Custom Tags in JSP Pages

Overview

Stephanie Bodoff

The standard JSP tags for invoking operations on JavaBeans components and performing request dispatching simplify JSP page development and maintenance. JSP technology also provides a mechanism for encapsulating other types of dynamic functionality in custom tags, which are extensions to the JSP language. Custom tags are usually distributed in the form of a tag library, which defines a set of related custom tags and contains the objects that implement the tags.

Some examples of tasks that can be performed by custom tags include operations on implicit objects, processing forms, accessing databases and other enterprise services such as e-mail and directories, and performing flow control. JSP tag libraries are created by developers who are proficient at the Java programming language and expert in accessing data and other services, and are used by webapp designers who can focus on presentation issues rather than being concerned with how to access enterprise services. As well as encouraging division of labor between library developers and library users, custom tags increase productivity by encapsulating recurring tasks so that they can be reused across more than one application.

Tag libraries are receiving a great deal of attention in the JSP technology community. For more information about tag libraries and for pointers to some freely-available libraries, see 

http://java.sun.com/products/jsp/taglibraries.html

 

What Is a Custom Tag?

A custom tag is a user-defined JSP language element. When a JSP page containing a custom tag is translated into a servlet, the tag is converted to operations on an object called a tag handler. The Web container then invokes those operations when the JSP page's servlet is executed.

Custom tags have a rich set of features. They can

 

The Example JSP Pages

This chapter describes the tasks involved in using and defining tags. The chapter illustrates the tasks with excerpts from the JSP version of the Duke's Bookstore application discussed in The Example JSP Pages rewritten to take advantage of two tag libraries: Struts and tutorial-template. The third section in the chapter, Examples, describes two tags in detail: the iterate tag from Struts and the set of tags in the tutorial-template tag library.

The Struts tag library provides a framework for building internationalized webapps that implement the Model-View-Controller design pattern. Struts includes a comprehensive set of utility custom tags for handling:

The Duke's Bookstore application uses tags from the Struts bean and logic sublibraries.

The tutorial-template tag library defines a set of tags for creating an application template. The template is a JSP page with placeholders for the parts that need to change with each screen. Each of these placeholders is referred to as a parameter of the template. For example, a simple template could include a title parameter for the top of the generated screen and a body parameter to refer to a JSP page for the custom content of the screen. The template is created with a set of nested tags--definition, screen, and parameter--that are used to build a table of screen definitions for Duke's Bookstore and with an insert tag to insert parameters from the table into the screen.

Figure 15-1 shows the flow of a request through the following Duke's Bookstore Web components:

Figure 15-1 Request Flow Through Duke's Bookstore Components

The source code for the Duke's Bookstore application is located in the docs/tutorial/examples/web/bookstore3 directory created when you unzip the tutorial bundle (see Running the Examples). To build, deploy, and run the example:

  1. Download Struts version 1.0 from 
    http://jakarta.apache.org/builds/jakarta-struts/
release/v1.0/
  2. Unpack Struts and copy struts-bean.tld, struts-logic.tld, and struts.jar from jakarta-struts-1.0/lib to $JWSDP_HOME/docs/tutorial/examples/web/bookstore3.
  3. In a terminal window, go to $JWSDP_HOME/docs/tutorial/examples/bookstore3.
  4. Run ant build. The build target will spawn any necessary compilations and copy files to the $JWSDP_HOME/docs/tutorial/examples/web/bookstore3/build directory.
  5. Make sure Tomcat is started.
  6. Run ant install. The install target notifies Tomcat that the new context is available.
  7. Start the PointBase database server and populate the database if you have not done so already (see Accessing Databases from Web Applications).
  8. Open the bookstore URL http://localhost:8080/bookstore3/enter.

See Common Problems and Their Solutions and Troubleshooting for help with diagnosing common problems.

 

Using Tags

This section describes how a JSP page uses tags and introduces the different types of tags.

To use a tag, a page author must do two things:

 

Declaring Tag Libraries

You declare that a JSP page will use tags defined in a tag library by including a taglib directive in the page before any custom tag is used: 

<%@ taglib uri="/WEB-INF/tutorial-template.tld" prefix="tt" %>

The uri attribute refers to a URI that uniquely identifies the tag library descriptor (TLD), described in Tag Library Descriptors. This URI can be direct or indirect. The prefix attribute 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-INF directory of the WAR or in a subdirectory of WEB-INF. You can reference a TLD directly and indirectly.

The following taglib directive directly references a TLD filename: 

<%@ taglib uri="/WEB-INF/tutorial-template.tld" prefix="tt" %>

This taglib directive uses a short logical name to indirectly reference the TLD: 

<%@ taglib uri="/tutorial-template" prefix="tt" %>

You map a logical name to an absolute location in the webapp deployment descriptor. To map the logical name /tutorial-template to the absolute location /WEB-INF/tutorial-template.tld, you add a taglib element to web.xml: 

<taglib>
  <taglib-uri>/tutorial-template</taglib-uri>
  <taglib-location>
    /WEB-INF/tutorial-template.tld
  </taglib-location>
</taglib>

 

Making the Tag Library Implementation Available

A tag library implementation can be made available to a webapp in two basic ways. The classes implementing the tag handlers can be stored in an unpacked form in the WEB-INF/classes subdirectory of the webapp. Alternatively, if the library is distributed as a JAR, it is stored the WEB-INF/lib directory of the webapp. A tag library shared between more than one application is stored in the $JWSDP_HOME/common/lib directory of the Java WSDP.

