前言：套接字(socket)编程能够实现服务器和客户端的通信，以下通过Socket编程结合多线程实现多人聊天室。
程序展示：

界面类

1.客户端界面 ClientView.java

public class ClientView extends JFrame implements ActionListener, KeyListener, Runnable {
    private JTextArea textArea;
    private JTextField textField, tfName;
    private JButton btnSend, btnId;
    private JLabel label;
    private JPanel jp1, jp2;
    public boolean isConnect = false;
    private Socket socket = null;
    private DataInputStream inputStream = null;
    private DataOutputStream outputStream = null;
    private JScrollPane scrollPane;
    private static ClientView view;

    public JTextArea getTextArea() {
        return textArea;
    }

    public DataInputStream getInputStream() {
        return inputStream;
    }
    public DataOutputStream getOutputStream() {
        return outputStream;
    }

    public static void main(String[] args) {
        view = new ClientView();
        ServiceView.clientViews.add(view);
        Thread thread = new Thread(view);
        thread.start();
    }

    public ClientView() {
        initView();
        try {
            socket = new Socket("localhost", 9090);//连接本地服务器

        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void initView() {
        textArea = new JTextArea(20, 20);
        textArea.setEditable(false);
        scrollPane = new JScrollPane(textArea);
        textField = new JTextField(15);
        textField.addKeyListener(this);
        btnSend = new JButton("发送");
        btnSend.addActionListener(this);
        label = new JLabel("昵称：");
        tfName = new JTextField(8);
        jp1 = new JPanel();
        jp2 = new JPanel();
        jp1.add(label);
        jp1.add(tfName);
        tfName.setText("用户0");
        jp1.setLayout(new FlowLayout(FlowLayout.CENTER));
        jp2.add(textField);
        jp2.add(btnSend);
        jp2.setLayout(new FlowLayout(FlowLayout.CENTER));

        add(jp1, BorderLayout.NORTH);
        add(scrollPane, BorderLayout.CENTER);
        add(jp2, BorderLayout.SOUTH);
        setTitle("聊天室");
        setSize(500, 500);
        setLocation(450, 150);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        addWindowListener(new WindowAdapter() { //窗口关闭后断开连接
            @Override
            public void windowClosing(WindowEvent e) {
                try {
                    if (socket != null)
                        socket.close();
                    if (inputStream!= null)
                        inputStream.close();
                    if (outputStream != null)
                        outputStream.close();
                } catch (IOException e1) {
                    e1.printStackTrace();
                }
            }
        });
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == btnSend) {
            sendMsg();
        }
    }

    private void sendMsg() {
        try {
            String s = textField.getText();
            if (!s.equals("")) { //发送数据
                textField.setText("");
                textArea.append("我(" + tfName.getText() +  "):\r\n" + s + "\r\n");
                outputStream = new DataOutputStream(socket.getOutputStream());
                outputStream.writeUTF(tfName.getText() + "#" + s); 
                outputStream.flush();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

    }



    @Override
    public void keyPressed(KeyEvent arg0) {
        if (arg0.getKeyCode() == KeyEvent.VK_ENTER) {
            sendMsg();
        }
    }

    @Override
    public void keyReleased(KeyEvent arg0) {

    }

    @Override
    public void keyTyped(KeyEvent arg0) {

    }

    @Override
    public void run() {
        try {
            inputStream = new DataInputStream(socket.getInputStream());

            while (true) {
                String[] s = inputStream.readUTF().split("#");
                textArea.append(s[0] + ":\r\n" + s[1] + "\r\n");
            }
        } catch (IOException e) {
        }

    }
}

2.服务器界面 ServiceView.java

public class ServiceView extends JFrame implements ActionListener{
    private JButton btnOpen, btnStop;
    private JLabel label;
    private Service service = null;
    public static ArrayList<ClientView> clientViews = new ArrayList<>();
    private static ServiceView view;

    public static ServiceView getView() {
        return view;
    }
    public static void main(String[] args) {
        view = new ServiceView();
    }

    public ServiceView() {
        initView();
    }

    private void initView() {
        btnOpen = new JButton("打开服务器");
        btnStop = new JButton("关闭服务器");
        btnStop.setEnabled(false);
        btnOpen.addActionListener(this);
        btnStop.addActionListener(this);
        label = new JLabel("服务器停止工作");
        add(label);
        add(btnOpen);
        add(btnStop);
        setTitle("服务器");
        setLayout(new GridLayout(3, 1, 0, 10));
        setSize(300, 300);
        setLocation(450, 150);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == btnOpen) {
            open();
        } else {
            stop();
        }
    }

    public void open() { //开启服务器
        service = new Service();
        Thread thread = new Thread(service);
        thread.start();
        label.setText("服务器正在运行");
        btnOpen.setEnabled(false);
        btnStop.setEnabled(true);
    }

    public void stop() { //关闭服务器
        label.setText("服务器已关闭");
        btnOpen.setEnabled(true);
        btnStop.setEnabled(false);
        try {
            synchronized (ClientMannager.sockets) { //关闭各个连接
                for (ChatSocket socket : ClientMannager.sockets) {
                    socket.getInputStream().close();
                    socket.getOutputStream().close();
                }
                ClientMannager.sockets.removeAllElements();
            }


            for (ClientView view : clientViews) {
                view.getInputStream().close();
                view.getOutputStream().close();
            }

            service.getServerSocket().close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

功能类

1.ChatSocket.java

/*使用socket获得数据流，达到传输数据的目的*/
public class ChatSocket implements Runnable{
    private Socket socket = null;
    private DataInputStream inputStream = null;
    private DataOutputStream outputStream = null;

    public DataInputStream getInputStream() {
        return inputStream;
    }

    public DataOutputStream getOutputStream() {
        return outputStream;
    }

    public ChatSocket(Socket socket) {
        this.socket = socket;
        try {
            inputStream = new DataInputStream(socket.getInputStream());
            outputStream = new DataOutputStream(socket.getOutputStream());
        } catch (IOException e) {
            e.printStackTrace();
        }

    }


    public void send(String send) { //向客户端发送数据
        try {
            outputStream.writeUTF(send);
            outputStream.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() { //循环读取客户端发来的数据
        String accept = null;

        while (true) {
            try {
                accept = inputStream.readUTF();

                ClientMannager.sendAll(this, accept);
            } catch (IOException e) {
                ClientMannager.sockets.remove(this);
            }
        }
    }
}

2.ClientMannager.java

/*客户端管理器*/
public class ClientMannager {

    private ClientMannager() {
    }

    public static Vector<ChatSocket> sockets = new Vector<>();

    //向其他客户端发送数据
    public static void sendAll(ChatSocket chatSocket, String send) {
        for (ChatSocket socket : sockets) {
            if (!chatSocket.equals(socket)) {
                socket.send(send);
            }
        }
    }
}

3.Service.java

/*服务器端，使用线程达到循环等待连接的目的*/
public class Service implements Runnable{

    private ServerSocket serverSocket = null;

    public ServerSocket getServerSocket() {
        return serverSocket;
    }

    @Override
    public void run() {
        try {
            serverSocket = new ServerSocket(9090); //创建端口
            while (true) { //循地接收客户端的连接
                Socket socket = serverSocket.accept();
                JOptionPane.showMessageDialog(ServiceView.getView(), "客户端连接端口", "TIP", JOptionPane.INFORMATION_MESSAGE);
                ChatSocket chatSocket = new ChatSocket(socket); //新客户端连接
                ClientMannager.sockets.add(chatSocket); //往客户端管理器里添加客户
                Thread thread = new Thread(chatSocket); //启用线程使服务器开始不断接收客户端信息
                thread.start();

            }
        } catch (IOException e) {
            e.printStackTrace();
            System.out.println("服务器关闭");
        }
    }

}