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> ## Types of Tags

> Simple tags are invoked using XML syntax. They have a start tag and an end tag, and possibly a body:

>     <tt:tag>
>       body
>     </tt:tag>

> A custom tag with no body is expressed as follows:

>     <tt:tag /> or <tt:tag></tt:tag>

> ### Tags with Attributes

> A simple tag can have attributes. Attributes customize the behavior of a custom tag just as parameters customize the behavior of a method. There are three types of attributes:

> - Simple attributes
> - Fragment attributes
> - Dynamic attributes

> #### Simple Attributes

> Simple attributes are evaluated by the container before being passed to the tag handler. Simple attributes are listed in the start tag and have the syntax `attr="value"`. You can set a simple attribute value from a `String` constant, or an expression language (EL) expression, or by using a `jsp:attribute` element (see jsp:attribute Element). The conversion process between the constants and expressions and attribute types follows the rules described for JavaBeans component properties in Setting JavaBeans Component Properties (page 134).

> The Duke's Bookstore page `bookcatalog.jsp` calls the `catalog` tag, which has two attributes. The first attribute, a reference to a book database object, is set by an EL expression. The second attribute, which sets the color of the rows in a table that represents the bookstore catalog, is set with a `String` constant.

>     <sc:catalog bookDB ="${bookDB}" color="#cccccc">

> #### Fragment Attributes

> A JSP fragment is a portion of JSP code passed to a tag handler that can be invoked as many times as needed. You can think of a fragment as a template that is used by a tag handler to produce customized content. Thus, unlike a simple attribute which is evaluated by the container, a fragment attribute is evaluated by a tag handler during tag invocation.

> To declare a fragment attribute, you use the `fragment` attribute of the `attribute` directive (see Declaring Tag Attributes in Tag Files) or use the `fragment` subelement of the `attribute` TLD element (see Declaring Tag Attributes for Tag Handlers). You define the value of a fragment attribute by using a `jsp:attribute` element. When used to specify a fragment attribute, the body of the `jsp:attribute` element can contain only static text and standard and custom tags; it cannot contain scripting elements (see Chapter 8).

> JSP fragments can be parametrized via expression language (EL) variables in the JSP code that composes the fragment. The EL variables are set by the tag handler, thus allowing the handler to customize the fragment each time it is invoked (see Declaring Tag Variables in Tag Files, and Declaring Tag Variables for Tag Handlers).

> The `catalog` tag discussed earlier accepts two fragments: `normalPrice`, which is displayed for a product that's full price, and `onSale`, which is displayed for a product that's on sale.

>     <sc:catalog bookDB ="${bookDB}" color="#cccccc">
>       <jsp:attribute name="normalPrice">
>         <fmt:formatNumber value="${price}" type="currency"/>
>       </jsp:attribute>
>       <jsp:attribute name="onSale">
>         <strike><fmt:formatNumber value="${price}"
>           type="currency"/></strike><br/>
>         <font color="red"><fmt:formatNumber value="${salePrice}"
>           type="currency"/></font>
>       </jsp:attribute>
>     </sc:catalog>

> The tag executes the `normalPrice` fragment, using the values for the `price` EL variable, if the product is full price. If the product is on sale, the tag executes the `onSale` fragment using the `price` and `salePrice` variables.

> #### Dynamic Attributes

> A dynamic attribute is an attribute that is not specified in the definition of the tag. Dynamic attributes are used primarily by tags whose attributes are treated in a uniform manner but whose names are not necessarily known at development time.

> For example, this tag accepts an arbitrary number of attributes whose values are colors and outputs a bulleted list of the attributes colored according to the values:

>     <colored:colored color1="red" color2="yellow" color3="blue"/>

> You can also set the value of dynamic attributes using an EL expression or using the `jsp:attribute` element.

> #### Deferred Value

> A deferred value attribute is one that accepts deferred value expressions, which are described in Value Expressions (page 112).

> #### Deferred Method

> A deferred method attribute is one that accepts deferred method expressions, which are described in Method Expressions (page 117).