 

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 />

Simple Tags

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 Tags with Attributes).

You can set an attribute value from a String constant 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:present tag determine whether the body of the tag is evaluated. In the following example, an attribute specifies a request parameter named Clear: 

<logic:present parameter="Clear">

The Duke's Bookstore application page catalog.jsp uses a runtime expression to set the value of the attribute that determines the collection of books over which the Struts logic:iterate tag 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.jsp uses the Struts logic:present tag 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>&nbsp;<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, bookdetails.jsp uses the bean:parameter tag to create the bookId scripting variable and set it to the value of the bookId request parameter. The jsp:setProperty statement also sets the bookId property of the bookDB object to the value of the bookId request parameter. The bean:define tag retrieves the value of the bookstore database property bookDetails and 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>

Cooperating Tags

Custom tags can cooperate with each other through shared objects.

In the following example, tag1 creates 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.jsp uses a set of cooperating tags to define the screens of the application. These tags are described in A Template Tag Library.

 

Defining Tags

To define a tag, you need to:

This section describes the properties of tag handlers and TLDs and explains how to develop tag handlers and library descriptor elements for each type of tag introduced in the previous section.

 

Tag Handlers

A tag handler is an object invoked by a Web container to evaluate a custom tag during the execution of the JSP page that references the tag. Tag handlers must implement either the Tag or BodyTag interface. Interfaces can be used to take an existing Java object and make it a tag handler. For newly created handlers, you can use the TagSupport and BodyTagSupport classes as base classes. These classes and interfaces are contained in the javax.servlet.jsp.tagext package.

Tag handler methods defined by the Tag and BodyTag interfaces are called by the JSP page's servlet at various points during the evaluation of the tag. When the start tag of a custom tag is encountered, the JSP page's servlet calls methods to initialize the appropriate handler and then invokes the handler's doStartTag method. When the end tag of a custom tag is encountered, the handler's doEndTag method is invoked. Additional methods are invoked in between when a tag handler needs to interact with the body of the tag. For further information, see Tags with Bodies. In order to provide a tag handler implementation, implement the methods, summarized in Table 15-1, that are invoked at various stages of processing the tag.

Tag Handler Type
 Methods
Simple
 doStartTag, doEndTag, release
Attributes
 doStartTag, doEndTag, set/getAttribute1...N, release
Body, Evaluation and No Interaction
 doStartTag, doEndTag, release
Body, Iterative Evaluation
 doStartTag, doAfterBody, doEndTag, release
Body, Interaction
 doStartTag, doEndTag, release, doInitBody, doAfterBody, release

A tag handler has access to an API that allows it to communicate with the JSP page. The entry point to the API is the page context object (javax.servlet.jsp.PageContext), through which a tag handler can retrieve all the other implicit objects (request, session, and application) accessible from a JSP page.

Implicit objects can have named attributes associated with them. Such attributes are accessed using [set|get]Attribute methods.

If the tag is nested, a tag handler also has access to the handler (called the parent) associated with the enclosing tag.

A set of related tag handler classes (a tag library) is usually packaged and deployed as a JAR archive.

 

Tag Library Descriptors

A tag library descriptor (TLD) is an XML document that describes a tag library. A TLD contains information about a library as a whole and about each tag contained in the library. TLDs are used by a Web container to validate the tags and by JSP page development tools.

TLD file names must have the extension .tld. TLD files are stored in the WEB-INF directory of the WAR file or in a subdirectory of WEB-INF.

A TLD must begin with an XML document prolog that specifies the version of XML and the document type definition (DTD): 

<?xml version="1.0" encoding="ISO-8859-1" ?>
<!DOCTYPE taglib PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag 
Library 1.2//EN"
"http://java.sun.com/dtd/web-jsptaglibrary_1_2.dtd">

Tomcat supports version 1.1 and 1.2 DTDs. However, this chapter documents the 1.2 version because you should use the newer version in any tag libraries that you develop. The template library TLD, tutorial-template.tld, conforms to the 1.2 version. The Struts library TLDs conform to the 1.1 version of the DTD, which has fewer elements and uses slightly different names for some of the elements.

