private void putFilter(String filterName, ZuulFilter<?, ?> filter, long lastModified) {
        if (!filterRegistry.isMutable()) {
            LOG.warn("Filter registry is not mutable, discarding {}", filterName);
            return;
        }
        SortedSet<ZuulFilter<?, ?>> set = hashFiltersByType.get(filter.filterType());
        if (set != null) {
            hashFiltersByType.remove(filter.filterType()); // rebuild this list
        }

        String nameAndType = filter.filterType() + ":" + filter.filterName();
        filtersByNameAndType.put(nameAndType, filter);

        filterRegistry.put(filterName, filter);
        filterClassLastModified.put(filterName, lastModified);
    }
@return the total number of Zuul filters
zuul-core/src/main/java/com/netflix/zuul/DynamicFilterLoader.java