Programming Jakarta Struts, 2nd Edition [Electronic resources] نسخه متنی

8.6 Bean Tags

The tags that are part of the Bean tag
library are used for accessing JavaBeans and their associated
properties as well as for defining new beans that are accessible to
the remainder of the page via scripting variables and page-scope
attributes. Convenient mechanisms to create new beans based on the
values of request cookies, headers, and parameters are also provided.
Table 8-9 lists the tags within the Bean tag
library.

cookie

define

header

include

message

page

parameter

resource

size

struts

write

Many of the tags in this tag library throw a JspException at runtime when they are used incorrectly (e.g., when you specify an invalid combination of tag attributes). JSP allows you to declare an "error page" in the <%@ page %> directive. If you want to process the actual exception that caused the problem, it is passed to the error page as a request attribute under key org.apache.struts.action.EXCEPTION.

8.6.1 The define Tag

This tag retrieves a
specified bean property and defines it as an attribute that is
accessible to the remainder of the current page. No type conversion
is performed on the returned property value unless it is a Java
primitive type, in which case it is wrapped in the appropriate
wrapper class (e.g., int is wrapped by
java.lang.Integer). The property value is stored
in the scope defined by the toScope variable.

8.6.2 The header Tag

This tag retrieves the value
of the specified request header and defines the result as a
page-scope attribute of type String. If no header
with the specified name can be located and no default value is
specified in the value attribute, a request-time
exception will be thrown. If the attribute
multiple is set to any non-null value, the
id attribute will contain the result of the call
to HttpServletRequest.getHeaders() rather than a
call to HttpServletRequest.getHeader().

8.6.3 The include Tag

This tag performs an internal
dispatch to the specified application component (or external URL) and
makes the response data from that request available as a bean of type
String. The value is stored into the
id attribute. This tag has a function similar to
that of the standard <jsp:include> tag,
except that the response data is stored in a page-scope attribute
instead of being written to the output stream. This allows you to
position the output as needed.

If the current request is part of a session, the generated request
for the include will also include the session ID.

The URL used to access the specified application component is
calculated based on which of the following attributes you specify
(you must specify exactly one of them):

forward

Use the value of this attribute as the name of a global
ActionForward to be looked up, and use the
application-relative or context-relative URI found there.

href

Use the value of this attribute unchanged (as this might link to a
resource external to the application, the session identifier is not
included).

page

Use the value of this attribute as an application-relative URI to the
desired resource. This value must start with a
"/".

8.6.4 The message Tag

The
message tag is one of the most widely used tags
within the Struts tag libraries. It retrieves an internationalized
message for the specified locale, using the specified message key,
and writes it to the output stream. You can supply up to five
parametric replacements (such as
"{0}").

The message key may be specified directly, using the
key attribute, or indirectly, using the
name and property attributes to
obtain it from a bean. The bundle attribute allows
you to specify the name of an application-scope bean under which a
MessageResources object can be found. If no
locale attribute is specified, the
Locale will be retrieved from the session using
the key Action.LOCALE_KEY.

The following is an example of the message tag:

<td><bean:message key="global.user.firstName"/>:</td>

8.6.5 The parameter Tag

This tag retrieves the
value of the specified request parameter and defines the result as a
page-scope attribute of type String. If any
non-null value is specified for the multiple
attribute, the result will be a String[] obtained
from calling getParameters() instead of
getParameter().

8.6.6 The resource Tag

This tag retrieves the
value of the specified web application resource and makes it
available as either an InputStream or a
String, depending on the value of the input
attribute. If the input attribute contains any
non-null value, an InputStream will be created.
Otherwise, the resource will be loaded as a
String.

This tag calls the getResourceAsStream() method using the name attribute. The name attribute must start with a /.

8.6.7 The write Tag

The write
tag is another important tag within the Bean tag library, and it gets
a great deal of use. It retrieves the value of the specified bean
property and renders it to the page output as a
String. The write tag uses the
following rules:

• If the format attribute is specified, the value
  will be formatted based on the format string and the default locale.
  The formatKey attribute can also be used. It
  should specify the name of a format string from the message resource
  bundle.

• The formatKey attribute is used to specify a
  format string from the resource bundle. The specific resource bundle
  and locale can also be specified. If the bundle and locale are not
  specified, the default resource bundle and current user locale will
  be used.

• Otherwise, the usual toString() conversions will
  be applied.

The following is an example of using the write tag:

<td>Hello <bean:write name="user" property="firstName"/></td>

If the ignore attribute is set to true and the bean specified by the name and scope attributes is not found, the tag will not throw an exception. If not set, or set to false, a runtime exception will be thrown.