The root of a TLD is the taglib element. The subelements of taglib are listed in Table 15-2:

Element Description
tlib-version The tag library's version
jsp-version The JSP specification version that the tag library requires
short-name Optional name that could be used by a JSP page authoring tool to create names with a mnemonic value
uri A URI that uniquely identifies the tag library
display-name Optional name intended to be displayed by tools
small-icon Optional small-icon that can be used by tools
large-icon Optional large-icon that can be used by tools
description Optional tag-specific information
listener See listener Element
tag See tag Element

listener Element

A tag library can specify some classes that are event listeners (see Handling Servlet Life Cycle Events). The listeners are listed in the TLD as listener elements, and the Web container will instantiate the listener classes and register them in a way analogous to listeners defined at the WAR level. Unlike WAR-level listeners, the order in which the tag library listeners are registered is undefined. The only subelement of the listener element is the listener-class element, which must contain the fully qualified name of the listener class.

tag Element

Each tag in the library is described by giving its name and the class of its tag handler, information on the scripting variables created by the tag, and information on the tag's attributes. Scripting variable information can be given directly in the TLD or through a tag extra info class (see Tags That Define Scripting Variables). Each attribute declaration contains an indication of whether the attribute is required, whether its value can be determined by request-time expressions, and the type of the attribute (see Attribute Element).

A tag is specified in a TLD in a tag element. The subelements of tag are listed in Table 15-3:

Element Description
name The unique tag name
tag-class The fully-qualified name of the tag handler class
tei-class Optional subclass of javax.servlet.jsp.tagext.TagExtraInfo. See Providing Information about the Scripting Variable.
body-content The body content type. See body-content Element and body-content Element.
display-name Optional name intended to be displayed by tools
small-icon Optional small-icon that can be used by tools
large-icon Optional large-icon that can be used by tools
description Optional tag-specific information
variable Optional scripting variable information. See Providing Information about the Scripting Variable.
attribute Tag attribute information. See Attribute Element.

The following sections describe the methods and TLD elements that you need to develop for each type of tag introduced in Types of Tags.

 

Simple Tags

Tag Handlers

The handler for a simple tag must implement the doStartTag and doEndTag methods of the Tag interface. The doStartTag method is invoked when the start tag is encountered. This method returns SKIP_BODY because a simple tag has no body. The doEndTag method is invoked when the end tag is encountered. The doEndTag method needs to return EVAL_PAGE if the rest of the page needs to be evaluated; otherwise, it should return SKIP_PAGE.

The simple tag discussed in the first section, 

<tt:simple />

would be implemented by the following tag handler: 

public SimpleTag extends TagSupport {
  public int doStartTag() throws JspException {
    try {
      pageContext.getOut().print("Hello.");
    } catch  Exception(ex) {
      throw new JspTagException("SimpleTag: " + 
        ex.getMessage());
    }
    return SKIP_BODY;
  }
  public int doEndTag() {
    return EVAL_PAGE;
  }
}

body-content Element

Tags without bodies must declare that their body content is empty using the body-content element: 

<body-content>empty</body-content>

 

Tags with Attributes

Defining Attributes in a Tag Handler

For each tag attribute, define a property and get and set methods that conform to the JavaBeans architecture conventions in the tag handler. For example, the tag handler for the Struts logic:present tag, 

<logic:present parameter="Clear">

contains the following declaration and methods: 

protected String parameter = null;
public String getParameter() {
  return (this.parameter);
}
public void setParameter String(parameter) {
  this.parameter = parameter;
}

Note that if your attribute is named id and your tag handler inherits from the TagSupport class, you do not need to define the property and set and get methods because these are already defined by TagSupport.

A tag attribute whose value is a String can name an attribute of one of the implicit objects available to tag handlers. An implicit object attribute would be accessed by passing the tag attribute value to the [set|get]Attribute method of the implicit object. This is a good way to pass scripting variable names to a tag handler where they are associated with objects stored in the page context (See Implicit Objects).

Attribute Element

For each tag attribute, specify whether the attribute is required, whether the value can be determined by an expression, and, optionally, the type of the attribute in an attribute element. For static values the type is always java.lang.String. If the rtexprvalue element is true or yes, then the type element defines the return type expected from any expression specified as the value of the attribute. 

<attribute>
  <name>attr1</name>
  <required>true|false|yes|no</required>
  <rtexprvalue>true|false|yes|no</rtexprvalue>
  <type>fully_qualified_type</type>
</attribute>

If a tag attribute is not required, a tag handler should provide a default value.

The tag element for the logic:present tag declares that the parameter attribute is not required (because the tag can also test for the presence of other entities such as bean properties) and that its value can be set by a runtime expression. 

<tag>
  <name>present</name>
  <tag-class>org.apache.struts.taglib.
    logic.PresentTag</tag-class>
  <body-content>JSP</body-content>
  ...
  <attribute>
    <name>parameter</name>
    <required>false</required>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
  ...
</tag>

Attribute Validation

The documentation for a tag library should describe valid values for tag attributes. When a JSP page is translated, a Web container will enforce any constraints contained in the TLD element for each attribute.

The attributes passed to a tag can also be validated at translation time with the isValid method of a class derived from TagExtraInfo. This class is also used to provide information about scripting variables defined by the tag (see Providing Information about the Scripting Variable).

The isValid method is passed the attribute information in a TagData object, which contains attribute-value tuples for each of the tag's attributes. Since the validation occurs at translation time, the value of an attribute that is computed at request time will be set to TagData.REQUEST_TIME_VALUE.