> #### Dynamic Attribute or Deferred Expression

> This kind of attribute can accept a String literal, a scriptlet expression, or an EL expression, including deferred expressions.

> #### jsp:attribute Element

> The `jsp:attribute` element allows you to define the value of a tag attribute in the body of an XML element instead of in the value of an XML attribute.

> For example, the Duke's Bookstore template page `screendefinitions.jsp` uses `jsp:attribute` to use the output of `fmt:message` to set the value of the `value` attribute of `tt:parameter`:

>     ...
>     <tt:screen id="/bookcatalog">
>       <tt:parameter name="title" direct="true">
>         <jsp:attribute name="value" >
>           <fmt:message key="TitleBookCatalog"/>
>         </jsp:attribute>
>       </tt:parameter>
>       <tt:parameter name="banner" value="/template/banner.jsp"
>         direct="false"/>
>       <tt:parameter name="body" value="/bookcatalog.jsp"
>         direct="false"/>
>     </tt:screen>
>     ...

> `jsp:attribute` accepts a `name` attribute and a `trim` attribute. The `name` attribute identifies which tag attribute is being specified. The optional `trim` attribute determines whether or not whitespace appearing at the beginning and end of the element body should be discarded. By default, the leading and trailing whitespace is discarded. The whitespace is trimmed when the JSP page is translated. If a body contains a custom tag that produces leading or trailing whitespace, that whitespace is preserved regardless of the value of the `trim` attribute.

> An empty body is equivalent to specifying "" as the value of the attribute.

> The body of `jsp:attribute` is restricted according to the type of attribute being specified:

> - For simple attributes that accept an EL expression, the body can be any JSP content.
> - For simple attributes that do not accept an EL expression, the body can contain only static text.
> - For fragment attributes, the body must not contain any scripting elements (see Chapter 8).

> ### Tags with Bodies

> A simple tag can contain custom and core tags, HTML text, and tag-dependent body content between the start tag and the end tag.

> In the following example, the Duke's Bookstore application page `bookshowcart.jsp` uses the JSTL `c:if` tag to print the body if the request contains a parameter named `Clear`:

>     <c:if test="${param.Clear}">
>       <font color="#ff0000" size="+2"><strong>
>       You just cleared your shopping cart!
>       </strong><br>&nbsp;<br></font>
>     </c:if>

> #### jsp:body Element

> You can also explicitly specify the body of a simple tag by using the `jsp:body` element. If one or more attributes are specified with the `jsp:attribute` element, then `jsp:body` is the only way to specify the body of the tag. If one or more `jsp:attribute` elements appear in the body of a tag invocation but you don't include a `jsp:body` element, the tag has an empty body.

> ### Tags That Define Variables

> A simple tag can define an EL variable that can be used within the calling page. In the following example, the `iterator` tag sets the value of the EL variable `departmentName` as it iterates through a collection of department names.

>     <tlt:iterator var="departmentName" type="java.lang.String"
>         group="${myorg.departmentNames}">
>       <tr>
>         <td><a href="list.jsp?deptName=${departmentName}">
>           ${departmentName}</a></td>
>       </tr>
>     </tlt:iterator>

> ### Communication between Tags

> Custom tags communicate with each other through shared objects. There are two types of shared objects: public and private.

> In the following example, the `c:set` tag creates a public EL variable called `aVariable`, which is then reused by `anotherTag`.

>     <c:set var="aVariable" value="aValue" />
>     <tt:anotherTag attr1="${aVariable}" />

> Nested tags can share private objects. In the next example, an object created by `outerTag` is available to `innerTag`. The inner tag retrieves its parent tag and then retrieves an object from the parent. Because the object is not named, the potential for naming conflicts is reduced.

>     <tt:outerTag>
>       <tt:innerTag />
>     </tt:outerTag>

> The Duke's Bookstore page `template.jsp` uses a set of cooperating tags that share public and private objects to define the screens of the application. These tags are described in A Template Tag Library.