如何防止套接字在具有多个线程的程序中变为空（Java套接字线程编程）

Question

Error message:

Exception in thread "Thread-1" java.lang.NullPointerException
    at DirectMessengerServer$2.run(DirectMessengerServer.java:72)

Screenshot of error

Line 72:

OutputStream os = socket.getOutputStream();

I am trying to get the same socket from the previous thread (ServerRecieve) but the socket is null at this point, I'm not sure how to prevent socket from being null or if there is a way to set the socket to something non-null globally that would work too if possible.

I am trying to create the server side of a program where messages are being sent and received simultaneously (like two people using a text messenger app on a phone)

Code of Server:

import java.io.*;
import java.net.*;
import java.util.*;

import javax.imageio.IIOException;
//import static java.nio.charset.StandardCharsets.*;
public class DirectMessengerServer
{
    private static Socket socket;
    boolean KeepRunning = true;
    void ServerRun(String[] args) 
    {
        Thread ServerRecieve = new Thread () 
        {
            public void run ()
            {   
                System.out.println("Server recieve thread is now running");
                try
                {
                    System.out.println("Try block begins..");
                    int port_number1= Integer.valueOf(args[1]);
                    System.out.println("Port number is: " + port_number1);
                    ServerSocket serverSocket = new ServerSocket(port_number1);
                    //SocketAddress addr = new InetSocketAddress(address, port_number1);
                    System.out.println( "Listening for connections on port: " + ( port_number1 ) );

                    while(KeepRunning)
                    {
                        //Reading the message from the client

                        socket = serverSocket.accept();    
                        InputStream is = socket.getInputStream();
                        InputStreamReader isr = new InputStreamReader(is);
                        BufferedReader br = new BufferedReader(isr);
                        String MessageFromClient = br.readLine();
                        System.out.println("Message received from client: "+ MessageFromClient);


                    }
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                finally
                {
                    try {
                        socket.close();
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
        };ServerRecieve.start();

          Thread ServerSend = new Thread () 
            {

                public void run ()
                {   
                    System.out.println("Server sending thread is now running");
                    //if(socket!=null)
                    //{
                        try
                        {         
                    //  int port_number1= Integer.valueOf(args[1]);

                     //  ServerSocket serverSocket = new ServerSocket(port_number1);
                        //socket = serverSocket.accept();    
                        //Send the message to the server
                        OutputStream os = socket.getOutputStream();
                        OutputStreamWriter osw = new OutputStreamWriter(os);
                        BufferedWriter bw = new BufferedWriter(osw);

                        //creating message to send from standard input
                        String newmessage = "";
                        try 
                        {
                            // input the message from standard input
                            BufferedReader input= new BufferedReader( 
                            new InputStreamReader(System.in));
                            String line = "";

                            line= input.readLine(); 
                                newmessage += line + " ";

                        }
                        catch ( Exception e )
                        {
                            System.out.println( e.getMessage() );
                        }
                        String sendMessage = newmessage;
                        bw.write(sendMessage + "\n");
                        bw.flush();
                        System.out.println("Message sent to client: "+sendMessage);

                        }

                        catch (IOException e) 
                        {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    finally
                    {

                    }
              //  }
                }
            };ServerSend.start();

    }
}

Full code of Client and Server and main file (more context):

Server unable to send messages to client (java thread socket programming)

My question is: How do I resolve the error on line 72 while keeping the socket the same as the one in the previous thread (ServerRecieve)?

Answer 1

You are assuming that System.in() is thread safe. You should put this section in a synchronized block like this

synchronized(input){    
    line= input.readLine();
}

You will have to move the declaration

BufferedReader input= new BufferedReader( 
                            new InputStreamReader(System.in));

to your master thread, and then pass the reference of input to your worker threads.