Accepting EL Expressions

This section describes how EL expressions can be used in custom tag attributes. In JSP 2.1 EL expressions can be used directly. But in JSP 1.2, you need to use alternative approaches.

In the previous sections, I only talked about how to define an attribute in a custom tag, how attribute values are passed to the tag class, and how attribute values are converted to correct data type required by the tag class. Now let's look at the possibility of entering EL (Expression Langage) expressions as attribute values in a custom tag, similar to JSTL tags.

If you are using JSP 2.1, EL expressions are supported directly in custom tag attributes now. But if you are using JSP 1.2 or older versions, you need to use alternative approaches listed below.

1. Using EL expressions directly as attribute values in JSP 2.1. You need to define the attribute in the TLD file with <rtexprvalue>true</rtexprvalue>, then enter the attribute value with the following format:

<hy:tag att="${expression}"/>

2. Using Java expression elements as attribute values in JSP 1.2. You need to define the attribute in the TLD file with <rtexprvalue>true</rtexprvalue>, then enter the attribute value with the following format:

<hy:tag att="<%=java_expression%>"/>

3. Using EL expressions as strings and evaluate them inside the tag class in JSP 1.2. You can enter an EL expression in an attribute value, received it by the setter method as a string, then evaluate it to the desire data type using the expression evaluation tool offer by the Java EE.

The 1st approach is the best approach if you are running a JSP 2.1 server. An example is provided in the next section.

The 2nd approach requires the JSP page to be written in a non-XML format. And I don't like that format. So I am not going to try that.

The 3rd approach seems to be interesting and powerful. You can use it to pass an object of any class as an attribute value to the tag class. See the example later

Last update: 2012.

Table of Contents

 About This Book

 JSP (JavaServer Pages) Overview

 Tomcat 7 Installation on Windows Systems

 JSP Scripting Elements

 Java Servlet Introduction

 JSP Implicit Objects

 Syntax of JSP Pages and JSP Documents

 JSP Application Session

 Managing Cookies in JSP Pages

 JavaBean Objects and "useBean" Action Elements

 Managing HTTP Response Header Lines

 Non-ASCII Characters Support in JSP Pages

 Performance of JSP Pages

 EL (Expression Language)

 Overview of JSTL (JSP Standard Tag Libraries)

 JSTL Core Library

 JSP Custom Tags

 JSP Java Tag Interface

Custom Tag Attributes

 Tag Attributes Are Tag Object Properties

 Setter Method Example -

 Value Type Conversion

 Value Type Conversion Example -

Accepting EL Expressions

 Accepting EL Expression Example

 Passing Expression as String Example

 Using jsp:attribute Action Elements

 Multiple Tags Working Together

 File Upload Test Application

 Outdated Tutorials


 PDF Printing Version