The tag <tt:twa attr1="value1"/> has the following TLD attribute element: 

<attribute>
  <name>attr1</name>
  <required>true</required>
  <rtexprvalue>true</rtexprvalue>
</attribute>

This declaration indicates that the value of attr1 can be determined at runtime.

The following isValid method checks that the value of attr1 is a valid Boolean value. Note that since the value of attr1 can be computed at runtime, isValid must check whether the tag user has chosen to provide a runtime value. 

public class TwaTEI extends TagExtraInfo {
  public boolean isValid(Tagdata data) {
    Object o = data.getAttribute("attr1");
    if (o != null && o != TagData.REQUEST_TIME_VALUE) {
      if (((String)o).toLowerCase().equals("true") || 
        ((String)o).toLowerCase().equals("false") ) 
        return true;
      else
        return false;
    }
    else
      return true;
  }
}

 

Tags with Bodies

Tag Handlers

A tag handler for a tag with a body is implemented differently depending on whether the tag handler needs to interact with the body or not. By interact, we mean that the tag handler reads or modifies the contents of the body.

Tag Handler Does Not Interact with the Body

If the tag handler does not need to interact with the body, the tag handler should implement the Tag interface (or be derived from TagSupport). If the body of the tag needs to be evaluated, the doStartTag method needs to return EVAL_BODY_INCLUDE; otherwise it should return SKIP_BODY.

If a tag handler needs to iteratively evaluate the body, it should implement the IterationTag interface or be derived from TagSupport. It should return EVAL_BODY_AGAIN from the doStartTag and doAfterBody methods if it determines that the body needs to be evaluated again.

Tag Handler Interacts with the Body

If the tag handler needs to interact with the body, the tag handler must implement BodyTag (or be derived from BodyTagSupport). Such handlers typically implement the doInitBody and the doAfterBody methods. These methods interact with body content passed to the tag handler by the JSP page's servlet.

Body content supports several methods to read and write its contents. A tag handler can use the body content's getString or getReader methods to extract information from the body, and the writeOut(out) method to write the body contents to an out stream. The writer supplied to the writeOut method is obtained using the tag handler's getPreviousOut method. This method is used to ensure that a tag handler's results are available to an enclosing tag handler.

If the body of the tag needs to be evaluated, the doStartTag method needs to return EVAL_BODY_BUFFERED; otherwise, it should return SKIP_BODY.

doInitBody Method

The doInitBody method is called after the body content is set but before it is evaluated. You generally use this method to perform any initialization that depends on the body content.

doAfterBody Method

The doAfterBody method is called after the body content is evaluated.

Like the doStartTag method, doAfterBody must return an indication of whether to continue evaluating the body. Thus, if the body should be evaluated again, as would be the case if you were implementing an iteration tag, doAfterBody should return EVAL_BODY_BUFFERED; otherwise, doAfterBody should return SKIP_BODY.

release Method

A tag handler should reset its state and release any private resources in the release method.

The following example reads the content of the body (which contains a SQL query) and passes it to an object that executes the query. Since the body does not need to be reevaluated, doAfterBody returns SKIP_BODY. 

public class QueryTag extends BodyTagSupport {
  public int doAfterBody() throws JspTagException {
    BodyContent bc = getBodyContent();
    // get the bc as string
    String query = bc.getString();
    // clean up
    bc.clearBody();
    try {
      Statement stmt = connection.createStatement();
      result = stmt.executeQuery(query);
    } catch  SQLException(e) {
      throw new JspTagException("QueryTag: " +
         e.getMessage());
    }
    return SKIP_BODY;
  }
}

body-content Element

For tags that have a body, specify the type of the body content using the body-content element: 

<body-content>JSP|tagdependent</body-content>

Body content containing custom and core tags, scripting elements, and HTML text is categorized as JSP. This is the value declared for the Struts logic:present tag. All other types of body content--for example--SQL statements passed to the query tag, would be labeled tagdependent.

Note that the value of the body-content element does not affect the interpretation of the body by the tag handler; the element is only intended to be used by an authoring tool for rendering the body content.

 

Tags That Define Scripting Variables

Tag Handlers

A tag handler is responsible for creating and setting the object referred to by the scripting variable into a context accessible from the page. It does this by using the pageContext.setAttribute(name, value, scope) or pageContext.setAttribute(name, value) methods. Typically an attribute passed to the custom tag specifies the name of the scripting variable object; this name can be retrieved by invoking the attribute's get method described in Using Scope Objects.

If the value of the scripting variable is dependent on an object present in the tag handler's context, it can retrieve the object using the pageContext.getAttribute(name, scope) method.

The usual procedure is that the tag handler retrieves a scripting variable, performs some processing on the object, and then sets the scripting variable's value using the pageContext.setAttribute(name, object) method.

The scope that an object can have is summarized in Table 15-4. The scope constrains the accessibility and lifetime of the object.

