一、需求分析

• 用户与用户之间1-1或1-n通讯

二、技术分析
（一）客户端

• 客户端的功能有两个：发消息和接消息
• 发消息：使用Socket技术的流式输出，配合打印流封装发送。
• 接信息：使用读取专用线程，搭配字符缓冲流读取。

（二）服务端

• 服务端的功能：转发消息。
• 实现转发功能：
  • 1、创建服务通道ServerSocket
  • 2、建立线程池ExecutorService->ThreadPoolExecutor
  • 3、等待连接
  • 4、向集合添加通道对象
  • 5、添加转发任务到线程池任务队列
  • 6、转发线程类的重写
    • 第 1 部分：接收信息： 字符缓冲流读取接收的信息。
    • 第 2 部分：转发信息： 遍历所有通道，获取输出流输出数据。

三、实现代码
客户端

class Client1{
    public static void main(String[] args) {
        try {
            //建立通道
            Socket sender = new Socket(InetAddress.getLocalHost().getHostAddress(),7777);

            new ClientServiceThread(sender).start();

            Scanner in = new Scanner(System.in);

            if (sender.isConnected()) System.out.println(sender.getRemoteSocketAddress() + " 已连接！");

            //输出流
            PrintStream outStream = new PrintStream(sender.getOutputStream(),true);
            String message;
            while(!(message = in.nextLine()).equals("exit")){
                outStream.println(message);
            }

        } catch (IOException e) {
            System.out.println("连接已断开");
        }
    }
}

class Client2{
    public static void main(String[] args) {
        try {
            //建立通道
            Socket sender = new Socket(InetAddress.getLocalHost().getHostAddress(),7777);

            new ClientServiceThread(sender).start();

            Scanner in = new Scanner(System.in);

            if (sender.isConnected()) System.out.println(sender.getRemoteSocketAddress() + " 已连接！");

            //输出流
            PrintStream outStream = new PrintStream(sender.getOutputStream(),true);
            String message;
            while(!(message = in.nextLine()).equals("exit")){
                outStream.println(message);
            }

        } catch (IOException e) {
            System.out.println("连接已断开");
        }
    }
}

class ClientServiceThread extends Thread{

    private Socket socket;

    public ClientServiceThread(){}

    public ClientServiceThread(Socket socket){
        this.socket = socket;
    }

    @Override
    public void run() {
        try {
            InputStream in = socket.getInputStream();

            BufferedReader br = new BufferedReader(new InputStreamReader(in));

            String message;

            while((message = br.readLine()) != null){
                System.out.println(socket.getRemoteSocketAddress() + " " + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(System.currentTimeMillis()));
                System.out.println(message);
            }
        } catch (IOException e) {
            System.out.println("服务器将您踢出群聊······");
        }
    }
}

服务端

class Server{
    public static List<Socket> sockets = new ArrayList<>();

    public static void main(String[] args) {
        try {
            ServerSocket socket = new ServerSocket(7777);
            ExecutorService service = new ThreadPoolExecutor(3,5,5, TimeUnit.SECONDS,new LinkedBlockingDeque<>(4),new ThreadPoolExecutor.AbortPolicy());

            while(true){
                Socket accept = socket.accept();
                System.out.println(accept.getRemoteSocketAddress() + "已上线！时间：" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(System.currentTimeMillis()));
                sockets.add(accept);
                service.execute(new ServerThread(accept));
            }

        } catch (IOException e) {
            System.out.println();
        }
    }
}

class ServerThread implements Runnable{

    private Socket socket;

    public ServerThread() {
    }

    public ServerThread(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            System.out.println(socket.getRemoteSocketAddress() + " 已上线，上线时间：" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(System.currentTimeMillis()));
            String message;
            while((message = reader.readLine())!=null){
                //获取信息
                System.out.println(socket.getRemoteSocketAddress() + " " + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
                System.out.println(message);

                //使用转发技术，转发其它客户端
                sendMessageAll(message);
            }
        } catch (IOException e) {
            System.out.println(socket.getRemoteSocketAddress() + "，下线了，时间：" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
            Server.sockets.remove(socket);
        }
    }

    private void sendMessageAll(String message) {
        for (Socket socket :Server.sockets) {
            try {
                PrintStream ps = new PrintStream(socket.getOutputStream(),true);
                ps.println(message);
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

效果图