Home TOC |
![]() ![]() ![]() |
Core Tags
The core tags 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.
Expression Tags
The
out
tag evaluates an expression and outputs the result of the evaluation to the currentJspWriter
object. It is the equivalent of the JSP syntax<%=
expression
%>
. For example,showcart.jsp
displays the number of items in a shopping cart as follows:<c:out value="${session.cart.numberOfItems}"/>The
set
tag sets the value of an attribute in any of the JSP scopes (page, request, session, application). If the attribute does not already exist, it is created.The JSP scoped attribute can be set either from attribute value:
<c:set id="foo" scope="session" value="..."/><c:set id="foo"> ... </c:set>To remove a scoped attribute, you use the
remove
tag.The JSTL expression language reduces the need for scripting. However, page authors will still have to deal with situations where some attributes of non-JSTL tags must be specified as expressions in the page's scripting language. The standard JSP element
jsp:useBean
is used to declare a scripting variable that can be used in a scripting language expression or scriptlet. For example,showcart.jsp
removes a book from a shopping cart using a scriptlet. The ID of the book to be removed is passed as a request parameter. The value of the request parameter is first set as a page attribute (to be used later by the JSTLsql:query
tag) and then declared as scripting variable and passed to thecart.remove
method:<c:set var="bookId" value="${param.Remove}"/> <jsp:useBean id="bookId" type="java.lang.String" /> <% cart.remove(bookId); %> <sql:query var="books" dataSource="${application.bookDS}"> select * from PUBLIC.books where id = ? <sql:param value="${bookId}" /> </sql:query>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 scoped variable identified by
var
, which always has page scope. If no exception occurred, the scoped variable identified byvar
is removed if it existed. If var is missing, the exception is simply caught and not saved.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.getQuantity()%> </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 fromcatalog.jsp
tests whether the request parameterAdd
is not null. 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="${param.Add != null}"> <c:set var="bid" value="${param.Add}"/> <c:declare id="bid" type="java.lang.String" /> <sql:query var="books" dataSource="${application.bookDS}"> select * from PUBLIC.books where id = ? <sql:param value="${bid}" /> </sql:query> <c:forEach var="bookRow" begin="0" items="${books.rows}"> <jsp:useBean id="bookRow" type="java.util.Map" /> <jsp:useBean id="addedBook" class="database.BookDetails" scope="page" /> ... <% cart.add(bid, addedBook); %> ... </c:if>The
choose
tag performs conditional block execution embedded by thewhen
sub tags. It renders the body of the firstwhen
tag whose test condition evaluates to true. If none of the test conditions of nestedwhen
tags evaluate totrue
, then the body of anotherwise
tag is evaluated, if present.Iterator Tags
The
forEach
tag allows you to iterate over a collection of objects.Here's the iteration from the previous section using the
forEach
tag:<c:forEach var="item" items="${session.cart.items}"> ... <tr> <td align="right" bgcolor="#ffffff"> <c:out value="${item.quantity}"/> </td> ... </c:forEach>URL Tags
The
jsp:include
element provides for the inclusion of static and dynamic resources in the same context as the current page. However,jsp:include
cannot access resources that reside outside of the Web application and causes unnecessary buffering when the resource included is fed into another elementIn the example below, the
transform
element uses the content of the included resource as the input of its transformation. Thejsp:include
element reads the content of the response, writes it to the body content of the enclosing transform element, which then re-reads the exact same content. It would be more efficient if thetransform
element could access the input source directly and avoid the buffering involved in the body content of the transform tag.<acme:transform> <jsp:include page="/exec/employeesList"/> <acme:transform/>The
import
tag is therefore the simple, generic way to access URL based resources whose content can then be included and or processed within the JSP page. Theparam
tag, analogous to thejsp:param
tag (see jsp:param Element), can be used withimport
to specify request parameters.In Session Tracking we discussed how an application must rewrite URLs to enable session tracking whenever the client turns off cookies. You can use the
url
tag to rewrite URLs returned from a JSP page. The tag includes the session ID in the URL only if cookies are disabled; otherwise, it returns the URL unchanged. Note that this feature requires the URL to be relative. Theurl
tag takesparam
subtags for including parameters in the returned URL. For example,catalog.jsp
rewrites the URL used to add a book to the shopping cart as follows:<c:url var="url" value="${pageContext.request.contextPath}/catalog" > <c:param name="Add" value="${bookId}" /> </c:url> <p><strong><a href="<c:out value='${url}'/>">
The redirect
tag sends an HTTP redirect to the client. Theredirect
tag takesparam
subtags for including parameters in the returned URL.
Home TOC |
![]() ![]() ![]() |