code: public static Object putIfAbsent(String key, Object value) {
          return value == null ? CONTEXT.remove(key) : CONTEXT.putIfAbsent(key, value);
      }
docs: 设置上下文信息（不存在才设置成功）
  
  @param key   the key
  @param value the value
  @return the object
  @see ConcurrentHashMap#putIfAbsent(Object, Object)