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<TITLE>Conditionally Generating Output Using JSTL in a JSP Page (Java Developers Almanac Example)
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<B><A HREF="../javax.servlet.jsp.jstl.core/pkg.html">javax.servlet.jsp.jstl.core</A></B><font color="#666666" class="xsmall-font">
[6 examples]
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<h3>e1069. Conditionally Generating Output Using JSTL in a JSP Page</h3>
Any non-JSP code in a JSP file is called template text and is
automatically written to the output stream. The JSTL tags <code><c:if></code>
and <code><c:choose></code> make it possible to dynamically generate template
text depending on a condition.
The <code><c:if></code> tag generates its body if the expression in the
<code>test</code> attribute evaluates to the boolean value <code>true</code>
or the string value <code>"true"</code>:
<pre> <%-- Declare the core library --%>
<%@ taglib uri="/WEB-INF/tld/c.tld" prefix="c" %>
<%-- Simple if conditions --%>
<c:if test='${param.p == "someValue"}'>
Generate this template text if p equals someValue
</c:if>
<c:if test='${param.p}'>
Generate this template text if p equals "true"
</c:if>
</pre>
An if/else action requires the use of the <code><c:choose></code> tag:
<pre> <%-- A simple if/else condition --%>
<c:choose>
<c:when test='${param.p == "someValue"}'>
Generate this template text if p equals someValue
</c:when>
<c:otherwise>
Otherwise generate this template text
</c:otherwise>
</c:choose>
</pre>
Multiple conditions can be evaluated using the <code><c:choose></code> tag:
<pre> <c:choose>
<c:when test='${param.p == "0"}'>
Generate this template text if p equals 0
</c:when>
<c:when test='${param.p == "1"}'>
Generate this template text if p equals 1
</c:when>
<c:otherwise>
Generate this template text if p equals anything else
</c:otherwise>
</c:choose>
</pre>
The <code><c:out></code> can also be used to conditionally generate template text.
If the <code>value</code> attribute is null, a <font color="#0066ff"><i>default value</i></font> (if specified)
is generated. The default value can be specified using the <code>default</code>
attribute or can be specified in the body content. This
example demonstrates both methods:
<pre> <%-- Default value in an attribute --%>
<c:out value='${param.p}' default="Generate this if p is null" />
<%-- Default value in the body content --%>
<c:out value='${param.p}'>
Generate this if p is null
</c:out>
</pre>
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<b>Related Examples</b></font></td>
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e1064. <a class="eglink" href="usejstl.html?l=rel">
Using the Java Standard Tag Library (JSTL) in a JSP Page
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e1065. <a class="eglink" href="uselang.html?l=rel">
Enabling the JSTL Expression Language in a JSP Page
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e1066. <a class="eglink" href="getparam.html?l=rel">
Getting a Request Parameter Using JSTL in a JSP Page
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e1067. <a class="eglink" href="attr.html?l=rel">
Saving Data Using JSTL in a JSP Page
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e1068. <a class="eglink" href="htmlfrag.html?l=rel">
Saving and Emitting HTML Fragments Using JSTL in a JSP Page
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