jspintro7.html

j2eePDF格式的电子书

  <tr align="left">    <td><a name="wp77123"> </a><div class="pCellBody">
<code class="cCode">${3 div 4}</code>
</div>
</td>
    <td><a name="wp77125"> </a><div class="pCellBody">
<code class="cCode">0.75</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77127"> </a><div class="pCellBody">
<code class="cCode">${10 mod 4}</code>
</div>
</td>
    <td><a name="wp77129"> </a><div class="pCellBody">
<code class="cCode">2</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77286"> </a><div class="pCellBody">
<code class="cCode">${!empty param.Add}</code>
</div>
</td>
    <td><a name="wp77288"> </a><div class="pCellBody">
True if the request parameter named <code class="cCode">Add</code> is null or an empty string<code class="cCode">.</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77245"> </a><div class="pCellBody">
<code class="cCode">${pageContext.request.contextPath}</code>
</div>
</td>
    <td><a name="wp77247"> </a><div class="pCellBody">
The context path
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77241"> </a><div class="pCellBody">
<code class="cCode">${sessionScope.cart.numberOfItems}</code>
</div>
</td>
    <td><a name="wp77243"> </a><div class="pCellBody">
The value of the <code class="cCode">numberOfItems</code> property of the session-scoped attribute named <code class="cCode">cart</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77237"> </a><div class="pCellBody">
<code class="cCode">${param[</code>&#39;<code class="cCode">mycom.productId</code>&#39;<code class="cCode">]}</code>
</div>
</td>
    <td><a name="wp77239"> </a><div class="pCellBody">
The value of the request parameter named <code class="cCode">mycom.productId</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77233"> </a><div class="pCellBody">
<code class="cCode">${header[&quot;host&quot;]}</code>
</div>
</td>
    <td><a name="wp77235"> </a><div class="pCellBody">
The host
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp81273"> </a><div class="pCellBody">
<code class="cCode">${departments[deptName]}</code>
</div>
</td>
    <td><a name="wp81275"> </a><div class="pCellBody">
The value of the entry named <code class="cCode">deptName</code> in the <code class="cCode">departments</code> map
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp84339"> </a><div class="pCellBody">
<code class="cCode">${requestScope[&#39;javax.servlet.<br />forward.servlet_path&#39;]}</code>
</div>
</td>
    <td><a name="wp84341"> </a><div class="pCellBody">
The value of the request-scoped attribute named <code class="cCode">javax.servlet.<br />forward.servlet_path</code>
</div>
</td>
</tr>
</table>
</div>
<p class="pBody">
</p>
<a name="wp77078"> </a><h3 class="pHeading2">
Functions
</h3>
<a name="wp71289"> </a><p class="pBody">
The JSP expression language allows you to define a function that can be invoked in an expression. Functions are defined using the same mechanisms as custom tags (See <a  href="JSPIntro9.html#wp73314">Using Custom Tags</a> and Chapter&nbsp;<a  href="JSPTags.html#wp74641">15</a>). 
</p>
<a name="wp71290"> </a><h4 class="pHeading3">
Using Functions
</h4>
<a name="wp71821"> </a><p class="pBody">
Functions can appear in template text and tag attribute values. 
</p>
<a name="wp71709"> </a><p class="pBody">
To use a function in a JSP page, you import the tag library containing the function using a <code class="cCode">taglib</code> directive. Then, you preface the function invocation with the prefix declared in the directive. 
</p>
<a name="wp71718"> </a><p class="pBody">
For example, the date example page <code class="cCode">index.jsp</code> imports the <code class="cCode">/functions</code> library and invokes the function <code class="cCode">equals</code> in an expression:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;%@ taglib prefix=&quot;f&quot; uri=&quot;/functions&quot;%&gt;
...
&nbsp;&nbsp;&nbsp;&nbsp;&lt;c:when
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;test=&quot;${f:equals(selectedLocaleString,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;localeString)}&quot; &gt;<a name="wp75108"> </a>
</pre></div>
<a name="wp71298"> </a><h4 class="pHeading3">
Defining Functions
</h4>
<a name="wp71300"> </a><p class="pBody">
To define a function you program it as a public static method in a public class. The <code class="cCode">mypkg.MyLocales</code> class in the <code class="cCode">date</code> example defines a function that tests the equality of two <code class="cCode">Strings</code> as follows:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
package mypkg;
public class MyLocales {

&nbsp;&nbsp;...
&nbsp;&nbsp;public static boolean equals( String l1, String l2 ) {
&nbsp;&nbsp;&nbsp;&nbsp;return l1.equals(l2);
&nbsp;&nbsp;}
}<a name="wp75123"> </a>
</pre></div>
<a name="wp71307"> </a><p class="pBody">
Then, you map the function name as used in the EL expression to the defining class and function signature in a TLD. The following <code class="cCode">functions.tld</code> file in the date example maps the <code class="cCode">equals</code> function to the class containing the implementation of the function <code class="cCode">equals</code> and the signature of the function:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
&lt;function&gt;
&nbsp;&nbsp;&lt;name&gt;equals&lt;/name&gt;
&nbsp;&nbsp;&lt;function-class&gt;mypkg.MyLocales&lt;/function-class&gt;
&nbsp;&nbsp;&lt;function-signature&gt;boolean equals( java.lang.String,
&nbsp;&nbsp;&nbsp;&nbsp;java.lang.String )&lt;/function-signature&gt;
&lt;/function&gt;<a name="wp75037"> </a>
</pre></div>
<a name="wp71919"> </a><p class="pBody">
A tag library can only have one <code class="cCode">function</code> element with any given <code class="cCode">name</code> element.
</p>
    </blockquote>

   <img src="images/blueline.gif" width="550" height="8" ALIGN="BOTTOM" NATURALSIZEFLAG="3" ALT="Divider">


    <table width="550" summary="layout" id="SummaryNotReq1">
      <tr>
	<td align="left" valign="center">
	<font size="-1">
	<a href="http://java.sun.com/j2ee/1.4/download.html#tutorial" target="_blank">Download</a>
	<br>
	<a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/faq.html" target="_blank">FAQ</a>
	<br>
	<a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/history.html" target="_blank">History</a>
	</td>
        <td align="center" valign="center">
<a accesskey="p" href="JSPIntro6.html"><img id="LongDescNotReq1" src="images/PrevArrow.gif" width="26" height="26" border="0" alt="Prev" /></a><a accesskey="c" href="J2EETutorialFront.html"><img id="LongDescNotReq1" src="images/UpArrow.gif" width="26" height="26" border="0" alt="Home" /></a><a accesskey="n" href="JSPIntro8.html"><img id="LongDescNotReq3" src="images/NextArrow.gif" width="26" height="26" border="0" alt="Next" /></a><a accesskey="i" href="J2EETutorialIX.html"></a>
        </td>
	<td align="right" valign="center">
	<font size="-1">
	<a href="http://java.sun.com/j2ee/1.4/docs/api/index.html" target="_blank">API</a>
	<br>
	<a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/search.html" target="_blank">Search</a>
	<br>
	<a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/sendusmail.html" target="_blank">Feedback</a></font>
	</font>
	</td>
      </tr>
    </table>

    <img src="images/blueline.gif" width="550" height="8" ALIGN="BOTTOM" NATURALSIZEFLAG="3" ALT="Divider">

<p><font size="-1">All of the material in <em>The J2EE(TM) 1.4 Tutorial</em> is 
<a href="J2EETutorialFront2.html">copyright</a>-protected and may not be published in other works
without express written permission from Sun Microsystems.</font>

  </body>
</html>