服务端3.1

package talkingthread;

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


public class ChatServer3 {
    static List<ChatThread> listClient = new ArrayList<>();

    public static class ChatThread implements Runnable{
        Socket socket;
        BufferedReader reader;
        PrintWriter writer;

        public ChatThread(Socket nsocket) throws IOException{
            socket = nsocket;
            writer  = new PrintWriter(socket.getOutputStream());
            reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        }

        public void sendMessage(String msg){
            writer.println(msg);
            writer.flush();
        }
        
        public void sendAll(String msg){
            for(ChatThread client : listClient){
                client.sendMessage(msg);
            }
        }

        @Override
        public void run() {
            String msg = "";
            try {
                while ((msg = reader.readLine()) != null) {
                    sendAll(msg);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }
    public static void main(String[] args) throws IOException {
        @SuppressWarnings("resource")
        ServerSocket server = new ServerSocket(10000);
        while (true) {
            Socket socket = server.accept();
            ChatThread thread = new ChatThread(socket);
            listClient.add(thread);
            Thread t = new Thread(thread); 
            t.start();
        }
    }
}

服务端3

package talkingthread;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;


public class ChatServer3 {
    static List<String> message = new ArrayList<String>();

    public static class ChatThread implements Runnable{
        Socket socket;
        BufferedReader reader;
        PrintWriter writer;

        public ChatThread(Socket nsocket){
            socket = nsocket;
        }

        public void sendMessage() throws IOException{
//          for (String client : message){
//              writer = new PrintWriter(socket.getOutputStream());
//              writer.println(client);
//              writer.flush();
//          }
            Iterator<String> iterator = message.iterator();
            while (iterator.hasNext()) {
                writer = new PrintWriter(socket.getOutputStream());
                writer.println(iterator.next());
                writer.flush();
            }
        }

        @Override
        public void run() {
            String msg = "";
            try {
                reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                while ((msg = reader.readLine()) != null) {
                    message.add(msg);
                    sendMessage();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }
    public static void main(String[] args) throws IOException {
        @SuppressWarnings("resource")
        ServerSocket server = new ServerSocket(10000);
        while (true) {
            Socket socket = server.accept();
            ChatThread thread = new ChatThread(socket);
            Thread t = new Thread(thread); 
            t.start();
        }
    }
}

客户端

package talkingthread;

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

public class ChatClient2 {
    private Thread readThread;
    private Thread writeThread;

    public ChatClient2() throws Exception, IOException{
        @SuppressWarnings("resource")
        Socket socket = new Socket("127.0.0.1", 10000);
        final BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        final PrintWriter out = new PrintWriter(socket.getOutputStream());
        final BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        readThread = new Thread(new Runnable(){
            @Override
            public void run() {
                String message = "";
                while(true){
                    try {
                        message = in.readLine();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    System.out.println(message);
                }
            }
        });
        readThread.start();
        writeThread = new Thread(new Runnable(){
            public void run() {
                String msg = "";
                while(true){
                    try {
                        msg = reader.readLine();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    out.println(msg);
                    out.flush();
                }
            }
        });
        writeThread.start();
    }
    public static void main(String[] args) throws Exception, IOException {
        new ChatClient2();
    }
}

问题1，客户端必须发出消息，服务端才会返回信息
问题2，服务端会返还之前所有的信息
将客户端socket记录下来，往每一个socket分别写信息。