服务器向客户端套接字发送消息

Question

I started working with sockets this week and I'm having a hard time. My goal is when the client sent a message the server responded with a notification. On the client side sending to the server has no problem, but when the server sends to the client nothing appears. Can anybody help me with this problem?

Client:

    Thread thread = new Thread(new myServerThread());
    thread.start();

class myServerThread implements Runnable {
    Socket socket;
    ServerSocket serverSocket;
    InputStreamReader inputStreamReader;
    BufferedReader bufferedReader;
    String message;
    Handler handler = new Handler();

    @Override
    public void run() {
        try {
            serverSocket = new ServerSocket(5000);
            while (true){
                socket = serverSocket.accept();
                inputStreamReader = new InputStreamReader(socket.getInputStream());
                bufferedReader = new BufferedReader(inputStreamReader);
                message = bufferedReader.readLine();
                    handler.post(new Runnable() {
                        @Override
                        public void run() {
                            Toast.makeText(getApplicationContext(), message,Toast.LENGTH_LONG).show();
                        }
                    });
            }

        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}
}

SERVER:

 String EMAIL = "Email";

   try {        
       serverSocket = new ServerSocket(6000);
       while(true){
       socket = serverSocket.accept();
       inputStreamReader = new InputStreamReader(socket.getInputStream());
       bufferedReader = new BufferedReader(inputStreamReader);
       email = bufferedReader.readLine();


       System.out.println(email);

       if(email.equals(EMAIL)){
           jTextArea1.setText(email);
           try {
            socket = new Socket("localHost", 5000);
            PrintWriter  printWriter = new PrintWriter(socket.getOutputStream());
            printWriter.write(message);
            printWriter.flush();
            printWriter.close();
            System.out.println("connected");


        } catch (IOException e) {
            e.printStackTrace();
        }

       }else{
       }

       }

   } catch (IOException ex) {
   }
    etEmail = findViewById(R.id.etvEmail);

Answer 1

First the short answer: Your Java codes are both working almost fine.

Anyway, you should always test your application against another program that is known to be OK. By testing one self-written program against another fresh self-written program, it is difficult to say which of both is broken. It seems that you are testing it with some GUI (Android?). Transforming the relevant part to a simple console application which you run on a regular PC makes troubleshooting much easier.

So let me show how I checked your code on my Linux laptop:

First copy your "client" code into a "Hello-World" template. I added some debug messages and a loop which allows the client to receive more than one single line of text:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;

class Main
{

    static class myServerThread implements Runnable
    {
        Socket socket;
        ServerSocket serverSocket;
        InputStreamReader inputStreamReader;
        BufferedReader bufferedReader;
        String message;

        @Override
        public void run()
        {
            try
            {
                serverSocket = new ServerSocket(5000);
                while (true)
                {
                    System.out.println("Accepting...");
                    socket = serverSocket.accept();
                    System.out.println("Connected...");
                    inputStreamReader = new InputStreamReader(socket.getInputStream());
                    bufferedReader = new BufferedReader(inputStreamReader);
                    String message = bufferedReader.readLine();
                    while (message!=null)
                    {
                        System.out.println("Received:" + message);
                        message = bufferedReader.readLine();
                    }
                    System.out.println("Closing...");
                    socket.close();
                }

            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args)
    {
        Thread thread = new Thread(new myServerThread());
        thread.start();
    }
}

I started the "client" and opened another command window to open a network connection with Netcat and send two lines of text:

nc localhost 5000
Hallo
Test

The related output of the "client" program was:

Accepting...
Connected...
Received:Hallo
Received:Test
Closing...
Accepting...

So the "client" part is running fine obviously.

Next I copied your "server" code into a "Hello World" template:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

class Main
{

    static class myServerThread implements Runnable
    {
        Socket socket;
        ServerSocket serverSocket;
        InputStreamReader inputStreamReader;
        BufferedReader bufferedReader;
        String message;

        @Override
        public void run()
        {
            String EMAIL = "Email";

            try
            {
                serverSocket = new ServerSocket(6000);
                while (true)
                {
                    System.out.println("Accepting...");
                    socket = serverSocket.accept();
                    System.out.println("Connected...");
                    inputStreamReader = new InputStreamReader(socket.getInputStream());
                    bufferedReader = new BufferedReader(inputStreamReader);

                    String message = bufferedReader.readLine();
                    while (message != null)
                    {
                        System.out.println("Received:" + message);

                        if (message.equals(EMAIL))
                        {
                            System.out.println("Sending...");
                            try
                            {
                                socket = new Socket("localHost", 5000);
                                PrintWriter printWriter = new PrintWriter(socket.getOutputStream());
                                printWriter.write(message);
                                printWriter.flush();
                                printWriter.close();
                                System.out.println("sending done");

                            }
                            catch (IOException e)
                            {
                                e.printStackTrace();
                            }

                        }
                        message = bufferedReader.readLine();
                    }
                    System.out.println("Closing...");
                    socket.close();
                }
            }
            catch (IOException ex)
            {
                ex.printStackTrace();
            }
        }
    }

    public static void main(String[] args)
    {
        Thread thread = new Thread(new myServerThread());
        thread.start();
    }
}

Again I added some debug messages and a loop, so the server can receive multiple lines of text. Since this Java program handles two connections, I had to open two command windows.

In the first command window I tell NetCat to accept (listen) connection on port 5000. That is where your "server" will send the "Email" message to:

nc -lp 5000

In the second command window I tell Netcat to connect to your "server" on port 6000, then I send two lines of text and then I press Ctrl-C to stop it:

nc localhost 6000
Test
Email
^C

The related output of the "server" program is:

Accepting...
Connected...
Received:Test
Received:Email
Sending...
sending done
Closing...
Accepting...

And my listening Netcat in the other (first) command windows produced this output:

stefan@stefanpc:/hdd/stefan$ nc -lp 5000
Emailstefan@stefanpc:/hdd/stefan$ 

So everything looks fine on my machine. Beside one small detail: There is no line break after the "Email" that your "server" sends to the client (in this case NetCat). So the fix is simple:

printWriter.write(message+"\n"); 

This final line break is required because your client consumes the input by readLine() .

NetCat is a very helpful tool to test plain text TCP socket communication. It is included in all Linux distributions. If you have difficulties to find a Windows executable, then take it from my homepage: http://stefanfrings.de/avr_tools/netcat-win32-1.12.zip

Please comment if that was helpful to you.