java类库详细讲解

				
				
				
				Conditionally Generating Output Using JSTL in a JSP Page
				(Java Developers Almanac Example)
				
				
				
				
				
				
								    BODY CODE  {font-family: Courier, Monospace;				           font-size: 11pt}				    TABLE, BODY				          {font-family: Verdana, Arial, Helvetica, sans-serif;				           font-size: 10pt}				    PRE   {font-family: Courier, Monospace;				           font-size: 10pt}				    H3    {font-family: Verdana, Arial, Helvetica, sans-serif;				           font-size: 11pt}				    A.eglink {text-decoration: none}				    A:hover.eglink {text-decoration: underline}				    -->
				
				
				
				
				
				The Java Developers Almanac 1.4
				
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				    javax.servlet.jsp.jstl.core
				         [6 examples]
				        
				
				  
				    e1068.  
				    Conditionally Generating Output Using JSTL in a JSP Page
				
				Any non-JSP code in a JSP file is called template text and is
				automatically written to the output stream.  The JSTL tags <c:if>
				and <c:choose> make it possible to dynamically generate template
				text depending on a condition.
				The <c:if> tag generates its body if the expression in the
				test attribute evaluates to the boolean value true
				or the string value "true":
				
				
				
				
				    <%-- 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>
				
				An if/else action requires the use of the <c:choose> tag:
				
				
				    <%-- 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>
				
				Multiple conditions can be evaluated using the <c:choose> tag:
				
				
				    <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>
				
				The <c:out> can also be used to conditionally generate template text.
				If the value attribute is null, a default value (if specified) 
				is generated. The default value can be specified using the default
				attribute or can be specified in the body content. This
				example demonstrates both methods:
				
				
				
				    <%-- 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>
				
				
				
				
				             Related Examples
				        
				
				
				
				
				e1063. 
				    Using the Java Standard Tag Library (JSTL) in a JSP Page
				
				
				
				e1064. 
				    Enabling the JSTL Expression Language in a JSP Page
				
				
				
				e1065. 
				    Getting a Request Parameter Using JSTL in a JSP Page
				
				
				
				e1066. 
				    Saving Data Using JSTL in a JSP Page
				
				
				
				e1067. 
				    Saving and Emitting HTML Fragments Using JSTL in a JSP Page
				
				
				
				
				
				
				
				
				© 2002 Addison-Wesley.