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j2eePDF格式的电子书

  <% cart.add(bid, addedBook); %>
...
&lt;/c:if&gt;<a name="wp73413"> </a>
</pre></div>
<a name="wp73414"> </a><p class="pBody">
The <code class="cCode">choose</code> tag performs conditional block execution by the embedded <code class="cCode">when</code> sub tags. It renders the body of the first <code class="cCode">when</code> tag whose test condition evaluates to true. If none of the test conditions of nested <code class="cCode">when</code> tags evaluate to <code class="cCode">true</code>, then the body of an <code class="cCode">otherwise</code> tag is evaluated, if present. 
</p>
<a name="wp81081"> </a><p class="pBody">
For example, the following sample code shows how to render text based on a customer's membership category. 
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;c:choose&gt; 
&nbsp;&nbsp;&lt;c:when test=&quot;${customer.category == &#39;trial&#39;}&quot; &gt; 
&nbsp;&nbsp;&nbsp;&nbsp;... 
&nbsp;&nbsp;&lt;/c:when&gt; 
&nbsp;&nbsp;&lt;c:when test=&quot;${customer.category == &#39;member&#39;}&quot; &gt; 
&nbsp;&nbsp;&nbsp;&nbsp;... 
&nbsp;&nbsp;&lt;/c:when&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&lt;c:when test=&quot;${customer.category == &#39;preferred&#39;}&quot; &gt; 
&nbsp;&nbsp;&nbsp;&nbsp;... 
&nbsp;&nbsp;&lt;/c:when&gt; 
&nbsp;&nbsp;&lt;c:otherwise&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;... 
&nbsp;&nbsp;&lt;/c:otherwise&gt; 
&lt;/c:choose&gt; <a name="wp81548"> </a>
</pre></div>
<a name="wp81533"> </a><p class="pBody">
The <code class="cCode">choose</code>, <code class="cCode">when</code>, and <code class="cCode">otherwise</code> tags can be used to construct an <code class="cCode">if</code>-<code class="cCode">then</code>-<code class="cCode">else</code> statement as follows: 
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;c:choose&gt; 
&nbsp;&nbsp;&lt;c:when test=&quot;${count == 0}&quot; &gt; 
&nbsp;&nbsp;&nbsp;&nbsp;No records matched your selection. 
&nbsp;&nbsp;&lt;/c:when&gt; 
&nbsp;&nbsp;&lt;c:otherwise&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;${count} records matched your selection. 
&nbsp;&nbsp;&lt;/c:otherwise&gt; 
&lt;/c:choose&gt;<a name="wp81544"> </a>
</pre></div>
<a name="wp63713"> </a><h4 class="pHeading3">
Iterator Tags
</h4>
<a name="wp66776"> </a><p class="pBody">
The <code class="cCode">forEach</code> tag allows you to iterate over a collection of objects. You specify the collection via the <code class="cCode">items</code> attribute, and the current item is available through a scope variable named by the <code class="cCode">item</code> attribute.
</p>
<a name="wp81610"> </a><p class="pBody">
A large number of collection types are supported by <code class="cCode">forEach</code>, including all implementations of <code class="cCode">java.util.Collection</code> and <code class="cCode">java.util.Map</code>. If the <code class="cCode">items</code> attribute is of type <code class="cCode">java.util.Map</code>, then the current item will be of type <code class="cCode">java.util.Map.Entry</code>, which has the following properties:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp81582"> </a><div class="pSmartList1"><li><code class="cCode">key</code> - the key under which the item is stored in the underlying <code class="cCode">Map</code></li></div>
<a name="wp81583"> </a><div class="pSmartList1"><li><code class="cCode">value</code> - the value that corresponds to the key</li></div>
</ul></div>
<a name="wp81572"> </a><p class="pBody">
Arrays of objects as well as arrays of primitive types (for example, <code class="cCode">int</code>) are also supported. For arrays of primitive types, the current item for the iteration is automatically wrapped with its standard wrapper class (for example, <code class="cCode">Integer</code> for <code class="cCode">int</code>, <code class="cCode">Float</code> for <code class="cCode">float</code>, and so on). 