Name
 Accessible From
 Lifetime
page
 Current page
 Until the response has been sent back to the user or the request is passed to a new page
request
 Current page and any included or forwarded pages
 Until the response has been sent back to the user
session
 Current request and any subsequent request from the same browser (subject to session lifetime)
 The life of the user's session
application
 Current and any future request from the same webapp
 The life of the application

Providing Information about the Scripting Variable

The example described in Tags That Define Scripting Variables defines a scripting variable book that is used for accessing book information: 

<bean:define id="book" name="bookDB" property="bookDetails"
  type="database.BookDetails"/>
<font color="red" size="+2">
  <%=messages.getString("CartRemoved")%>
  <strong><jsp:getProperty name="book"
      property="title"/></strong> 
<br>&nbsp;<br>
</font>

When the JSP page containing this tag is translated, the Web container generates code to synchronize the scripting variable with the object referenced by the variable. To generate the code, the Web container requires certain information about the scripting variable:

There are two ways to provide this information: by specifying the variable TLD subelement or by defining a tag extra info class and including the tei-class element in the TLD. Using the variable element is simpler, but slightly less flexible.

variable Element

The variable element has the following subelements:

One of name-given or name-from-attribute is required. The following subelements are optional:

The implementation of the Struts bean:define tag conforms to the JSP specification version 1.1, which requires you to define a tag extra info class. The JSP specification version 1.2 adds the variable element. You could define the following variable element for the bean:define tag: 

<tag>
  <variable>
    <name-from-attribute>id</name-from-attribute>
    <variable-class>database.BookDetails</variable-class>
    <declare>true</declare>
    <scope>AT_BEGIN</scope>
  </variable>
</tag>
TagExtraInfo Class

You define a tag extra info class by extending the class javax.servlet.jsp.TagExtraInfo. A TagExtraInfo must implement the getVariableInfo method to return an array of VariableInfo objects containing the following information:

The Web container passes a parameter called data to the getVariableInfo method that contains attribute-value tuples for each of the tag's attributes. These attributes can be used to provide the VariableInfo object with a scripting variable's name and class.

The Struts tag library provides information about the scripting variable created by the bean:define tag in the DefineTei tag extra info class. Since the name (book) and class (database.BookDetails) of the scripting variable are passed in as tag attributes, they can be retrieved with the data.getAttributeString method and used to fill in the VariableInfo constructor. To allow the scripting variable book to be used in the rest of the page, the scope of book is set to be AT_BEGIN. 

public class DefineTei extends TagExtraInfo {
  public VariableInfo[] getVariableInfo(TagData data) {
  String type = data.getAttributeString("type");
    if (type == null)
      type = "java.lang.Object";
    return new VariableInfo[] {
      new VariableInfo(data.getAttributeString("id"),
        type,
        true,
        VariableInfo.AT_BEGIN)
    };
  }
}

The fully qualified name of the tag extra info class defined for a scripting variable must be declared in the TLD in the tei-class subelement of the tag element. Thus, the tei-class element for DefineTei would be as follows: 

<tei-class>
  org.apache.struts.taglib.bean.DefineTagTei
</tei-class>

 

Cooperating Tags

Tags cooperate by sharing objects. JSP technology supports two styles of object sharing.

The first style requires that a shared object be named and stored in the page context (one of the implicit objects accessible to both JSP pages and tag handlers). To access objects created and named by another tag, a tag handler uses the pageContext.getAttribute(name, scope) method.

In the second style of object sharing, an object created by the enclosing tag handler of a group of nested tags is available to all inner tag handlers. This form of object sharing has the advantage that it uses a private namespace for the objects, thus reducing the potential for naming conflicts.

To access an object created by an enclosing tag, a tag handler must first obtain its enclosing tag with the static method TagSupport.findAncestorWithClass(from, class) or the TagSupport.getParent method. The former method should be used when a specific nesting of tag handlers cannot be guaranteed. Once the ancestor has been retrieved, a tag handler can access any statically or dynamically created objects. Statically created objects are members of the parent. Private objects can also be created dynamically. Such objects can be stored in a tag handler with the setValue method and retrieved with the getValue method.

The following example illustrates a tag handler that supports both the named and private object approaches to sharing objects. In the example, the handler for a query tag checks whether an attribute named connection has been set in the doStartTag method. If the connection attribute has been set, the handler retrieves the connection object from the page context. Otherwise, the tag handler first retrieves the tag handler for the enclosing tag, and then retrieves the connection object from that handler. 

public class QueryTag extends BodyTagSupport {
  private String connectionId;
  public int doStartTag() throws JspException {
    String cid = getConnection();
    if (cid != null) {
    // there is a connection id, use it
      connection =(Connection)pageContext.
        getAttribute(cid);
    } else {
      ConnectionTag ancestorTag =
        (ConnectionTag)findAncestorWithClass(this,
          ConnectionTag.class);
      if (ancestorTag == null) {
        throw new JspTagException("A query without
          a connection attribute must be nested
          within a connection tag.");
      }
      connection = ancestorTag.getConnection();
    }
  }
}

The query tag implemented by this tag handler could be used in either of the following ways: 

