java network programming -TCP- and more people talking (question and answer)

Online chat room
server:

public class Chat {

public static void main(String[]args) throws IOException
{
    System.out.println("服务器启动中...");
    //创建服务器
    ServerSocket server=new ServerSocket(9999);
    //阻塞式等待连接，当客户端Socket创建好以后才开启

    while(true)
    {
    Socket client=server.accept();
    System.out.println("一个客户端建立了连接");

    new Thread(()->{
        DataInputStream dis = null;
        DataOutputStream dos = null;
        try {
            dis = new DataInputStream(client.getInputStream());
            dos = new DataOutputStream(client.getOutputStream());
        } catch (IOException e1) {
            e1.printStackTrace();
        }

        boolean flag=true;
        while(flag) {

        String msg = null;
        try {
            msg = dis.readUTF();
            dos.writeUTF(msg);
            dos.flush();
        } catch (IOException e) {

            //当断掉客户端连接时，不用循环再读取数据
            flag=false;
        }

        //返回消息

        }
        try {
            if(null!=dos)
            {
            dos.close();
            }
        } catch (IOException e) {

            e.printStackTrace();
        }
        try {
            if(null!=dis)
            {
            dis.close();
            }
        } catch (IOException e) {

            e.printStackTrace();
        }
        try {
            if(null!=client)
            {
            client.close();
            }
        } catch (IOException e) {

            e.printStackTrace();
        }
    }).start();

    }
    }

}

Client:

public class Client {

public static void main(String[]args) throws UnknownHostException, IOException
{
    System.out.println("客户端启动中...");

    Socket client=new Socket("localhost",9999);
    //客户端发送消息
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    DataOutputStream dos=new DataOutputStream(client.getOutputStream());
    DataInputStream dis =new DataInputStream(client.getInputStream());
    boolean flag=true;

    while(flag) {
    System.out.println("请输入消息");
    String msg=br.readLine();
    dos.writeUTF(msg);
    dos.flush();
    //获取消息

     msg=dis.readUTF();
     System.out.println(msg);
    }

     dos.close();
     dis.close();
     client.close();

}
}