple one.

At the end of every try-catch block in your server put

socketOut.println("$$$");

In client declare socketIn as BufferedReader and instantiate as

socketIn = new BufferedReader(new InputStreamReader(connection.getInputStream()));

In client also change your loop loop to

int command = keyIn.nextInt();
do {
    socketOut.flush();
    socketOut.writeInt(command);
    String line;
    while (!(line = socketIn.readLine()).equals("$$$"))
        System.out.println(line);
} while ((command = keyIn.nextInt()) != 0);

This will also fix your double checking for keyIn.nextInt() . In your code you're calling it twice per loop (apart from first iteration).