freemarker中对null值的处理

喜欢ヅ旅行 2023-10-07 18:03 15阅读 0赞

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## 1. freemarker不支持null。 ##

如果值为null会报错，如下：

FreeMarker template error (DEBUG mode; use RETHROW in production!):
    The following has evaluated to null or missing:
    ==> setmeal.sex [in template "mobile_setmeal.ftl" at line 41, column 42]
    
    ----
    Tip: It's the step after the last dot that caused this error, not those before it.
    ----
    Tip: If the failing expression is known to be legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
    ----
    
    ----
    FTL stack trace ("~" means nesting-related):
    - Failed at: #if setmeal.sex == "0" [in template "mobile_setmeal.ftl" at line 41, column 37]
    ----
    
    Java stack trace (for programmers):
    ----
    freemarker.core.InvalidReferenceException: [... Exception message was already printed; see it above ...]
    at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:131)
    at freemarker.core.EvalUtil.compare(EvalUtil.java:195)
    at freemarker.core.EvalUtil.compare(EvalUtil.java:112)
    at freemarker.core.ComparisonExpression.evalToBoolean(ComparisonExpression.java:64)
    at freemarker.core.IfBlock.accept(IfBlock.java:46)
    at freemarker.core.Environment.visit(Environment.java:324)
    at freemarker.core.MixedContent.accept(MixedContent.java:54)
    at freemarker.core.Environment.visitByHiddingParent(Environment.java:345)

## 2. 当值为null的处理 ##

### 2.1 过滤不显示 ###

**在属性后面加感叹号即可过滤null和空字符串，如果name为null则为空，不会报错**

示例：

Hello${name!}World

结果：HelloWorld

**if和??**

示例：

<#if age??>
         年龄不为空:${age}
         <#else>
         年龄为空
    </#if>

结果：年龄为空

**$和!**

如果age为null，默认给'0'

示例：

${age!'0'}

结果：0

## 2.2 设置默认值 ##

示例：

<#assign info={"mobile":"13812345678",'address':'北京市昌平区'} >
    ${info.mobile999!"null"}

结果：null

### 2.3 判断是否存在值 ###

示例：

<#assign info={"mobile":"13812345678",'address':'北京市昌平区'} >
    ${info.mobile999?if_exists}

结果：空，这样显示就没有问题了

### 2.4 忽略null值 ###

假设前提：userName为null  
$\{userName\} error  
$\{userName!\} 空白  
$\{userName!'tivon'\} tivon

假设前提：user.name为null  
$\{user.name\}，异常  
$\{(user.name)!\},显示空白  
$\{user.name!'vakin'\}，若user.name不为空则显示本身的值，否则显示vakin  
$\{user.name?default('vakin')\}，同上  
$\{user.name???string('不为空','为空')\},为空

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