</p>
<a name="wp81581"> </a><p class="pBody">
Implementations of <code class="cCode">java.util.Iterator</code> and <code class="cCode">java.util.Enumeration</code> are supported but these must be used with caution. <code class="cCode">Iterator</code> and <code class="cCode">Enumeration</code> objects are not resettable so they should not be used within more than one iteration tag. Finally, <code class="cCode">java.lang.String</code> objects can be iterated over if the string contains a list of comma separated values (for example: Monday,Tuesday,Wednesday,Thursday,Friday).
</p>
<a name="wp81619"> </a><p class="pBody">
Here's the shopping cart iteration from the previous section with the <code class="cCode">forEach</code> tag:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;c:forEach var=&quot;item&quot; items=&quot;${sessionScope.cart.items}&quot;&gt;
&nbsp;&nbsp;...
&nbsp;&nbsp;&lt;tr&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;&lt;td align=&quot;right&quot; bgcolor=&quot;#ffffff&quot;&gt; 
&nbsp;&nbsp;&nbsp;&nbsp;${item.quantity}
&nbsp;&nbsp;&lt;/td&gt;
&nbsp;&nbsp;...
&lt;/c:forEach&gt;<a name="wp81620"> </a>
</pre></div>
<a name="wp81567"> </a><p class="pBody">
The <code class="cCode">forTokens</code> tag is used to iterate over a collection of tokens separated by a delimiter.
</p>
<a name="wp64122"> </a><h3 class="pHeading2">
URL Tags
</h3>
<a name="wp64935"> </a><p class="pBody">
The <code class="cCode">jsp:include</code> element provides for the inclusion of static and dynamic resources in the same context as the current page. However, <code class="cCode">jsp:include</code> cannot access resources that reside outside of the Web application and causes unnecessary buffering when the resource included is used by another element.
</p>
<a name="wp64951"> </a><p class="pBody">
In the example below, the <code class="cCode">transform</code> element uses the content of the included resource as the input of its transformation. The <code class="cCode">jsp:include</code> element reads the content of the response, writes it to the body content of the enclosing transform element, which then re-reads the exact same content. It would be more efficient if the <code class="cCode">transform</code> element could access the input source directly and avoid the buffering involved in the body content of the transform tag.
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;acme:transform&gt;
&nbsp;&nbsp;&lt;jsp:include page=&quot;/exec/employeesList&quot;/&gt;
&lt;acme:transform/&gt;<a name="wp64956"> </a>
</pre></div>
<a name="wp69696"> </a><p class="pBody">
The <code class="cCode">import</code> tag is therefore the simple, generic way to access URL-based resources whose content can then be included and or processed within the JSP page. For example, in <a  href="JSTL5.html#wp63716">XML Tags</a>, <code class="cCode">import</code> is used to read in the XML document containing book information and assign the content to the scoped variable <code class="cCode">xml</code>:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;c:import url=&quot;/books.xml&quot; var=&quot;xml&quot; /&gt;
&lt;x:parse doc=&quot;${xml}&quot; var=&quot;booklist&quot; 
&nbsp;&nbsp;scope=&quot;application&quot; /&gt;<a name="wp75969"> </a>
</pre></div>
<a name="wp75967"> </a><p class="pBody">
The <code class="cCode">param</code> tag, analogous to the <code class="cCode">jsp:param</code> tag (see <a  href="JSPIntro11.html#wp66264">jsp:param Element</a>), can be used with <code class="cCode">import</code> to specify request parameters.
</p>
<a name="wp73424"> </a><p class="pBody">
In <a  href="Servlets11.html#wp64784">Session Tracking</a> we discussed how an application must rewrite URLs to enable session tracking whenever the client turns off cookies. You can use the <code class="cCode">url</code> tag to rewrite URLs returned from a JSP page. The tag includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged. Note that this feature requires the URL to be <em class="cEmphasis">relative</em>. The <code class="cCode">url</code> tag takes <code class="cCode">param</code> subtags for including parameters in the returned URL. For example, <code class="cCode"><a  href="../examples/web/bookstore4/web/bookcatalog.txt" target="_blank">bookcatalog.jsp</a></code> rewrites the URL used to add a book to the shopping cart as follows:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;c:url var=&quot;url&quot; value=&quot;/catalog&quot; &gt;
&nbsp;&nbsp;&lt;c:param name=&quot;Add&quot; value=&quot;${bookId}&quot; /&gt;
&lt;/c:url&gt;
&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;${url}&quot;&gt;<a name="wp74593"> </a>
</pre></div>
<a name="wp73452"> </a><p class="pBody">
<code class="cCode">The redirect</code> tag sends an HTTP redirect to the client. The <code class="cCode">redirect</code> tag takes <code class="cCode">param</code> subtags for including parameters in the returned URL.
