How to implement a db listener in Java

I have a solution for Oracle. You don’t need to create your own since now that Oracle bought Java it released a listener for it. As far as I know this does not use polling internally, instead notifications are pushed to the Java side (probably based on some trigger):

public interface oracle.jdbc.dcn.DatabaseChangeListener 
extends java.util.EventListener {
    void onDatabaseChangeNotification(oracle.jdbc.dcn.DatabaseChangeEvent arg0);
}

And you can implement it like this (this is just a sample):

public class DBListener implements DatabaseChangeListener {
    private DbChangeNotification toNotify;

    public BNSDBListener(DbChangeNotification toNotify) {
        this.toNotify = toNotify;
    }

    @Override
    public void onDatabaseChangeNotification(oracle.jdbc.dcn.DatabaseChangeEvent e) {
        synchronized( toNotify ) {
            try {
                toNotify.notifyDBChangeEvent(e); //do sth
            } catch (Exception ex) {
                Util.logMessage(CLASSNAME, "onDatabaseChangeNotification", 
                    "Errors on the notifying object.", true);
                Util.printStackTrace(ex);
                Util.systemExit();                                       
            }
        }       
    }
}

EDIT:
You can use the following class to register: oracle.jdbc.OracleConnectionWrapper

public class oracle.jdbc.OracleConnectionWrapper implements oracle.jdbc.OracleConnection {...}

Say you create a method somewhere:

public void registerPushNotification(String sql) {
    oracle.jdbc.driver.OracleConnection oracleConnection = ...;//connect to db

    dbProperties.setProperty(OracleConnection.DCN_NOTIFY_ROWIDS, "true");
    dbProperties.setProperty(OracleConnection.DCN_QUERY_CHANGE_NOTIFICATION, "true");

    //this is what does the actual registering on the db end
    oracle.jdbc.dcn.DatabaseChangeRegistration dbChangeRegistration= oracleConnection.registerDatabaseChangeNotification(dbProperties);

    //now you can add the listener created before my EDIT
    listener = new DBListener(this);
    dbChangeRegistration.addListener(listener);

    //now you need to add whatever tables you want to monitor
    Statement stmt = oracleConnection.createStatement();
    //associate the statement with the registration:
    ((OracleStatement) stmt).setDatabaseChangeRegistration(dbChangeRegistration); //look up the documentation to this method [http://docs.oracle.com/cd/E11882_01/appdev.112/e13995/oracle/jdbc/OracleStatement.html#setDatabaseChangeRegistration_oracle_jdbc_dcn_DatabaseChangeRegistration_]

    ResultSet rs = stmt.executeQuery(sql); //you have to execute the query to link it to the statement for it to be monitored
    while (rs.next()) { ...do sth with the results if interested... }

    //see what tables are being monitored
    String[] tableNames = dbChangeRegistration.getTables();
    for (int i = 0; i < tableNames.length; i++) {
        System.out.println(tableNames[i]    + " has been registered.");
    }
    rs.close();
    stmt.close();
}

This example does not include try-catch clauses or any exception handling.