<tt:connection id="con01" ....> ... </tt:connection>
<tt:query id="balances" connection="con01"> 
  SELECT account, balance FROM acct_table 
    where customer_number = <%= request.getCustno()%> 
</tt:query>

<tt:connection ...>
  <x:query id="balances"> 
    SELECT account, balance FROM acct_table 
      where customer_number = <%= request.getCustno()%> 
  </x:query>
</tt:connection>

The TLD for the tag handler must indicate that the connection attribute is optional with the following declaration: 

<tag>
  ...
  <attribute>
    <name>connection</name>
    <required>false</required>
  </attribute>
</tag>

 

Examples

The custom tags described in this section demonstrate solutions to two recurring problems in developing JSP applications: minimizing the amount of Java programming in JSP pages and ensuring a common look and feel across applications. In doing so, they illustrate many of the styles of tags discussed in the first part of the chapter.

 

An Iteration Tag

Constructing page content that is dependent on dynamically generated data often requires the use of flow control scripting statements. By moving the flow control logic to tag handlers, flow control tags reduce the amount of scripting needed in JSP pages.

The Struts logic:iterate tag retrieves objects from a collection stored in a JavaBeans component and assigns them to a scripting variable. The body of the tag retrieves information from the scripting variable. While elements remain in the collection, the iterate tag causes the body to be reevaluated.

JSP Page

Two Duke's Bookstore application pages, catalog.jsp and showcart.jsp, use the logic:iterate tag to iterate over collections of objects. An excerpt from catalog.jsp is shown below. The JSP page initializes the iterate tag with a collection named(by the property attribute) of the bookDB bean. The iterate tag sets the book scripting variable on each iteration over the collection. The bookId property of the book variable is exposed as another scripting variable. Properties of both variables are used to dynamically generate a table containing links to other pages and book catalog information. 

<logic:iterate name="bookDB" property="books" 
  id="book" type="database.BookDetails">
  <bean:define id="bookId" name="book" property="bookId"
    type="java.lang.String"/>

  <tr> 
  <td bgcolor="#ffffaa"> 
  <a href="<%=request.getContextPath()%>
    /bookdetails?bookId=<%=bookId%>">
    <strong><jsp:getProperty name="book"
    property="title"/>&nbsp;</strong></a></td> 

  <td bgcolor="#ffffaa" rowspan=2> 
  <jsp:setProperty name="currency" property="amount"
    value="<%=book.getPrice()%>"/>
  <jsp:getProperty name="currency" property="format"/>
  &nbsp;</td> 

  <td bgcolor="#ffffaa" rowspan=2> 
  <a href="<%=request.getContextPath()%>
    /catalog?Add=<%=bookId%>">
    &nbsp;<%=messages.getString("CartAdd")%>
    &nbsp;</a></td></tr> 

  <tr> 
  <td bgcolor="#ffffff"> 
  &nbsp;&nbsp;<%=messages.getString("By")%> <em>
    <jsp:getProperty name="book"
      property="firstName"/>&nbsp;
    <jsp:getProperty name="book"
      property="surname"/></em></td></tr>
</logic:iterate>

Tag Handler

The implementation of the Struts logic:iterate tag conforms to the capabilities of the JSP version 1.1 specification, which requires you to extend the BodyTagSupport class. The JSP version 1.2 specification adds features (described in Tag Handler Does Not Interact with the Body) that simplify programming tags that iteratively evaluate their body. The following discussion is based on an implementation that uses these features.

The logic:iterate tag supports initializing the collection in several ways: from a collection provided as a tag attribute or from a collection that is a bean or a property of a bean. Our example uses the latter method. Most of the code in doStartTag is concerned with constructing an iterator over the collection object. The method first checks if the handler's collection property is set and, if not, proceeds to checking the bean and property attributes. If the name and property attributes are both set, doStartTag calls a utility method that uses JavaBeans introspection methods to retrieve the collection. Once the collection object is determined, the method constructs the iterator.

If the iterator contains more elements, doStartTag sets the value of the scripting variable to the next element and then indicates that the body should be evaluated; otherwise it ends the iteration by returning SKIP_BODY.

After the body has been evaluated, the doAfterBody method retrieves the body content and writes it to the out stream. The body content object is then cleared in preparation for another body evaluation. If the iterator contains more elements, doAfterBody again sets the value of the scripting variable to the next element and returns EVAL_BODY_AGAIN to indicate that the body should be evaluated again. This causes the reexecution of doAfterBody. When there are no remaining elements, doAfterBody terminates the process by returning SKIP_BODY. 

