code: private RpcContent findRpcContent() {
          synchronized (freeList) {
              RpcContent cacheContent = freeList.getFirst();
              if (!cacheContent.isUsed()) {
                  cacheContent.init();
                  freeList.remove(cacheContent);
                  return cacheContent;
              }
          }
          
          RpcContent rpcContent = new RpcContent(getWaitTime());
          rpcContent.init();
          return rpcContent;
      }
docs: 空闲队列处理
  
  @return RpcContent