freemarker 循环遍历map（踩坑）

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哎呀，真是痛苦啊，用以前用过的方式进行循环遍历，总是报错，代码和错误如下：
代码：

    <#list stringMap?keys as strKey>
        　<#list stringMap[strKey] as system>
              ${system}
          </#list>
    </#list>

错误：

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is freemarker.template.TemplateModelException: Getting the number of items or enumerating the items is not supported on this method+sequence (wrapper: f.e.b.SimpleMethodModel) value.
(Hint 1: Maybe you wanted to call this method first and then do something with its return value. Hint 2: Getting items by intex possibly works, hence it’s a “+sequence”.)

真的是让人头疼，找了两个小时也没有找到错误，我真是没有折了，折腾了半天，结果打印出来的key值你真的是想象不到：
key值：

getClass getOrDefault computeIfAbsent values replace replaceAll remove containsValue put empty compute hashCode putAll get merge class keySet abc entrySet forEach containsKey isEmpty clear computeIfPresent size equals clone toString putIfAbsent

想象不到吧，竟然把map自带的方法打印出来了，这当然是错的了，所以我就像大多数的程序猿一样，继续去网上找答案，终于皇天不负有心人啊，找到了解决办法，代码如下：

 <#list stringMap.keySet() as strKey>
        　<#list stringMap[strKey] as system>
              ${system}
          </#list>
    </#list>

虽然是一个小错误或者是小知识，也浪费了lz不少的时间，希望我在这里提供出来，能帮大家避免这个坑！