private static <V> boolean containsKey(final Map<String, V> cache, final String key) {
        return Objects.nonNull(cache) && cache.containsKey(key);
    }
determines whether the string is path variable.

@param key path string
@return true or false
shenyu-plugin/shenyu-plugin-base/src/main/java/org/apache/shenyu/plugin/base/trie/ShenyuTrie.java