本不想使用swing这种过时技术，只不过学校实验要求。。。。
最后结果如图：

附上代码：

import javax.swing.*;
import javax.swing.border.EmptyBorder;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
public class Client extends JFrame implements Runnable {
     Socket socket;
    ServerSocket server;
    private static JPanel contentPane;
    private static JTextField inputArea;
    protected static JTextArea recordPane;
    private JFrame frame;
    public Client() {
        try {
            frame=new JFrame("客户端");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setBounds(100, 100, 420, 300);
            contentPane = new JPanel();
            contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
            frame.setContentPane(contentPane);
            contentPane.setLayout(null);
            //发送按钮
            JButton btnSend = new JButton("发送");
            JButton btnExit = new JButton("退出");

            btnSend.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent arg0) {
                    new sendThread().start();
                }
            });
            btnExit.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    frame.dispatchEvent(new WindowEvent(frame,WindowEvent.WINDOW_CLOSING) );
                }
            });
            btnSend.setBounds(270, 210, 60, 50);
            btnExit.setBounds(340, 210, 60, 50);
            contentPane.add(btnSend);
            contentPane.add(btnExit);
            //聊天记录面板
            recordPane = new JTextArea();
            recordPane.setBounds(9, 9, 400, 190);
            contentPane.add(recordPane);
            //输入框
            inputArea = new JTextField();
            inputArea.setBounds(2, 210, 250, 75);
            contentPane.add(inputArea);
            inputArea.setColumns(10);
            frame.setVisible(true);
            try {
                socket = new Socket("127.0.0.1",8080);
                server=new ServerSocket(8081);
                recordPane.append("客户已启动!\n");
            } catch (Exception e){
                e.printStackTrace();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    @Override
    public void run() {
        try {
            while (true) {
                InputStream in = socket.getInputStream();
                InputStreamReader isr = new InputStreamReader(in,"UTF-8");
                BufferedReader br = new BufferedReader(isr);
                String line = null;
                while((line = br.readLine())!=null) {
                    recordPane.append("蛋蛋说:"+line+"\n");
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
class sendThread extends Thread{
        @Override
        public void run() {
            super.run();
            try {
                OutputStream out = socket.getOutputStream();
                OutputStreamWriter osw = new OutputStreamWriter(out,"UTF-8");
                BufferedWriter bw = new BufferedWriter(osw);
                PrintWriter pw = new PrintWriter(bw,true);
                    String line=inputArea.getText();
                    if(!line.equals("")) {
                        inputArea.setText("");
                        pw.println(line);
                        recordPane.append("我说:"+line+"\n");
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) {
        Client client = new Client();
        new Thread(client).start();
    }
}

下面是Server类

import javax.swing.*;
import javax.swing.border.EmptyBorder;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

public class Server extends  JFrame implements Runnable {
    ServerSocket server;
    Socket socket;
    private static JPanel contentPane;
    private static JTextField inputArea;
    protected static JTextArea recordPane;
    private JFrame frame;
    public Server() {
        frame=new JFrame("服务端");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setBounds(100, 100, 420, 300);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        frame.setContentPane(contentPane);
        contentPane.setLayout(null);
        //发送按钮
        JButton btnSend = new JButton("发送");
        JButton btnExit = new JButton("退出");
        btnSend.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                new sendThread().start();
            }
        });
        btnExit.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                frame.dispatchEvent(new WindowEvent(frame,WindowEvent.WINDOW_CLOSING) );
            }
        });
        btnSend.setBounds(270, 210, 60, 50);
        btnExit.setBounds(340, 210, 60, 50);
        contentPane.add(btnSend);
        contentPane.add(btnExit);
        //聊天记录面板
        recordPane = new JTextArea();
        recordPane.setBounds(9, 9, 400, 190);
        contentPane.add(recordPane);
        //输入框
        inputArea = new JTextField();
        inputArea.setBounds(2, 210, 250, 75);
        contentPane.add(inputArea);
        inputArea.setColumns(10);
        frame.setVisible(true);
        try {
            System.out.println("正在启动服务端...");
            server = new ServerSocket(8080);
            System.out.println("服务端启动完毕!");

        } catch (Exception e){
            e.printStackTrace();
        }
    }
    @Override
    public void run() {
        try {
            while(true){
                socket = server.accept();
                recordPane.append("服务器已启动!\n");
                InputStream in = socket.getInputStream();
                InputStreamReader isr = new InputStreamReader(in,"UTF-8");
                BufferedReader br = new BufferedReader(isr);
                String line = null;
                while((line = br.readLine())!=null){
                    recordPane.append("狗狗说:"+line+"\n");
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
class sendThread extends Thread{
        @Override
        public void run() {
            super.run();
            try {
                if(socket!=null){
                    OutputStream out = socket.getOutputStream();
                    OutputStreamWriter osw;
                    BufferedWriter bw;
                    PrintWriter pw;
                        osw = new OutputStreamWriter(out,"UTF-8");
                        bw = new BufferedWriter(osw);
                        pw = new PrintWriter(bw,true);
                        String line=inputArea.getText();
                        if(!line.equals("")) {
                            inputArea.setText("");
                            pw.println(line);
                            recordPane.append("我说:"+line+"\n");
                        }
                }
            }catch (Exception e){
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) {
        Server server = new Server();
        new Thread(server).start();
    }
}

这个聊天室其实就服务端使用两个线程，一个线程让Java swing里响应触发（也就是点击发送，获取到了你在文本框输入的值，然后发送）。客户端的两个线程也是类似，一个无限循环获取服务端来的值，一个点击发送响应得到你写的值，使用outputstream 写回到服务端