code: @SuppressWarnings("unchecked")
      public final static <T extends Object> T getContext(String key) {
          Map<String, Object> ctx = CTX_HOLDER.get();
          if (ctx == null) {
              return null;
          }
          return (T) ctx.get(key);
      }
docs: 从线程上下文中获取内容
  
  @param key