# 7.1 简单条件：if与unless

有时，你会需要模版的某部分在满足特定条件时，才在结果中显示。

比如，想象一下：我们想要在产品表格的某一列里，展示每一个产品的评论数量，并且如果有评论的话，还要显示一条指向评论详细页面的链接。

为此，我们使用`th:if`属性：

```markup
<table>
  <tr>
    <th>NAME</th>
    <th>PRICE</th>
    <th>IN STOCK</th>
    <th>COMMENTS</th>
  </tr>
  <tr th:each="prod : ${prods}" th:class="${prodStat.odd}? 'odd'">
    <td th:text="${prod.name}">Onions</td>
    <td th:text="${prod.price}">2.41</td>
    <td th:text="${prod.inStock}? #{true} : #{false}">yes</td>
    <td>
      <span th:text="${#lists.size(prod.comments)}">2</span> comment/s
      <a href="comments.html" 
         th:href="@{/product/comments(prodId=${prod.id})}" 
         th:if="${not #lists.isEmpty(prod.comments)}">view</a>
    </td>
  </tr>
</table>
```

这里有相当多可以看的地方，让我们看到最重要的一行上：

```markup
<a href="comments.html"
   th:href="@{/product/comments(prodId=${prod.id})}" 
   th:if="${not #lists.isEmpty(prod.comments)}">view</a>
```

这会创建一条指向评论页的链接（URL是`/product/comments`），并把`prodId`参数设置到产品`id`上，但是只会在产品有任何评论的情况下。

让我们看一看生成的标记：

```markup
<table>
  <tr>
    <th>NAME</th>
    <th>PRICE</th>
    <th>IN STOCK</th>
    <th>COMMENTS</th>
  </tr>
  <tr>
    <td>Fresh Sweet Basil</td>
    <td>4.99</td>
    <td>yes</td>
    <td>
      <span>0</span> comment/s
    </td>
  </tr>
  <tr class="odd">
    <td>Italian Tomato</td>
    <td>1.25</td>
    <td>no</td>
    <td>
      <span>2</span> comment/s
      <a href="/gtvg/product/comments?prodId=2">view</a>
    </td>
  </tr>
  <tr>
    <td>Yellow Bell Pepper</td>
    <td>2.50</td>
    <td>yes</td>
    <td>
      <span>0</span> comment/s
    </td>
  </tr>
  <tr class="odd">
    <td>Old Cheddar</td>
    <td>18.75</td>
    <td>yes</td>
    <td>
      <span>1</span> comment/s
      <a href="/gtvg/product/comments?prodId=4">view</a>
    </td>
  </tr>
</table>
```

完美！这就是我们想要的。

注意：`th:if`属性不仅仅会对布尔条件求值。它的能力超出了布尔条件的范围。他会依据以下规则，将特定的表达式求值为`true`：

* 如果值不为null：
  * 如果值是布尔值，并且为true
  * 如果值是数字，并且不为0
  * 如果值是字符，并且不为0
  * 如果值是字符串，并且不为“false”， “off”或者“no”
  * 如果值不是布尔值，数字，字符或者字符串
* （如果值为null，`th:if`会求值为false)

`th:if`有一个相反的属性：`th:unless`。我们可以把它用在先前的例子里，代替在OGNL表达式里使用`not`：

```markup
<a href="comments.html"
   th:href="@{/comments(prodId=${prod.id})}" 
   th:unless="${#lists.isEmpty(prod.comments)}">view</a>
```


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