code: public void addEngine(ReqMethod method, LinkedEngine engine) {
          for (ReqMethod md : method.parse()) {
              LinkedEngine[] methodEngines = engines[md.ordinal()];
              if (methodEngines.length == 0) {
                  methodEngines = new LinkedEngine[] { engine };
              } else {
                  methodEngines = Arrays.copyOf(methodEngines, methodEngines.length + 1);
                  methodEngines[methodEngines.length - 1] = engine;
              }
              engines[md.ordinal()] = methodEngines;
              engineCount++;
          }
          clearCache();
      }
docs: 添加一个 {@link Engine} ；如果所给的 method 是 {@link ReqMethod#ALL}，则优先级最低。
  
  @param method
  @param engine