public class IterateTag extends TagSupport {
  protected Iterator iterator = null;
  protected Object collection = null;
  protected String id = null;
  protected String name = null;
  protected String property = null;
  protected String type = null;
  public int doStartTag() throws JspException {
    Object collection = this.collection;
    if (collection == null) {
      try {
        Object bean = pageContext.findAttribute(name);
        if (bean == null) {
          ... throw an exception
        }
        if (property == null)
          collection = bean;
        else
          collection =
            PropertyUtils.
              getProperty(bean, property);
        if (collection == null) {
          ... throw an exception
        }
      } catch 
        ... catch exceptions thrown 
          by PropertyUtils.getProperty
      }
    }
    // Construct an iterator for this collection
    if (collection instanceof Collection)
      iterator = ((Collection) collection).iterator();
    else if (collection instanceof Iterator)
      iterator = (Iterator) collection;
      ...
    }
    // Store the first value and evaluate, 
    // or skip the body if none
    if (iterator.hasNext()) {
      Object element = iterator.next();
      pageContext.setAttribute(id, element);
      return (EVAL_BODY_AGAIN);
    } else
      return (SKIP_BODY);
  }
  public int doAfterBody() throws JspException {
    if (bodyContent != null) {
      try {
        JspWriter out = getPreviousOut();
        out.print(bodyContent.getString());
        bodyContent.clearBody();
      } catch  IOException(e) {
        ...
      }
    }
    if (iterator.hasNext()) {
      Object element = iterator.next();
      pageContext.setAttribute(id, element);
      return (EVAL_BODY_AGAIN);
    } else
      return (SKIP_BODY);
    }
  }
}

Tag Extra Info Class

Information about the scripting variable is provided in the IterateTei tag extra info class. The name and class of the scripting variable are passed in as tag attributes and used to fill in the VariableInfo constructor. 

public class IterateTei extends TagExtraInfo {
  public VariableInfo[] getVariableInfo(TagData data) {
  String type = data.getAttributeString("type");
  if (type == null)
    type = "java.lang.Object";

  return new VariableInfo[] {
    new VariableInfo(data.getAttributeString("id"),
      type,
      true,
      VariableInfo.AT_BEGIN)
    };
  }
}

 

A Template Tag Library

A template provides a way to separate the common elements that are part of each screen from the elements that change with each screen of an application. Putting all the common elements together into one file makes it easier to maintain and enforce a consistent look and feel in all the screens. It also makes development of individual screens easier because the designer can focus on portions of a screen that are specific to that screen while the template takes care of the common portions.

The template is a JSP page with placeholders for the parts that need to change with each screen. Each of these placeholders is referred to as a parameter of the template. For example, a simple template could include a title parameter for the top of the generated screen and a body parameter to refer to a JSP page for the custom content of the screen.

The template uses a set of nested tags--definition, screen, and parameter--to define a table of screen definitions and uses an insert tag to insert parameters from a screen definition into a specific application screen.

JSP Page

The template for the Duke's Bookstore example, template.jsp, is shown below. This page includes a JSP page that creates the screen definition and then uses the insert tag to insert parameters from the definition into the application screen. 

<%@ taglib uri="/tutorial-template.tld" prefix="tt" %>
<%@ page errorPage="errorpage.jsp" %>
<%@ include file="screendefinitions.jsp" %><html>
  <head>
    <title>
      <tt:insert definition="bookstore"
        parameter="title"/>
    </title>
  </head>
    <tt:insert definition="bookstore"
      parameter="banner"/>
    <tt:insert definition="bookstore" 
      parameter="body"/>
  </body>
</html>

screendefinitions.jsp creates a screen definition based on a request attribute selectedScreen: 

<tt:definition name="bookstore" 
  screen="<%= (String)request.
    getAttribute(\"selectedScreen\") %>">
  <tt:screen id="/enter">
    <tt:parameter name="title" 
      value="Duke's Bookstore" direct="true"/>
    <tt:parameter name="banner" 
      value="/banner.jsp" direct="false"/>
    <tt:parameter name="body" 
      value="/bookstore.jsp" direct="false"/>
  </tt:screen>
  <tt:screen id="/catalog">
    <tt:parameter name="title" 
    value="<%=messages.getString("TitleBookCatalog")%>"
    direct="true"/>
    ...
</tt:definition>

The template is instantiated by the Dispatcher servlet. Dispatcher first gets the requested screen and stores it as an attribute of the request. This is necessary because when the request is forwarded to template.jsp, the request URL doesn't contain the original request (for example, /bookstore3/catalog) but instead reflects the path (/bookstore3/template.jsp) of the forwarded page. Finally, the servlet dispatches the request to template.jsp: 

public class Dispatcher extends HttpServlet {
  public void doGet(HttpServletRequest request, 
      HttpServletResponse response) {
    request.setAttribute("selectedScreen",
      request.getServletPath());
    RequestDispatcher dispatcher =
      request.getRequestDispatcher("/template.jsp");
    if (dispatcher != null)
      dispatcher.forward(request, response);
  }
  public void doPost(HttpServletRequest request, 
        HttpServletResponse response) {
    request.setAttribute("selectedScreen",
      request.getServletPath());
    RequestDispatcher dispatcher =
      request.getRequestDispatcher("/template.jsp");
    if (dispatcher != null)
      dispatcher.forward(request, response);
  }
}

Tag Handlers

The template tag library contains four tag handlers--DefinitionTag, ScreenTag, ParameterTag, and InsertTag--that demonstrate the use of cooperating tags. DefinitionTag, ScreenTag, and ParameterTag comprise a set of nested tag handlers that share public and private objects. DefinitionTag creates a public object named definition that is used by InsertTag.