</p>
<a name="wp85708"> </a><h3 class="pHeading2">
Miscellaneous Tags
</h3>
<a name="wp85666"> </a><p class="pBody">
The <code class="cCode">catch</code> tag provides a complement to the JSP error page mechanism. It allows page authors to recover gracefully from error conditions that they can control. Actions that are of central importance to a page should <em class="cEmphasis">not</em> be encapsulated in a <code class="cCode">catch</code>, so their exceptions will propagate to an error page. Actions with secondary importance to the page should be wrapped in a <code class="cCode">catch</code>, so they never cause the error page mechanism to be invoked.
</p>
<a name="wp85667"> </a><p class="pBody">
The exception thrown is stored in the variable identified by <code class="cCode">var</code>, which always has page scope. If no exception occurred, the scoped variable identified by <code class="cCode">var</code> is removed if it existed. If <code class="cCode">var</code> is missing, the exception is simply caught and not saved.
</p>
<a name="wp85668"> </a><p class="pBody">
The <code class="cCode">out</code> tag evaluates an expression and outputs the result of the evaluation to the current <code class="cCode">JspWriter</code> object. The syntax and attributes are 
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;c:out value=&quot;<code class="cVariable">value</code>&quot; [escapeXml=&quot;{true|false}&quot;] 
&nbsp;&nbsp;[default=&quot;<code class="cVariable">defaultValue</code>&quot;] /&gt;<a name="wp85669"> </a>
</pre></div>
<a name="wp85670"> </a><p class="pBody">
If the result of the evaluation is a <code class="cCode">java.io.Reader</code> object, data is first read from the <code class="cCode">Reader</code> object and then written into the current <code class="cCode">JspWriter</code> object. The special processing associated with <code class="cCode">Reader</code> objects improves performance when large amount of data must be read and then written to the response.
</p>
<a name="wp85705"> </a><p class="pBody">
If <code class="cCode">escapeXml</code> is true, the character conversions listed in <a  href="JSTL4.html#wp85678">Table 14-4</a> are applied:</p><div align="left">
<table border="1" summary="Character Conversions" id="wp85678">
  <caption><a name="wp85678"> </a><div class="pTableTitle">
Table 14-4   Character Conversions
</div>
</caption>
  <tr align="center">    <th><a name="wp85682"> </a><div class="pCellHeading">
Character
</div>
</th>
    <th><a name="wp85684"> </a><div class="pCellHeading">
Character Entity Code
</div>
</th>
</tr>
  <tr align="left">    <td><a name="wp85686"> </a><div class="pCellBody">
<code class="cCode">&lt;</code>
</div>
</td>
    <td><a name="wp85688"> </a><div class="pCellBody">
<code class="cCode">&amp;lt;</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp85690"> </a><div class="pCellBody">
<code class="cCode">&gt;</code>
</div>
</td>
    <td><a name="wp85692"> </a><div class="pCellBody">
<code class="cCode">&amp;gt;</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp85694"> </a><div class="pCellBody">
<code class="cCode">&amp;</code>
</div>
</td>
    <td><a name="wp85696"> </a><div class="pCellBody">
<code class="cCode">&amp;amp;</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp85698"> </a><div class="pCellBody">
&#39;
</div>
</td>
    <td><a name="wp85700"> </a><div class="pCellBody">
<code class="cCode">&amp;#039;</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp85702"> </a><div class="pCellBody">
<code class="cCode">&quot;</code>
</div>
</td>
    <td><a name="wp85704"> </a><div class="pCellBody">
<code class="cCode">&amp;#034;</code>
</div>
</td>
</tr>
</table>
</div>
<p class="pBody">
</p>
    </blockquote>

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