# 5.5 定值布尔属性

HTML有布尔属性的概念。布尔属性没有值，属性存在意味着值是“true”。在XHTML里，这些属性只有一个值，这个值就是它自己。 例如，`checked`：

```markup
<input type="checkbox" name="option2" checked /> <!-- HTML -->
<input type="checkbox" name="option1" checked="checked" /> <!-- XHTML -->
```

标准方言包含允许你通过求值条件设置属性的属性。如果求值为true，属性将会被设置为它的定值。如果为false，属性将不会被设置：

```markup
<input type="checkbox" name="active" th:checked="${user.active}" />
```

以下定值布尔属性存在标准方言里：

|                   |              |               |
| ----------------- | ------------ | ------------- |
| th:async          | th:autofocus | th:autoplay   |
| th:checked        | th:controls  | th:declare    |
| th:default        | th:defer     | th:disabled   |
| th:formnovalidate | th:hidden    | th:ismap      |
| th:loop           | th:multiple  | th:novalidate |
| th:nowrap         | th:open      | th:pubdate    |
| th:readonly       | th:required  | th:reversed   |


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