Recipe 23.10 Accessing Request Parameters with the EL

Problem

You want to access a request parameter using the EL in a JSP.

Solution

Use the param implicit object in your JSP code.

Discussion

The JSTL provides an implicit object named param that you can use to get a request parameter. Simply follow the term "param" with a period and the parameter name. Use this terminology with the EL to output the value of a request parameter with the c:out tag. Example 23-11 displays a greeting with the visitor's name. The request might look like:

http://localhost:8080/home/welcome.jsp?name=Bruce%20Perry

If the URL does not include the "name" parameter, the JSP displays the message "Hello Esteemed Visitor."

Example 23-11. Using the JSTL in a JSP to display the result of a request parameter

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>

<html>
<head><title>Accessing a Scoped Value</title></head>
<body>
<h2>Hello

<c:choose>

<c:when test="${empty param.name}">
 Esteemed Visitor
 </c:when>

<c:otherwise>

<c:out value="${param.name}" />

</c:otherwise>

</c:choose>

</h2>

</body>
</html>

The code tests whether the request contains a value for name by using the empty EL keyword:

<c:when test="${empty param.name}">

The c:choose , c:when, and c:otherwise tags are like if/then/else statements in Java code. If the request parameter name does not have a value, the browser will display "Esteemed Visitor". Otherwise, it displays the value of name.

Figure 23-6 shows a JSP displaying the message, including the parameter value.

Figure 23-6. Humble output of a JSP using the param JSTL implicit object

Chapter 18 on working with the client request; the Jakarta Project's Taglibs site: http://jakarta.apache.org/taglibs/index.html; Sun Microsystem's JSTL information page: http://java.sun.com/products/jsp/jstl/; Recipe 23.3 on using the core tags; Recipe 23.4 and Recipe 23.5 on using the XML tags; Recipe 23.6 on using the formatting tags; Recipe 23.7 and Recipe 23.8 on using the SQL JSTL tags; Recipe 23.11-Recipe 23.14 on using the EL to access request headers, cookies, and JavaBean properties.