code: public void initialize() {
          if (cacheSize <= 0) {// 不考虑cacheSize为0的情况，强制使用LRU cache机制
              cacheSize = DEFAULT_CACHE_SIZE;
          }
  
          functions = new HashMap<String, Object>();
          engine = new ScriptJexlEngine();
          engine.setCache(cacheSize);
          engine.setSilent(true);
          engine.setFunctions(functions); // 注册functions
      }
docs: 初始化function