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<td bgcolor="ffffff" width="15">&nbsp;</td><td bgcolor="ffffff" width="12">&nbsp;</td><td valign="top" align="left" bgcolor="ffffff" width="*">
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>声明 bean</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 页（共8 页）</nobr></font></td>
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<p>
<code>&lt;jsp:useBean&gt;</code> 动作用于声明，JSP 页面将使用存储在指定作用域中、并可从中访问的 bean。</p>
<p>例如，下面的标签在应用程序作用域中，声明了类型 <code>UserBean</code> 和 <code>id user</code> 的 bean：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
&lt;jsp:useBean id="user" class="UserBean" scope="application"/&gt;
</code>
</pre>
<p>
<code>id</code> 属性的值是用于引用其他 JSP 元素和 scriptlet 中的 bean 的标识符。bean 的作用域可以是 <code>application</code>、<code>session</code>、<code>request</code> 或 <code>page</code>。<code>id</code> 属性是强制的，而 <code>scope</code> 是可选的，<code>scope</code> 的默认值是 <code>page</code>。</p>
<p>其他的可能属性是 <code>class</code>、<code>type</code> 和 <code>beanName</code>。在 <code>&lt;jsp:useBean&gt;</code> 动作中，这些属性的子集需要以下面组合之一出现：</p>
<ul>
<li>
<code>class</code>
</li>
<li>
<code>class</code> 和 <code>type</code>
</li>
<li>
<code>type</code>
</li>
<li>
<code>beanName</code> 和 <code>type</code>
</li>
</ul>
<p>
<b>使用 class 属性</b>
<br>
下面标签使用 <code>class</code> 属性。这导致 JSP 引挚试图使用 <code>id</code> user 在应用程序作用域中，查找 <code>UserBean</code> 类的一个实例。如果不能找到匹配实例，就用 <code>id user</code> 创建一个新实例，并存储在应用程序作用域中。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
&lt;jsp:useBean id="user" class="UserBean" scope="application"/&gt;
</code>
</pre>
<p>
<b>使用 class 和 type 属性</b>
<br>
如果在下面的代码中使用 <code>class</code> 和 <code>type</code>，JSP 引挚就尝试查找 <code>PersonBean</code> 类型的 bean 实例。但如果不能找到匹配的 bean，就通过初始化 <code>UserBean</code> 类创建一个新实例。在该例中，<code>UserBean</code> 必须是类型为 <code>PersonBean</code> 的一个类。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
&lt;"jsp:useBean id="user" type="PersonBean" class="UserBean" scope="application"/&gt;
</code>
</pre>
<p>
<b>使用 type 属性</b>
<br>
在没有 <code>class</code> 或 <code>beanName</code> 属性下，可以使用 <code>type</code> 属性，如下面例子所示：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
&lt;jsp:useBean id="user" type="PersonBean" scope="session"/&gt;
</code>
</pre>
<p>这将导致 JSP 引挚在提及的作用域中查找指定类型的 bean。在本例中，没有现有的 bean 匹配类型，将不创建任何新的 bean 实例，并抛出 <code>InstantiationException</code>。</p>
<p>
<b>使用 type 和 beanName 属性</b>
<br>

<code>beanName</code> 属性可以引用类名或包含 bean 的序列化文件的名称。当使用 <code>beanName</code>时，bean 是通过 <code>java.beans.Beans.instantiate</code> 方法实例化的。如果 <code>beanName</code> 代表序列化模板，它就使用类加载器读取序列化表单。否则，通常实例化 bean。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
&lt;"jsp:useBean id="user" type="PersonBean" beanName="User.ser" scope="session"/&gt;
</code>
</pre>
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