EL Expressions

Overview

An EL expressions is an expression enclosed with ${ and }, i.e., the syntax is ${expr}. For example,

 <element attr1="${bean.property}".../>
 ${map[entry]}
 <another-element>${3+counter} is ${empty map}</another-element>

When an EL expression is used as an attribute value, it could return any kind of objects as long as the attribute allows. For example, the following expressions will be evaluated to boolean and int, respectively.

 <window if="${some > 10}"><!-- boolean -->
   <progressmetter value="${progress}"/><!-- integer -->

If the class does not match, ZK Loader will try to coerce it to the correct one. If failed, an exception is thrown.

Multiple EL expressions could be specified in a single attribute:

<window title="${foo.name}: ${foo.version}">

Example

EL Expression	Result
${1 > (4/2)}	false
${100.0 == 100}	true
${'a' < 'b'}	true
${'hip' gt 'hit'}	false
${1.2E4 + 1.4}	12001.4
${3 div 4}	0.75
${10 div 4}	2
${empty param.add}	true if the request parameter named add is null or an empty string
${param['mycom.productId']}	The value of the request parameter named mycom.productId

• The example is from JSP Tutorial.

Difference from Java

• A string can be enclosed with single quote or double double. In other words, 'abc' and "abc" are the same.
• The empty operator is useful for testing null and empty (string, list and map), such as ${empty param.add}.
• The . operator can be used to access a property of an object (assuming there is a get method of the same name), or a value of a map, such as ${foo.value.name}.
• The [] operator can be used to access an item of a list or array, a value of a map, and a property of an object (assuming there is a get method of the same name), such as ${ary[5]} and ${wnd['title']}.
• null is returned if the value is not found and the index is out-of-bound.

For more information please refer to Operators and Literals.

Version History

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