Question

I’m working with Struts 2 and when I’m accessing ValueStack variables I don’t know whether to use % or # or $. I try all of them until I find the correct one.

Can anybody explain what is the difference between them?

Answer 1

Use of # (pound sign)

OGNL is used to refer to objects in the ActionContext as follows:

• objectName: object in the ValueStack (default/root object in the OGNL context), such as an Action property
• #objectName: object in the ActionContext but outside of the ValueStack, specifically…
  • #objectName: ActionContext object that has been created using the Struts2 data tags with the default action scope (e.g., <s:set name="foo" value="'Testing'" />, referenced by <s:property value="#foo" />)
  • #parameters.objectName: request parameter
  • #request.objectName: request-scoped attribute
  • #session.objectName: session-scoped attribute
  • #application.objectName: application-scoped attribute
  • #attr.objectName: attribute in page, request, session, or application scope (searched in that order)

The scoped map references above (parameters, request, session, and application) can be made one of two ways:

• #scopeName.objectName or
• #scopeName['objectName']

Use of % (percent sign)

%{ OGNL expression } is used to force OGNL evaluation of an attribute that would normally be interpreted as a String literal.

Example: <s:property value="myProperty" default="%{myDynamicDefaultValue}" />

Use of @ (at sign)

The @ symbol is used to make references to static properties and methods. Note that you may need to enable this in your Struts2 properties: struts.ognl.allowStaticMethodAccess=true

Examples:

@my.package.ClassName@MY_STATIC_PROPERTY
@my.package.ClassName@myStaticMethod

Use of $ (dollar sign)

Struts2 OGNL does not make special use of the dollar sign. However, it can be used to evaluate normal JSTL expressions. For example:

Struts2: <h1><s:property value="#pageTitle" /></h1>
(is equivalent to…)
JSTL: <h1>${pageTitle}</h1>