This section describes how a page author specifies that a JSP page is using a tag library and introduces the different types of tags.
Declaring Tag Libraries
You declare that a JSP page will use tags defined in a tag library by including a
taglibdirective in the page before any custom tag is used:<%@ taglib uri="/WEB-INF/tutorial-template.tld" prefix="tt" %>
uriattribute refers to a URI that uniquely identifies the tag library descriptor (TLD), described in the section Tag Library Descriptors. This URI can be direct or indirect. The
prefixattribute defines the prefix that distinguishes tags defined by a given tag library from those provided by other tag libraries.
Tag library descriptor file names must have the extension
.tld. TLD files are stored in the
WEB-INFdirectory of the WAR or in a subdirectory of
WEB-INF. You can reference a TLD directly and indirectly.
taglibdirective directly references a TLD filename:<%@ taglib uri="/WEB-INF/tutorial-template.tld" prefix="tt" %>
taglibdirective uses a short logical name to indirectly reference the TLD:<%@ taglib uri="/tutorial-template" prefix="tt" %>
A logical name must be mapped to an absolute location in the Web application deployment descriptor. To map the logical name
/tutorial-templateto the absolute location
- Select the File Refs tab
- Click the Add button in the JSP Tag Libraries subpane.
- Enter the relative URI
/tutorial-templatein the Coded Reference field.
- Enter the absolute location
/WEB-INF/tutorial-template.tldin the Tag Library field.
Types of Tags
JSP custom tags are written using XML syntax. They have a start tag and end tag, and possibly a body:<tt:tag> body </tt:tag>
A custom tag with no body is expressed as follows:<tt:tag />
A simple tag contains no body and no attributes:<tt:simple />
Tags With Attributes
A custom tag can have attributes. Attributes are listed in the start tag and have the syntax
attr="value". Attribute values serve to customize the behavior of a custom tag just as parameters are used to customize the behavior of a method.
You specify the types of a tag's attributes in a tag library descriptor (see Tag Library Descriptors).
You can set an attribute value from a
Stringconstant or a runtime expression. The conversion process between the constants and runtime expressions and attribute types follows the rules described for JavaBeans component properties in Setting JavaBeans Component Properties.
The attributes of the Struts
logic:presenttag determine whether the body of the tag is evaluated. In the following example, an attribute specifies a request parameter named
The Duke's Bookstore application page
catalog.jspuses a runtime expression to set the value of the attribute that determines the collection of books over which the Struts
logic:iteratetag iterates:<logic:iterate collection="<%=bookDB.getBooks()%>" id="book" type="database.BookDetails">
Tags with Bodies
A custom tag can contain custom and core tags, scripting elements, HTML text, and tag-dependent body content between the start and end tag.
In the following example, the Duke's Bookstore application page
showcart.jspuses the Struts
logic:presenttag to clear the shopping cart and print a message if the request contains a parameter named
Clear:<logic:present parameter="Clear"> <% cart.clear(); %> <font color="#ff0000" size="+2"><strong> You just cleared your shopping cart! </strong><br> <br></font> </logic:present>
Choosing between Passing Information as Attributes or Body
As shown in the last two sections, it is possible to pass a given piece of data as an attribute of the tag or as the tag's body. Generally speaking, any data that is a simple string or can be generated by evaluating a simple expression is best passed as an attribute.
Tags That Define Scripting Variables
A custom tag can define a variable that can be used in scripts within a page. The following example illustrates how to define and use a scripting variable that contains an object returned from a JNDI lookup. Examples of such objects include enterprise beans, transactions, databases, environment entries, and so on:<tt:lookup id="tx" type="UserTransaction" name="java:comp/UserTransaction" /> <% tx.begin(); %>
In the Duke's Bookstore application, several pages use bean-oriented tags from Struts to define scripting variables. For example,
bean:parametertag to create the
bookIdscripting variable and set it to the value of the
bookIdrequest parameter. The
jsp:setPropertystatement also sets the
bookIdproperty of the
bookDBobject to the value of the
bookIdrequest parameter. The
bean:definetag retrieves the value of the bookstore database property
bookDetailsand defines the result as the scripting variable
book:<bean:parameter id="bookId" name="bookId" /> <jsp:setProperty name="bookDB" property="bookId"/> <bean:define id="book" name="bookDB" property="bookDetails" type="database.BookDetails"/> <h2><jsp:getProperty name="book" property="title"></h2>
Customer tags can cooperate with each other through shared objects. In the following example,
tag1creates an object called
obj1, which is then reused by
tag2.<tt:tag1 attr1="obj1" value1="value" /> <tt:tag2 attr1="obj1" />
In the next example, an object created by the enclosing tag of a group of nested tags is available to all inner tags. Since the object is not named, the potential for naming conflicts is reduced. This example illustrates how a set of cooperating nested tags would appear in a JSP page.<tt:outerTag> <tt:innerTag /> </tt:outerTag>
The Duke's Bookstore page
template.jspuses a set of cooperating tags to define the screens of the application. These tags are described in the section A Template Tag Library.