Recipe3.3.Displaying Indexed Properties

Recipe 3.3. Displaying Indexed Properties

Problem

On a JSP page, you need to access data from an indexed property of an object.

Solution

Use bean.property[index] to access the indexed value, as shown in Example 3-1.

Example 3-1. Accessing indexed properties

<@taglib uri=http://jakarta.apache.org/struts/tags-bean" prefix="bean"%> <ul>   <li><bean:write name="foo" property="bar.baz[0]"/></li>   <li><bean:write name="foo" property="bar.baz[1]"/></li>   <li><bean:write name="foo" property="bar.baz[2]"/></li> </ul>

JSTL supports access to indexed properties, as shown in Example 3-2.

Example 3-2. Accessing indexed properties (JSTL)

<@taglib uri="http://java.sun.com/jstl/core" prefix="c"%> <ul>   <li><c:out value="${foo.bar.baz[0]}"/></li>   <li><c:out value="${foo.bar.baz[1]}"/></li>   <li><c:out value="${foo.bar.baz[1]}"/></li> </ul>

Discussion

Indexed properties are one of the most misunderstood aspects of the Struts tags. An indexed property is a JavaBean property that represents a set of values, not a single scalar value. Indexed properties are accessed using a getter method of the following form:

public Foo getSomeProperty (int index) { ... }

Likewise, indexed properties are set using a setter method of this form:

public void setFoo(int index, Foo someProperty) { ... }

Consider a JavaBean representing a calendar. The CalendarHolder class shown in Example 3-3 has a nested property representing the months in a calendar named monthSet.

Example 3-3. Calendar JavaBean

package com.oreilly.strutsckbk; public class CalendarHolder {          private MonthSet monthSet;     public CalendarHolder( ) {         monthSet = new MonthSet( );     }          public MonthSet getMonthSet( ) {         return monthSet;     } }

The MonthSet class, shown in Example 3-4, is a class that has an indexed property, month representing the month names ("January," "February," and so forth).

Example 3-4. Class with indexed property

package com.oreilly.strutsckbk; public class MonthSet {          static String[] months = new String[] {             "January", "February", "March", "April",               "May", "June", "July", "August",               "September", "October", "November", "December"     };          public String[] getMonths( ) {         return months;     }     public String getMonth(int index) {         return months[index];     }          public void setMonth(int index, String value) {         months[index] = value;     } }

The goal is to access the indexed property month of the monthSet property of the CalendarHolder instance in a JSP page as shown in the following snippet from a JSP:

<jsp:useBean  /> <ul>     <li><bean:write name="calendar" property="monthSet.month[0]"/></li>     <li><bean:write name="calendar" property="monthSet.month[1]"/></li>     <li><bean:write name="calendar" property="monthSet.month[2]"/></li>     </ul>

If the specific indexed property to display was determined dynamicallythat is, the index to use was set using a JSP scripting variableyou would need to use scriptlet to generate the property value as follows:

You have selected month number <bean:write name="monthIndex"/>:  <bean:write name="calendar"          property='<%= "monthSet.month[" + monthIndex + "]" %>'

Using the scriptlet approach makes for an extremely hard to read and even harder to maintain JSP page. If you were using JSTL, however, this becomes much cleaner:

You have selected month number <c:out value="${monthIndex}"/>:  <c:out value="${calendar.monthSet.month[monthIndex]}"/>

More commonly, indexed properties are accessed dynamically in a loop. Say you want to display the list of months using the Struts logic:iterate tag. This tag iterates over Collections and arrays. Here's how you would display all the months in an ordered list:

<ol> <logic:iterate  name="calendar" property="monthSet.months">     <li><bean:write name="monthName"/></li> </logic:iterate> </ol>

Again, JSTL can be used as an alternative. The JSTL c:forEach tag is a bit easier to use than the Struts logic:iterate tag. Here's how you would generate the same ordered list using JSTL:

<ol> <c:forEach var="monthName" items="${calendar.monthSet.months}">     <li><c:out name="${monthName}"/></li> </c:forEach> </ol>

Recipe3.3.Displaying Indexed Properties