Web 应用程序的 Java Oracle 数据库更改通知问题

Question

I am using following code for registering and listening to Oracle database change notifications. This code is working fine when i run it as a standalone java program. It is receiving the notification from the database and printing as expected.

  public class DBChangeNotification {
         
    static final String USERNAME = "XXX";
    static final String PASSWORD = "YYY";
    static String URL = "jdbc:oracle:thin:@xxxx:xxxx:xxxx";

    public static void main(String[] args) {
        DBChangeNotification demo = new DBChangeNotification();
        try {
            demo.run();
        } catch (SQLException mainSQLException) {
            mainSQLException.printStackTrace();
        }
    }
    
    public void run() throws SQLException {
        OracleConnection conn = connect();
        Properties prop = new Properties();
        prop.setProperty(OracleConnection.DCN_NOTIFY_ROWIDS, "true");
        prop.setProperty(OracleConnection.DCN_QUERY_CHANGE_NOTIFICATION, "true");
        prop.setProperty(OracleConnection.DCN_BEST_EFFORT, "true");

        DatabaseChangeRegistration dcr = conn.registerDatabaseChangeNotification(prop);
        try {
            // add the listenerr:
            DCNDemoListener list = new DCNDemoListener(this);
            dcr.addListener(list);

            // second step: add objects in the registration:
            Statement stmt = conn.createStatement();
            // associate the statement with the registration:
            ((OracleStatement) stmt).setDatabaseChangeRegistration(dcr);
            ResultSet rs = stmt.executeQuery("select * from xxxxxxxx where yyyy='zzzzz'");
            while (rs.next()) {
            }
            String[] tableNames = dcr.getTables();
            for (int i = 0; i < tableNames.length; i++) {
                System.out.println(tableNames[i] + " is part of the registration.");
            }
            rs.close();
            stmt.close();
        } catch (SQLException ex) {
            // if an exception occurs, we need to close the registration in order
            // to interrupt the thread otherwise it will be hanging around.
            if (conn != null) {
                conn.unregisterDatabaseChangeNotification(dcr);
            }
            ex.printStackTrace();
            throw ex;
        } finally {
            try {
                // Note that we close the connection!
                conn.close();
            } catch (Exception innerex) {
                innerex.printStackTrace();
            }
        }
    }
    /**
     * Creates a connection the database.
     */
    OracleConnection connect() throws SQLException {
        OracleDriver dr = new OracleDriver();
        Properties prop = new Properties();
        prop.setProperty("user", DBChangeNotification.USERNAME);
        prop.setProperty("password", DBChangeNotification.PASSWORD);
        return (OracleConnection) dr.connect(DBChangeNotification.URL, prop);
    }
}

/**
 * DCN listener: it prints out the event details in stdout.
 */
class DCNDemoListener implements DatabaseChangeListener {

    DBChangeNotification demo;
    DCNDemoListener(DBChangeNotification dem) {
        System.out.println("DCNDemoListener");
        demo = dem;
    }
    @Override
    public void onDatabaseChangeNotification(DatabaseChangeEvent e) {
        Thread t = Thread.currentThread();
        System.out.println("DCNDemoListener: got an event (" + this + " running on thread " + t + ")");
        System.out.println(e.toString());
        synchronized (demo) {
            demo.notify();
        }
    }
}

My requirement is to use this feature in a web application. Web application when started in the server, has to listen to data change notifications (may be on a separate thread) and notify the application through a websocket client. I have added the following code in contextInitialized method of servlet context listener, so that it will start as soon as the application starts.

public class MyServletContextListener implements ServletContextListener {
 DBChangeNotification demo;
 
  @Override
  public void contextDestroyed(ServletContextEvent arg0) {
    //Notification that the servlet context is about to be shut down. 
  }

  @Override
  public void contextInitialized(ServletContextEvent arg0) {
      demo = new DBChangeNotification();
        try {
            demo.run();
        } catch (SQLException mainSQLException) {
            mainSQLException.printStackTrace();
        }
  }
}

I did not see any notifications received by the web application when database change event occurs in the registered table. Please help me in resolving the issue. I do not know whether this is a correct approach or not.... may please suggest any alternative except continuous polling. I need to start something in the server as soon as i receive notification from database. Thank you.

Answer 1

It might be that you're running your code on an Oracle instance that doesn't have the Notification API available. Check this SO for more info