The tags are designed to display dynamic data. To create a input field that displays the property "postalCode", we'd pass the String "postalCode" to the textfield tag.
<s:textfield name="postalCode"/>
If there is a "postalCode" property on the value stack, its value will be set to the input field. When the field is submitted back to the framework, the value of the control will be set back to the "postalCode" property.
Sometimes, we want to pass the dynamic data to a tag. For example, we might want to display a label with the input field, and we might want to obtain the label from the application's messages resources. Accordingly, the framework will parse expressions found in the tag attributes, so that we can merge dynamic data into the tag attributes at runtime. The expression escape sequence is "%{ ... }". Any text embedded in the escape sequence is evalulated as an expression.
<s:textfield label="%{getText("postalCode.label")}" name="postalCode"/>
The expression language (OGNL) lets us call methods and evaluate properties. The method getText is provided by ActionSupport, which is the base class for most Actions. Since the Action is on the stack, we can call any of its methods from an expression, including getText.
Non-String Attributes
The HTTP protocol is text-based, but some tags have non-String attribute types, like bool or int. To make using non-String attributes intuitative, the framework evaulates all non-String attributes as an expression. In this case, you do not need to use the escape notation. (But, if you do anyway , the framework will just strip it off.)
<s:select label="%{getText("state.label")}" name="state" multiple="true"/>
Since the attribute multiple maps to a boolean property, the framework does not interpret the value as a String. The value is evaluated as an expression and automtically converted to a boolean.
Since it's easy to forget which attributes are String and which are non-String, you can still use the escape notation.
<s:select label="%{getText("state.label")}" name="state" multiple="%{true}"/>
<s:select label="%{getText("state.label")}" name="state" multiple="allowMultiple"/>
<s:select label="%{getText("state.label")}" name="state" multiple="%{allowMultiple}"/>
value is an Object!
Most often, the value attribute is set automatically, since name attribute usually tells the framework which property to call to set the value. But, if there is a reason to set the value directory, be advised that value is an Object NOT a String.
Since value is not a String, whatever is passed to value is evaluated as an expression - NOT a String literal.
<s:textfield label="%{getText("state.label")}" name="state" value="CA"/>
If a textfield is passed the value attribute "CA", the framework will look for a property named getCa. Generally, this is not what we mean. What we mean to do is pass a literal String. In the expression language, literals are placed within quotes
<s:textfield label="%{getText("state.label")}" name="state" value="%{'CA'}" />
Another approach would be to use the idiom value="'CA'", but, in this case, using the expression notation is recommended.
Boiled down, the tag attributes are evaluated using three rules.
- All String attribute types are parsed for the "%{ ... }" notation.
- All non-String attribute types are not parsed, but evaluated directly as an expression
- The exception to rule #2 is that if the non-String attribute uses the escape notion "{%{}", the notation is ignored as redundant, and the content evaluated.