simple chat program using java and sockets

Question

I have a problem using sockets in Java: the server doesn't respond and no exception is thrown.

Server Code:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;

class Server {
    public static void main(String args[]) {
        final int time = 75;
        //boolean CHAT_SESSION_ALIVE = false; 
        int port = 9999;

        try {
            System.out.println("Starting chat server using the port : " + port);
            ServerSocket srvr = new ServerSocket(port);
            Socket skt = srvr.accept();
            System.out.println("Server has connected with client         " + skt.getInetAddress());
            //CHAT_SESSION_ALIVE = true;

            PrintWriter out = new PrintWriter(skt.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(skt.getInputStream()));

            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        try {
                            if (in.ready()) {
                                String msg = in.readLine();
                                System.out.println("receive message: '" + msg + "'");
                                Thread.sleep(time);
                            }
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                }
            }).start();

            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        try {
                            Thread.sleep(time);
                            String msg = new Scanner(System.in).nextLine();
                            System.out.println("Sending message: '" + msg + "'");
                            out.print(msg);
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                }
            }).start();

            //in.close();
            //out.close();
            //skt.close();
            //srvr.close();
        } catch (Exception e) {
            System.out.print(e);
        }
    }
}

Client Code:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;

class Client {
    public static void main(String args[]) {
        final int time = 75;
        //boolean CHAT_SESSION_ALIVE = false;
        int port = 9999;
        String hostIP = "127.0.0.1";

        try {
            Socket skt = new Socket(hostIP, port);
            System.out.println("Client has connected with server " + hostIP + ":" + port);
            //CHAT_SESSION_ALIVE = true;

            PrintWriter out = new PrintWriter(skt.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(skt.getInputStream()));

            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        try {
                            if (in.ready()) {
                                String msg = in.readLine();
                                System.out.println("receive message: '" + msg + "'");
                                Thread.sleep(time);
                            }
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                }
            }).start();

            new Thread(new Runnable() {
                @Override
                public void run() {
                    while (true) {
                        try {
                            String msg = new Scanner(System.in).nextLine();
                            System.out.println("Sending message: '" + msg + "'");
                            out.print(msg);
                            Thread.sleep(time);
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                }
            }).start();

            //in.close();
            //out.close();
            //skt.close();
        } catch (Exception e) {
            System.out.print(e);
        }
    }
}

The server output:

Starting chat server using the port : 9999
Server has connected with client /127.0.0.1

The client output:

Client has connected with server 127.0.0.1:9999
simple message
Sending message: 'simple message'

Please explain why the server isn't working correctly.

Answer 1

Scanner.nextLine returns the line without the new line delim. The server is using BufferedReader.readLine , which expects a new line (or may block if it does not receive one). Solution: append the delimiter when sending messages. If using print , you must explicitly flush:

out.print(msg + "\n"); 
out.flush();//explicitly flush the stream

or use the println method to have it add the new line for you (and makes use of autoflush true flag passed to the PrintWriter constructor):

out.println(msg);//auto flushing 

Answer 2

在两个代码中，在PrintWriter out的实例化之后立即放置一个out.flush()