Apache Jakarta and Beyond: A Java Programmeramp;#039;s Introduction [Electronic resources] نسخه متنی

18.6. Iteration in a Page

It was mentioned previously that the expression language can handle compound values, meaning that an expression like

${aBean.values[1]}

would be legal if aBean has a property called values and that values is an array or List with at least two elements.

It is unusual to need to access a particular element in an array, but it is more common to have to repeat some action for every element, regardless of how many there are. There is a tag in the standard library that handles such iteration, called c:forEach. Listing 18.12 uses the c:forEach tag to show some information from a serialized bean holding information about an album, including the set of tracks.

Listing 18.12. The forEach tag

<%@ taglib prefix="c"
uri="http://java.sun.com/jstl/core" %>
<jsp:useBean
id="album"
beanName="tinderbox"
type="com.awl.jspbook.chapter19.AlbumInfo"/>
<h1><c:out value="${album.name}"/></h1>
Artist: <jsp:getProperty name="album"
property="artist"/><p>
Year: <c:out value="${album.year}"/></p>
Here are the tracks:
<ul>
<c:forEach items="${album.tracks}" var="track">
<li><c:out value="${track}"/>
</c:forEach>
</ul>

This example also illustrates a new feature of the jsp:useBean tag: the ability to load a serialized bean. This is accomplished by specifying a beanName, which will cause jsp:useBean to load a file with that name ending in .ser from the CLASSPATH. The result of Listing 18.12 is shown in Figure 18.2.

The c:forEach tag takes a number of parameters. The first is the items to iterate over, which is specified by a script. The second is a name to use as a variable; within the body of the c:forEach, this variable will be set to each element in the array in turn. This variable can be accessed by the expression language just as a bean, which means among other things that the c:out tag can be used to display it.