private static function _init()
    {
        if (!self::$_isInitialized) {
            self::$_sources = array();

            // Get list of default connection string sources.
            $default = ConnectionStringSource::getDefaultSources();
            foreach ($default as $name => $provider) {
                self::$_sources[$name] = $provider;
            }

            self::$_isInitialized = true;
        }
    }