if.html

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<TITLE>Conditionally Generating Output Using JSTL in a JSP Page
(Java Developers Almanac Example)
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<META CONTENT="Patrick Chan" NAME="AUTHOR">
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    <B><A HREF="../javax.servlet.jsp.jstl.core/pkg.html">javax.servlet.jsp.jstl.core</A></B><font color="#666666" SIZE="-2">
        &nbsp;[6 examples]
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  <h3>
    e1068.  
    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>&lt;c:if&gt;</code>
and <code>&lt;c:choose&gt;</code> make it possible to dynamically generate template
text depending on a condition.
The <code>&lt;c:if&gt;</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>
    &lt;%-- Declare the core library --%&gt;
    &lt;%@ taglib uri="/WEB-INF/tld/c.tld" prefix="c" %&gt;
    
    &lt;%-- Simple if conditions --%&gt;
    &lt;c:if test='${param.p == "someValue"}'&gt;
        Generate this template text if p equals someValue
    &lt;/c:if&gt;
    
    &lt;c:if test='${param.p}'&gt;
        Generate this template text if p equals "true"
    &lt;/c:if&gt;
</pre>
An if/else action requires the use of the <code>&lt;c:choose&gt;</code> tag:

<pre>
    &lt;%-- A simple if/else condition --%&gt;
    &lt;c:choose&gt;
        &lt;c:when test='${param.p == "someValue"}'&gt;
            Generate this template text if p equals someValue
        &lt;/c:when&gt;
        &lt;c:otherwise&gt;
            Otherwise generate this template text
        &lt;/c:otherwise&gt;
    &lt;/c:choose&gt;
</pre>
Multiple conditions can be evaluated using the <code>&lt;c:choose&gt;</code> tag:

<pre>
    &lt;c:choose&gt;
        &lt;c:when test='${param.p == "0"}'&gt;
            Generate this template text if p equals 0
        &lt;/c:when&gt;
        &lt;c:when test='${param.p == "1"}'&gt;
            Generate this template text if p equals 1
        &lt;/c:when&gt;
        &lt;c:otherwise&gt;
           Generate this template text if p equals anything else
        &lt;/c:otherwise&gt;
    &lt;/c:choose&gt;
</pre>
The <code>&lt;c:out&gt;</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>
    &lt;%-- Default value in an attribute --%&gt;
    &lt;c:out value='${param.p}' default="Generate this if p is null" /&gt;
    
    &lt;%-- Default value in the body content --%&gt;
    &lt;c:out value='${param.p}'&gt;
        Generate this if p is null
    &lt;/c:out&gt;
</pre>
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            &nbsp;Related Examples
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e1063. <a class="eglink" href="usejstl.html?l=rel">
    Using the Java Standard Tag Library (JSTL) in a JSP Page
</a>
<br>

e1064. <a class="eglink" href="uselang.html?l=rel">
    Enabling the JSTL Expression Language in a JSP Page
</a>
<br>

e1065. <a class="eglink" href="getparam.html?l=rel">
    Getting a Request Parameter Using JSTL in a JSP Page
</a>
<br>

e1066. <a class="eglink" href="attr.html?l=rel">
    Saving Data Using JSTL in a JSP Page
</a>
<br>

e1067. <a class="eglink" href="htmlfrag.html?l=rel">
    Saving and Emitting HTML Fragments Using JSTL in a JSP Page
</a>
<br>


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