使用Freemarker评估带有null值的JSon

When dealing with a JSon that some values are null Freemarker give an error in ?eval.

在处理JSon时，某些值为null Freemarker在？eval中给出错误。

with mapper.setSerializationInclusion(Inclusion.NON_NULL) i can avoid this but i miss this information on the generated JSon.

使用mapper.setSerializationInclusion（Inclusion.NON_NULL）我可以避免这种情况，但我想念生成的JSon上的这些信息。

There's a way to achieve the evaluation with this null values?

有一种方法可以用这个空值来实现评估吗？

<#assign test = "{\"foo\":\"bar\"}">
<#assign m = test?eval>
${m.foo}  <#-- prints: bar -->

Fail in eval

评估失败

<#assign test = "{\"foo\":null}">
<#assign m = test?eval> <#-- fail in eval -->
${m.foo}  

1 个解决方案

#1

Unfortunately (or... infuriatingly), FTL doesn't know the concept of null (although this might will change with 2.4). So even if you manage to create a Map from that JSON where the foo key exists but the associated value is null (like you create a such Map in Java), ${m.foo} will still fail. Surely you can write ${m.foo!'null'}, but that will print null even if there's no foo key at all. So maybe it's better if you provide a default value for the null-s during the JSON evaluation:

不幸的是（或......令人愤怒），FTL不知道null的概念（尽管这可能会随着2.4而改变）。因此，即使您设法从存在foo密钥的JSON创建Map，但关联的值为null（就像您在Java中创建这样的Map），$ {m.foo}仍然会失败。当然你可以写$ {m.foo！'null'}，但即使根本没有foo键也会打印null。因此，如果在JSON评估期间为null-s提供默认值，可能会更好：

<#function parseJSON json>
  <#local null = 'null'> <#-- null is not a keyword in FTL -->
  <#return json?eval>
</#function>

${parseJSON("{\"foo\":null}").foo}  <#-- prints null -->

Now, however, you can't tell the difference between "null" and null. If that's a problem, you could chose some odd default, like '@@@null' or even use a macro as indicator value and then use ?is_macro to test if the value was null (that hack is useful as JSON evaluation can't produce macros)...

但是，现在，您无法区分“null”和null。如果这是一个问题，你可以选择一些奇怪的默认值，比如'@@@ null'，或者甚至使用一个宏作为指标值然后使用？is_macro来测试该值是否为null（该hack是有用的，因为JSON评估不能产生宏）...