The tag syntax in WebWork is extremely easy to understand. To quickly get started, all you need to know is that all attributes are applied as Strings initially. They are then parsed for the syntax %{ ... }, and anything in between the braces is evaluated against the value stack.

Like most things in life, it turns out that this isn't quite that simple. Specifically, there are actually three rules to be aware of:

1. All String attribute types are parsed for the %{ ... } characters.
2. All non-String attribute types are not parsed, but instead evaluated directly as an OGNL expression
3. The exception to rule #2 is that if the non-String attribute starts with %{ and ends with }, those characters are cut off before evaluating the expression.

The best way to understand these rules is by looking at some examples.

Some Examples

The most basic example explaining how the tag syntax works is as follows. This example shows off rule #1 only:

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.

<saf: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.

<saf

<ww:textfield label="%{getText("statepostalCode.label")}" name="statepostalCode"/>

In this example, the label is dynamically evaluated and set to the outcome of the OGNL expression getText("state.label"), which will in turn invoke the Internationalization system are retrieve the value of the i18n key state.label. The name, being a String attribute, is simply set to the string state.

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.)The next example shows off rule #2:

<ww<saf:select label="%{getText("state.label")}" name="state" multiple="true"/>

While this looks very similar to the last example, the key thing to recognize is that the multiple attribute is of type Boolean, which means it falls under rule #2. Generally you won't even notice this, because true as an OGNL expression evaluated to true, which is what you want.

...

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.

<saf:select label="%{getText("state.label")}" name="state" multiple="%{true}"/>

<ww:select label="%{getText("state.label")}" name="state" multiple="allowMultiple"/>

<ww:select label="%{getText("state.label")}" name="state" multiple="%{allowMultiple}"/>

Because the attribute is of type Boolean and starts and ends with the correct characters from rule #3, it is reduced to the expression allowMultiple, which is evaluated against the value stack, returning a true of false value, just like in the previous example.

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. There is one trick example to keep an eye on, however. For example, the following is probably incorrect:

<ww:textfield label="%{getText("state.label")}" name="state" value="CA"/>

This example will only work if the expression CA can result in something, meaning that your action has a method getCA(), which is probably not what you expected. This is because the value attribute is of type Object and therefore rule #2 applies. If the desire is to set a static String as the initial value, you would need to supply an OGNL expression that returns a String. For example, this is the correct way to do it:If a textfield is passed the value attribute "CA", the framework will look for a property naemd {{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

<ww:textfield label="%{getText("state.label")}" name="state" value="%{'CA'}" />

While you could set the value attribute as just Another approach would be to use the idiom value="'CA'", we recommend the parsed expressions so that, in the future when WebWork supports parsed attributes for all types, your code will still work, but, in this case, using the expression notation is recommended.

Boiled down, the tag attributes are evaluated using three rules.

1. All String attribute types are parsed for the "%{ ... }" notation.
2. All non-String attribute types are not parsed, but evaluated directly as an expression
3. 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.