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Welcome ${user!}!
Welcome ${user!'your name'}!

如果user找不到值,会输出
Welcome !
Welcome your name!



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3.  Why is FreeMarker so picky about null-s and missing variables, and what to do with it?
 

To recapitulate what's this entry is about: FreeMarker by default treats an attempt to access a non-existent variable or a null value (this two is the same for FreeMarker) as error, which aborts the template execution.

First of all, you should understand the reason of being picky. Most scripting languages and template languages are rather forgiving with missing variables (and with null-s), and they usually treat them as empty string and/or 0 and/or logical false. This behavior has several problems:

  • It potentially hiders accidental mistakes, like a typo in a variable name, or when the template author refers to a variable that the programmer doesn't put into the data model, or for which the programmer uses a different name. Human is prone to do such accidental mistakes, computers are not, so missing this opportunity that the template engine can show these errors is a bad business. Even if you very carefully check the output of the templates during development, it is easy to look over mistakes like <#if hasDetp>print dept here...</#if>, which would then silently never print the dept of the visitor, since you have mistyped the variable name (it should be hasDept). Also think about maintenance, when you later modify your application... most probably you will not re-check templates that carefully each time.

  • Makes dangerous assumptions. The script language or template engine knows nothing about the application domain, so when it decides the value of something it don't know to be 0/false, it is a quite irresponsible and arbitrary thing. Just because it is not know what's your current bank account balance, can we just say it is $0 (and how easy it is to accidentally write Balance: ${balanace})? Just because it is not known if a patient has penicillin allergy, we can just say he/she doesn't have (and how easy it is to accidentally write <#if hasPenicilinAllergy>Warning...<#else>Allow...</#if>; there is a typo in this, if you didn't see)? Just consider the implications of such mistakes for a moment. They can be quite severe and troubling. Showing an error page is often better than showing incorrect information that formally looks good.

Being not picky is mostly sweeping under the carpet in this case (not facing with the problems), which of course most people feels more convenient, but still... we believe that in most cases being strict will save your time and increase your software quality in the long run.

On the other hand, we recognize that there are cases where you don't want FreeMarker to be that picky with good reason, and there is solution for them:

  • It's often normal that your data model contains null-s or have optional variables. In such cases use these operators. If you use them too often, try to rethink your data model, because depending on them too much is not just results in awkward verbose templates, but increases the probability of hiding errors and printing arbitrary incorrect output (for the reasons described earlier).

  • On a production server you may rather want to show an incomplete/damaged page than an error page. In this case you can use other error handler than the default. Error handlers can be made that rather skip the problematic part than abort the whole page rendering. Note, however, that although the error handlers don't give arbitrary default values to variables, for pages that show critical information it's maybe still better to show an error page. (Another feature you may interested in: the attempt/recover directives)

15.  What about null and the FreeMarker template language?
 

The FreeMarker template language doesn't know the Java language null at all. It doesn't have null keyword, and it can't test if something is null or not. When it technically faces with a null, it treats it exactly as a missing variable. For example, both if x is null in the data model and if it's not present at all, ${x!'missing'} will print ``missing'', you can't tell the difference. Also, if for example you want to test if a Java method has returned null, just write something like <#if foo.bar()??>.

You may interested in the rationale behind this. From the viewpoint of the presentation layer a null and non-existent thing is almost always the same. The difference between this two is usually just a technical detail, which is rather the result of implementation details than of the application logic. That you can't compare something to null (unlike in Java); it doesn't make sense to compare something with null in a template, since the template language doesn't do identity comparison (like the Java == operator when you compare two objects) but the more common sense value comparison (like Java's Object.equals(Object); that doesn't work with null either). And how could FreeMarker tell if something concrete equals with something that is missing and thus unknown? Or if two missing (unknown) things are equal? Of course these questions can't be answered.

There is at least one problem with this null-unaware approach. When you call a Java method from a template, you may want to pass a null value as argument (since the method was designed to be used in Java language, where the concept of null is known). In this case you can exploit a bug of FreeMarker (that we will not fix until we provide a correct solution for passing null values to a method): if you specify a missing variable as the argument, then it will not cause an error, but a null will be passed to the method instead. Like foo.bar(nullArg) will call the bar method with null as argument, assuming that there is no varaible exists with ``nullArg'' name.

posted on 2007-05-09 10:26 交口称赞 阅读(11751) 评论(5)  编辑  收藏 所属分类: freemarker

FeedBack:
# re: freemarker的空值和默认值 2007-05-09 10:29 交口称赞
Welcome ${user!}!
Welcome ${user!'your name'}!

但是这两种写法在freemarker editor中都提示错误。。。。。。。。  回复  更多评论
  
# re: freemarker的空值和默认值 2007-05-09 14:32 mmwy
${user?if_exists}

${user?default('your name')}
  回复  更多评论
  
# re: freemarker的空值和默认值 2007-05-09 15:41 交口称赞
呵呵
多谢楼上

!比较方便

楼上的写法比较规范,大家各取所需吧



  回复  更多评论
  
# re: freemarker的空值和默认值 2007-05-09 15:42 交口称赞
还有谁知道怎么在ftl模板里面直接用资源文件?  回复  更多评论
  
# re: freemarker的空值和默认值 2014-09-13 12:34 zuidaima
freemarker demo教程源代码下载地址:http://zuidaima.com/share/kfreemarker-p1-s1.htm  回复  更多评论
  

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