In doStartTag, DefinitionTag creates a public object named screens that contains a hash table of screen definitions. A screen definition consists of a screen identifier and a set of parameters associated with the screen. 

public int doStartTag() {
  HashMap screens = null;
  screens = (HashMap) pageContext.getAttribute("screens", 
    pageContext.APPLICATION_SCOPE);
  if (screens == null)
    pageContext.setAttribute("screens", new HashMap(), 
      pageContext.APPLICATION_SCOPE);
  return EVAL_BODY_INCLUDE;
}

The table of screen definitions is filled in by ScreenTag and ParameterTag from text provided as attributes to these tags. Table 15-6 shows the contents of the screen definitions hash table for the Duke's Bookstore application

Screen Id
 Title
 Banner
 Body
/enter
 Duke's Bookstore
 /banner.jsp
 /bookstore.jsp
/catalog
 Book Catalog
 /banner.jsp
 /catalog.jsp
/bookdetails
 Book Description
 /banner.jsp
 /bookdetails.jsp
/showcart
 Shopping Cart
 /banner.jsp
 /showcart.jsp
/cashier
 Cashier
 /banner.jsp
 /cashier.jsp
/receipt
 Receipt
 /banner.jsp
 /receipt.jsp
.

In doEndTag, DefinitionTag creates a public object of class Definition, selects a screen definition from the screens object based on the URL passed in the request, and uses it to initialize the Definition object. 

public int doEndTag()throws JspTagException {
  try {
    Definition definition = new Definition();
    HashMap screens = null;
    ArrayList params = null;
    TagSupport screen = null;
    screens = (HashMap) 
      pageContext.getAttribute("screens",
        pageContext.APPLICATION_SCOPE);
    if (screens != null)
      params = (ArrayList) screens.get(screenId);
    else
      ...
    if (params == null)
      ...
    Iterator ir = null;
    if (params != null)
      ir = params.iterator();
    while ((ir != null) && ir.hasNext())
      definition.setParam((Parameter) ir.next());
      // put the definition in the page context
    pageContext.setAttribute(
      definitionName, definition);
  } catch  Exception(ex) {
    ex.printStackTrace();
  }
  return EVAL_PAGE;
}

If the URL passed in the request is /enter, the Definition contains the items from the first row of Table 15-6:

Title
 Banner
 Body
Duke's Bookstore
 /banner.jsp
 /bookstore.jsp

The definition for the URL /enter is shown in Table 15-7. The definition specifies that the value of the Title parameter, Duke's Bookstore, should be inserted directly into the output stream, but the values of Banner and Body should be dynamically included.

Parameter Name
 Parameter Value
 isDirect
title
 Duke's Bookstore
 true
banner
 /banner.jsp
 false
body
 /bookstore.jsp
 false

InsertTag uses Definition to insert parameters of the screen definition into the response. In the doStartTag method, it retrieves the definition object from the page context. 

public int doStartTag() {
  // get the definition from the page context
  definition = (Definition) pageContext.
    getAttribute(definitionName);
  // get the parameter
  if (parameterName != null && definition != null)
    parameter = (Parameter)definition.
      getParam(parameterName);
  if (parameter != null)
    directInclude = parameter.isDirect();
  return SKIP_BODY;
}

The doEndTag method inserts the parameter value. If the parameter is direct, it is directly inserted into the response; otherwise, the request is sent to the parameter, and the response is dynamically included into the overall response. 

public int doEndTag()throws JspTagException {
  try {
    if (directInclude && parameter != null)
      pageContext.getOut().print(parameter.getValue());
    else {
      if ((parameter != null) && 
        (parameter.getValue() != null))
        pageContext.include(parameter.getValue());
    }
  } catch  Exception(ex) {
    throw new JspTagException(ex.getMessage());
  }
  return EVAL_PAGE;
}

How Is a Tag Handler Invoked?

The Tag interface defines the basic protocol between a tag handler and a JSP page's servlet. It defines the life cycle and the methods to be invoked when the start and end tags are encountered.

The JSP page's servlet invokes the setPageContext, setParent, and attribute setting methods before calling doStartTag. The JSP page's servlet also guarantees that release will be invoked on the tag handler before the end of the page.

Here is a typical tag handler method invocation sequence: 

ATag t = new ATag();
t.setPageContext(...);
t.setParent(...);
t.setAttribute1(value1);
t.setAttribute2(value2);
t.doStartTag();
t.doEndTag();
t.release();

The BodyTag interface extends Tag by defining additional methods that let a tag handler access its body. The interface provides three new methods:

A typical invocation sequence is: 

t.doStartTag();
out = pageContext.pushBody();
t.setBodyContent(out);
// perform any initialization needed after body content is set
t.doInitBody();
t.doAfterBody();
// while doAfterBody returns EVAL_BODY_BUFFERED we 
// iterate body evaluation
...
t.doAfterBody();
t.doEndTag();
t.pageContext.popBody();
t.release();

 

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