The framework provides a tag library decoupled from the view
technology. In this section, we describe each tag in general terms,
such as the attributes it supports, what the behaviors are, and so
forth. Most tags are supported in all template languages (see JSP Tags, Velocity Tags, and FreeMarker Tags),
but some are currently only specific to one language. Whenever a tag
doesn't have complete support for every language, it is noted on the
tag's reference page.
The types of tags can be broken in to two types: generic and UI.
Besides function and responsibility, the biggest difference between the
two is that the HTML tags support templates and themes.
In addition to the general tag reference, we also provide examples for
using these generic tags in each of the support languages.
Tag类型能被分成两种：Generic和UI。除了功能和指责，他们之间的最大不同在预HTML标签支持模板和主题。除了通用的标签参考之外，我们提供了例子。
Be sure to read the Tag Syntax document to learn how tag attribute syntax works.
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.
标签被设计用来展示动态数据。
<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.
如果value栈里有一个postalCode属性，他的值将被放到input字段，当字段被提交回框架，值将被设置到postalCode属性（注：有点服务端和客户端绑定的味道）
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.
有时，我们象传递动态数据到tag。例如，我们象显示Label和input在一起，我们从系统的资源文件获取Lable，框架将解析表达式，所以我们能把动态数据整合到标签属性上。表达式样子是“%{}”.在大括弧里的表达式将被计算。
<s:textfield key="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 key="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 key="state.label" name="state" multiple="%{true}"/>
<s:select key="state.label" name="state" multiple="allowMultiple"/>
<s:select key="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 directly, 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 key="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 key="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.
Expression Language Notations
<p>Username: ${user.username}</p>
|
A JavaBean object in a standard context in Freemarker, Velocity, or JSTL EL (Not OGNL). |
<s:textfield name="username"/>
|
A username property on the Value Stack. |
<s:url id="es" action="Hello">
<s:param name="request_locale">
es
</s:param>
</s:url>
<s:a href="%{es}">Espanol</s:a>
|
Another way to refer to a property placed on the Value Stack. |
<s:property
name="#session.user.username" />
|
The username property of the User object in the Session context. |
<s:select
label="FooBar" name="foo"
list="#{'username':'trillian',
'username':'zaphod'}" />
|
A static Map, as in put("username","trillian"). |