java - 每次在数据库中创建新条目时添加 ListView

Question

我的学生项目需要帮助。目前我正在使用 JavaFX、MQTT 和 Mysql 制作聊天应用程序。我想制作一个用户列表(哪些用户在线)

我尝试这样做，以便新用户通过特殊主题发送消息，并且所有用户都将“ secret ”订阅该主题，它将接收消息(使用回调)并调用方法 insertingMysql() 和 updateList( )。但不知怎的，它不起作用。

这是我提供用户名的地方

public static void logging(String username) throws MqttException {
    if(username != null && !username.isEmpty()) {
        MysqlDatabase.insertingMysql(username);
        window.close();
    }
}

这是添加ListView

public static void addItem(String item) {
    userList.getItems().add(item);
}

这是我遇到问题的部分(我认为)

public class MysqlDatabase {

static String topic_body = "chat";;
static String topic_ext = "userList";

static MqttCallback sqlCallback = new MqttCallback() {
    @Override
    public void connectionLost(Throwable thrwbl) {
    }

    @Override
    public void messageArrived(String string, MqttMessage mm) throws Exception {
        String username = new String(mm.getPayload());
        System.out.println(username); // Checking

        updatingList();
    }

    @Override
    public void deliveryComplete(IMqttDeliveryToken imdt) {
    }
};

public static void sqlConnection() {
    try {
        Class.forName("com.mysql.jdbc.Driver");
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    System.out.println("Connected");
}

public static void mqttConnection(String username) throws MqttException {
    MqttClient sqlClient = new MqttClient("tcp://iot.eclipse.org:1883", username);
    sqlClient.connect();
    sqlClient.subscribe(topic_body + topic_ext + "#");
    sqlClient.setCallback(sqlCallback);

    MqttMessage sqlMessage = new MqttMessage();
    sqlMessage.setPayload((username).getBytes());
    sqlClient.publish(topic_body + "userlist", sqlMessage);
}

public static void insertingMysql(String username) throws MqttException {
    sqlConnection();
    String host = "jdbc:mysql://localhost/test";
    String user = "root";
    String password = "";
    try {
        Connection connect = DriverManager.getConnection(host, user, password);
        PreparedStatement statement = (PreparedStatement) connect.prepareStatement("INSERT INTO chat_test(username)VALUES(?)");
        statement.setString(1, username);
        statement.executeUpdate();
        statement.close();
        System.out.println("Inserted to database!");
        mqttConnection(username);
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        System.out.println("Error");
        e.printStackTrace();
    }
}

public static void updatingList() {
    sqlConnection();
    String host = "jdbc:mysql://localhost/test";
    String user = "root";
    String password = "";
    try {
        Connection connect = DriverManager.getConnection(host, user, password);
        PreparedStatement statement = (PreparedStatement) connect.prepareStatement("SELECT * FROM chat_test");
        ResultSet rs = statement.executeQuery();
        while(rs.next()) {
            String uName = rs.getString("username");
            System.out.println("Username: " + uName);
            JavaFXChat.addItem(uName);
        }
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        System.out.println("Error");
        e.printStackTrace();
    }
}

}

Answer 1

这里有 2 个选项:

1. 数据库触发器。这是当表发生更改时在数据库服务器上运行的代码。通常这些触发器是 SQL 语句，但这篇博客 ( https://patternbuffer.wordpress.com/2012/09/14/triggering-shell-script-from-mysql/ ) 似乎讨论了一个支持运行脚本的插件，该脚本可以发送 MQTT 消息，宣布新用户上线。

2. 使用 MQTT 消息和遗嘱功能。用户上线后会向已知主题发布消息，离线时也会发布类似的消息。如果由于问题导致连接断开，LWT 可用于自动发布离线消息。 (LWT的详细信息可以在这里找到http://www.hivemq.com/blog/mqtt-essentials-part-9-last-will-and-testament