Question

我必须做一个聊天客户端/服务器项目，但我在ChatServer1类的pr1.println(msg)上得到一个Null Pointer Exception。任何帮助将不胜感激。

public class ChatClient1 extends JFrame{

    private JPanel contentPane;
    private JPanel panel_1 = new JPanel();
    private static JTextArea textArea = new JTextArea();
    private static JTextField Txt1 = new JTextField();
    private JButton DisconnectBtn = new JButton("DISCONNECT");
    private static JButton SendTxt = new JButton("SEND");
    private JLabel lbl1 = new JLabel("Message to send");

    ServerSocket serversocket;
    static BufferedReader br1;
    static PrintWriter pr1;
    Socket socket;


    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    ChatServer1 frame = new ChatServer1();
                    frame.setVisible(true);
                    frame.setTitle("CLIENT");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

    }

    /**
     * Create the frame.
     */
    public ChatClient1() {
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 542, 383);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        panel_1.setBackground(Color.GRAY);
        panel_1.setBounds(0, 0, 536, 355);
        contentPane.add(panel_1);
        panel_1.setLayout(null);

        DisconnectBtn.setBounds(29, 220, 183, 33);
        panel_1.add(DisconnectBtn);

        textArea.setBounds(235, 11, 291, 242);
        panel_1.add(textArea);

        Txt1.setBounds(29, 303, 387, 41);
        panel_1.add(Txt1);
        Txt1.setColumns(10);

        SendTxt.setBounds(437, 303, 89, 41);
        panel_1.add(SendTxt);

        lbl1.setBounds(29, 278, 123, 14);
        panel_1.add(lbl1);

        try {
            Socket socket = new Socket("LocalHost" , 5000);
            pr1 = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()), true);
            br1 = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            while(true){
                textArea.append("Server: " + br1.readLine() + '\n' );
            }

        } catch (IOException e) {

        }
        SendTxt.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String msg = Txt1.getText();
                pr1.println(msg);
                pr1.flush();
                textArea.append(msg + '\n');
                Txt1.setText("");

            }
        });
    }
}

ChatServer1类：

public class ChatServer1 extends JFrame{

    private JPanel contentPane;
    private JPanel panel_1 = new JPanel();
    private static JTextArea textArea = new JTextArea();
    private static JTextField Txt1 = new JTextField();
    private JButton DisconnectBtn = new JButton("DISCONNECT");
    private static JButton SendTxt = new JButton("SEND");
    private JLabel lbl1 = new JLabel("Message to send");

    static BufferedReader br1;
    static PrintWriter pr1;
    Socket socket;


    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    ChatServer1 frame = new ChatServer1();
                    frame.setVisible(true);
                    frame.setTitle("SERVER");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

    }
    /**
     * Create the frame.
     */
    public ChatServer1() {
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 542, 383);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        panel_1.setBackground(Color.GRAY);
        panel_1.setBounds(0, 0, 536, 355);
        contentPane.add(panel_1);
        panel_1.setLayout(null);

        DisconnectBtn.setBounds(29, 220, 183, 33);
        panel_1.add(DisconnectBtn);

        textArea.setBounds(235, 11, 291, 242);
        panel_1.add(textArea);

        Txt1.setBounds(29, 303, 387, 41);
        panel_1.add(Txt1);
        Txt1.setColumns(10);

        SendTxt.setBounds(437, 303, 89, 41);
        panel_1.add(SendTxt);

        lbl1.setBounds(29, 278, 123, 14);
        panel_1.add(lbl1);

        try {
            ServerSocket serversocket = new ServerSocket(5000);
            socket = serversocket.accept();
            pr1 = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()),true);
            br1 = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            while(true){
                textArea.append("Client: " + br1.readLine() + '\n' );
            }

        } catch (IOException e) {

        }
        SendTxt.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String msg = Txt1.getText();
                pr1.println(msg);
                pr1.flush();
                textArea.append(msg + '\n');
                Txt1.setText("");

            }
        });
    }
}

Answer 1

你的问题在于ChatServer1代码，这里有以下内容：

ServerSocket serversocket = new ServerSocket（5000）; socket = serversocket.accept（）;

当你调用accept时，它正在等待和监听传入的连接，所以从来没有实际到达你的pr1实例。因此pr1保持为null。易于修复只需将pr1实例移动到接受调用之上，如下所示：

DELETE FROM your_table
WHERE rowid not in
(SELECT MIN(rowid)
FROM your_table
GROUP BY column1, column2, column3);

希望这有帮助！

Java：空指针异常

1 个答案: