代码演示如下：

客户端：

public class Client {
    public static void main(String[] args) throws Exception{
        System.out.println("客户端开启！");
        //1.创建Socket对象，并同时请求与服务端程序的连接。
        Socket socket = new Socket("127.0.0.1",8888);

        //创建一个独立的线程，负责随时从socket中接收服务端发送过来的消息
        new ClientReaderThread(socket).start();

        //2.从socket通信管道中得到一个字节输出流。用来发数据给服务端程序
        OutputStream os = socket.getOutputStream();
        //3.把低级的字节输出流包装成数据输出流
        DataOutputStream dos = new DataOutputStream(os);

        Scanner sc = new Scanner(System.in);
        while (true) {
            System.out.println("请输入：");
            String msg = sc.nextLine();

            //一旦用户输入了exit，就退出客户端程序
            if ("exit".equals(msg)){
                System.out.println("退出成功！");
                dos.close();
                socket.close();
                break;
            }
            //4.开始写数据出去了
            dos.writeUTF(msg);
            dos.flush();
        }
    }
}

服务端：

public class Server {
    public static List<Socket> onLineSockets = new ArrayList<>();
    public static void main(String[] args) throws Exception{
        System.out.println("服务端开启！");
        //1.创建ServerSocket的对象，同时为服务端注册端口。
        ServerSocket serverSocket = new ServerSocket(8888);
        while (true) {
            //2.使用serverSocket对象，调用一个accept方法，等待客户端的连接请求
            Socket socket = serverSocket.accept();
            onLineSockets.add(socket);
            System.out.println("有人上线了：" + socket.getRemoteSocketAddress());
            //3.把这个客户端对应的socket通信管道，交给一个对立的线程负责处理
            new ServerReaderThread(socket).start();
        }
    }
}

客户端线程类：

public class ClientReaderThread extends Thread{
    private Socket socket;
    public ClientReaderThread(Socket socket){
        this.socket = socket;
    }
    @Override
    public void run(){
        try {
            InputStream is = socket.getInputStream();
            DataInputStream dis = new DataInputStream(is);
            while (true) {
                try {
                    String msg = dis.readUTF();
                    System.out.println(msg);
                } catch (Exception e) {
                    dis.close();
                    socket.close();
                    break;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

服务端线程类：

public class ServerReaderThread extends Thread{
    private Socket socket;
    public ServerReaderThread(Socket socket){
        this.socket = socket;
    }
    @Override
    public void run(){
        try {
            InputStream is = socket.getInputStream();
            DataInputStream dis = new DataInputStream(is);
            while (true) {
                try {
                    String msg = dis.readUTF();
                    System.out.println(msg);
                    //把这个消息分发给客户端进行接收
                    sendMsgToALL(msg);
                } catch (Exception e) {
                    System.out.println("有人下线了："+ socket.getRemoteSocketAddress());
                    dis.close();
                    socket.close();
                    break;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void sendMsgToALL(String msg) throws Exception {
        // 发送给全部在线的socket管道接收
        for (Socket onLineSocket : Server.onLineSockets) {
            OutputStream os = onLineSocket.getOutputStream();
            DataOutputStream dos = new DataOutputStream(os);
            dos.writeUTF(msg);
            dos.flush();
        }
    }
}