Download
FAQ History |
![]() ![]() ![]() |
API
Search Feedback |
Core Tags
Table 17-2 summarizes the core tags, which include those related to expressions, flow control, and a generic way to access URL-based resources whose content can then be included or processed within the JSP page.
Variable Support Tags
The
set
tag sets the value of an EL variable or the property of an EL variable in any of the JSP scopes (page, request, session, application). If the variable does not already exist, it is created.The JSP EL variable or property can be set either from attribute value:
or from the body of the tag:
For example, the following sets a EL variable named
bookID
with the value of the request parameter namedRemove
:To remove an EL variable, you use the
remove
tag. When the bookstore JSP pagebookreceipt.jsp
is invoked, the shopping session is finished, so thecart
session attribute is removed as follows:Flow Control Tags
To execute flow control logic, a page author must generally resort to using scriptlets. For example, the following scriptlet is used to iterate through a shopping cart:
<% Iterator i = cart.getItems().iterator(); while (i.hasNext()) { ShoppingCartItem item = (ShoppingCartItem)i.next(); ... %> <tr> <td align="right" bgcolor="#ffffff"> ${item.quantity} </td> ... <% } %>Flow control tags eliminate the need for scriptlets. The next two sections have examples that demonstrate the conditional and iterator tags.
Conditional Tags
The
if
tag allows the conditional execution of its body according to value of a test attribute. The following example frombookcatalog.jsp
tests whether the request parameterAdd
is empty. If the test evaluates totrue
, the page queries the database for the book record identified by the request parameter and adds the book to the shopping cart:<c:if test="${!empty param.Add}"> <c:set var="bid" value="${param.Add}"/> <jsp:useBean id="bid" type="java.lang.String" /> <sql:query var="books" dataSource="${applicationScope.bookDS}"> select * from PUBLIC.books where id = ? <sql:param value="${bid}" /> </sql:query> <c:forEach var="bookRow" begin="0" items="${books.rows}"> <jsp:useBean id="bookRow" type="java.util.Map" /> <jsp:useBean id="addedBook" class="database.BookDetails" scope="page" /> ... <% cart.add(bid, addedBook); %> ... </c:if>The
choose
tag performs conditional block execution by the embeddedwhen
sub tags. It renders the body of the firstwhen
tag whose test condition evaluates to true. If none of the test conditions of nestedwhen
tags evaluate totrue
, then the body of anotherwise
tag is evaluated, if present.For example, the following sample code shows how to render text based on a customer's membership category.
<c:choose> <c:when test="${customer.category == 'trial'}" > ... </c:when> <c:when test="${customer.category == 'member'}" > ... </c:when> <c:when test="${customer.category == 'preferred'}" > ... </c:when> <c:otherwise> ... </c:otherwise> </c:choose>The
choose
,when
, andotherwise
tags can be used to construct anif
-then
-else
statement as follows:<c:choose> <c:when test="${count == 0}" > No records matched your selection. </c:when> <c:otherwise> ${count} records matched your selection. </c:otherwise> </c:choose>Iterator Tags
The
forEach
tag allows you to iterate over a collection of objects. You specify the collection via theitems
attribute, and the current item is available through a scope variable named by theitem
attribute.A large number of collection types are supported by
forEach
, including all implementations ofjava.util.Collection
andjava.util.Map
. If theitems
attribute is of typejava.util.Map
, then the current item will be of typejava.util.Map.Entry
, which has the following properties:Arrays of objects as well as arrays of primitive types (for example,
int
) are also supported. For arrays of primitive types, the current item for the iteration is automatically wrapped with its standard wrapper class (for example,Integer
forint
,Float
forfloat
, and so on).Implementations of
java.util.Iterator
andjava.util.Enumeration
are supported but these must be used with caution.Iterator
andEnumeration
objects are not resettable so they should not be used within more than one iteration tag. Finally,java.lang.String
objects can be iterated over if the string contains a list of comma separated values (for example: Monday,Tuesday,Wednesday,Thursday,Friday).Here's the shopping cart iteration from the previous section with the
forEach
tag:<c:forEach var="item" items="${sessionScope.cart.items}"> ... <tr> <td align="right" bgcolor="#ffffff"> ${item.quantity} </td> ... </c:forEach>The
forTokens
tag is used to iterate over a collection of tokens separated by a delimiter.URL Tags
The
jsp:include
element provides for the inclusion of static and dynamic resources in the same context as the current page. However,jsp:include
cannot access resources that reside outside of the Web application and causes unnecessary buffering when the resource included is used by another element.In the example below, the
transform
element uses the content of the included resource as the input of its transformation. Thejsp:include
element reads the content of the response, writes it to the body content of the enclosing transform element, which then re-reads the exact same content. It would be more efficient if thetransform
element could access the input source directly and avoid the buffering involved in the body content of the transform tag.The
import
tag is therefore the simple, generic way to access URL-based resources whose content can then be included and or processed within the JSP page. For example, in XML Tags,import
is used to read in the XML document containing book information and assign the content to the scoped variablexml
:The
param
tag, analogous to thejsp:param
tag (see jsp:param Element, page 682), can be used withimport
to specify request parameters.In Session Tracking (page 643) we discussed how an application must rewrite URLs to enable session tracking whenever the client turns off cookies. You can use the
url
tag to rewrite URLs returned from a JSP page. The tag includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged. Note that this feature requires the URL to be relative. Theurl
tag takesparam
subtags for including parameters in the returned URL. For example,bookcatalog.jsp
rewrites the URL used to add a book to the shopping cart as follows:<c:url var="url" value="/catalog" > <c:param name="Add" value="${bookId}" /> </c:url> <p><strong><a href="${url}">
The redirect
tag sends an HTTP redirect to the client. Theredirect
tag takesparam
subtags for including parameters in the returned URL.Miscellaneous Tags
The
catch
tag provides a complement to the JSP error page mechanism. It allows page authors to recover gracefully from error conditions that they can control. Actions that are of central importance to a page should not be encapsulated in acatch
, so their exceptions will propagate to an error page. Actions with secondary importance to the page should be wrapped in acatch
, so they never cause the error page mechanism to be invoked.The exception thrown is stored in the variable identified by
var
, which always has page scope. If no exception occurred, the scoped variable identified byvar
is removed if it existed. Ifvar
is missing, the exception is simply caught and not saved.The
out
tag evaluates an expression and outputs the result of the evaluation to the currentJspWriter
object. The syntax and attributes areIf the result of the evaluation is a
java.io.Reader
object, data is first read from theReader
object and then written into the currentJspWriter
object. The special processing associated withReader
objects improves performance when large amount of data must be read and then written to the response.If
escapeXml
is true, the character conversions listed in Table 17-3 are applied:
Table 17-3 Character Conversions Character Character Entity Code<
<
>
>
&
&
''
"
"
Download
FAQ History |
![]() ![]() ![]() |
API
Search Feedback |
All of the material in The Java(TM) Web Services Tutorial is copyright-protected and may not be published in other works without express written permission from Sun Microsystems.