public Class getExtension(String namespace, String function) {
        if (!namespace.isEmpty()) {
            return extensionMap.get(namespace + SiddhiConstants.EXTENSION_SEPARATOR + function);
        } else {
            return extensionMap.get(function);
        }
    }
Abstract Holder class to retrieve added extensions and store them in a map.
modules/siddhi-core/src/main/java/io/siddhi/core/util/extension/holder/AbstractExtensionHolder.java