Java Thymeleaf：检查是否定义了变量
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Thymeleaf: check if a variable is defined
提问by Andrea
How can I check if a variable is definedin Thymeleaf?
我如何检查是否变量定义在Thymeleaf？
Something like this in Javascript:
在 Javascript 中是这样的：
if (typeof variable !== 'undefined') { }
or this in PHP:
或在 PHP 中：
if (isset($var)) { }
Is there an equivalent in Thymeleaf?
Thymeleaf 中是否有等价物？
采纳答案by Trynkiewicz Mariusz
Yes, you can easily check if given property exists for your document using following code. Note, that you're creating divtag if condition is met:
是的，您可以使用以下代码轻松检查文档是否存在给定属性。请注意，div如果满足条件，您正在创建标签：
<div th:if="${variable != null}" th:text="Yes, variable exists!">
I wonder, if variable exists...
</div>
If you want using variable's field it's worth checking if this field exists as well
如果您想使用variable's 字段，则值得检查该字段是否也存在
<div th:if="${variable != null && variable.name != null}" th:text="${variable.name}">
I wonder, if variable.name exists...
</div>
Or even shorter, without using if statement
甚至更短，不使用 if 语句
<div th:text="${variable?.name}">
I wonder, if variable.name exists...
</div>`
But using this statement you will end creating divtag whether variableor variable.nameexist
但是，使用这种说法你会最终创建div标签是否variable还是variable.name存在
You can learn more about conditionals in thymeleaf here
您可以在此处了解有关 thymeleaf 中的条件的更多信息
回答by Lay Leangsros
Short form:
简写：
<div th:if="${currentUser}">
<h3>Name:</h3><h3 th:text="${currentUser.id}"></h3>
<h3>Name:</h3><h3 th:text="${currentUser.username}"></h3>
</div>
回答by Lay Leangsros
In order to tell whether the context contains a given variable, you can ask the context variable map directly. This lets one determine whether the variable is specified at all, as opposed to the only the cases where it's defined but with a value of null.
为了判断上下文是否包含给定变量，可以直接询问上下文变量映射。这让我们可以确定是否完全指定了变量，而不是仅在定义了变量但值为 null 的情况下。
Thymeleaf 2
百里香叶 2
Use the #varsobject's containsKeymethod:
使用#vars对象的containsKey方法：
<div th:if="${#vars.containsKey('myVariable')}" th:text="Yes, $myVariable exists!"></div>
Thymeleaf 3
百里香叶 3
Use the #ctxobject's containsVariablemethod:
使用#ctx对象的containsVariable方法：
<div th:if="${#ctx.containsVariable('myVariable')}" th:text="Yes, $myVariable exists!"></div>
回答by Aleksandar Nikolic
You can use conditional operators. This will write variable if exists or empty string:
您可以使用条件运算符。如果存在或空字符串，这将写入变量：
<p th:text="${variable}?:''"></p>
