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.
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| title | Creating a dynamic input fieldxml |
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<s:textfield name="postalCode"/>
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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.
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| title | Using an expression to set the labelxml |
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<s:textfield key="postalCode.label" name="postalCode"/>
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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.)
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| title | Evaluating booleansxml |
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<s:select key="state.label" name="state" multiple="true"/>
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Since it's easy to forget which attributes are String and which are non-String, you can still use the escape notation.
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| title | Evaluating booleans (verbose)xml |
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<s:select key="state.label" name="state" multiple="%{true}"/>
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| title | Evaluating booleans (with property)xml |
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<s:select key="state.label" name="state" multiple="allowMultiple"/>
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| title | Evaluating booleans (verbose with property)xml |
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<s:select key="state.label" name="state" multiple="%{allowMultiple}"/>
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Since value is not a String, whatever is passed to value is evaluated as an expression - NOT a String literal.
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| title | Probably wrong!xml |
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<s:textfield key="state.label" name="state" value="ca"/>
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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
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| title | Passing a literal value the right wayxml |
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<s:textfield key="state.label" name="state" value="%{'ca'}" />
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