Synchronization Examples

The following examples illustrate how variable synchronization works between a tag file and its calling page. All the example JSP pages and tag files reference the JSTL core tag library with the prefix c. The JSP pages reference a tag file located in /WEB-INF/tags with the prefix my.

AT_BEGIN Scope

In this example, the AT_BEGIN scope is used to pass the value of the variable named x to the tag’s body and at the end of the tag invocation.

<%-- callingpage.jsp --%>
<c:set var="x" value="1"/>
${x} <%-- (x == 1) --%>
<my:example>
    ${x} <%-- (x == 2) --%>
</my:example>
${x} <%-- (x == 4) --%>
<%-- example.tag --%>
<%@ variable name-given="x" scope="AT_BEGIN" %>
${x} <%-- (x == null) --%>
<c:set var="x" value="2"/>
<jsp:doBody/>
${x} <%-- (x == 2) --%>
<c:set var="x" value="4"/>

NESTED Scope

In this example, the NESTED scope is used to make a variable named x available only to the tag’s body. The tag sets the variable to 2, and this value is passed to the calling page before the body is invoked. Because the scope is NESTED and because the calling page also had a variable named x, its original value, 1, is restored when the tag completes.

<%-- callingpage.jsp --%>
<c:set var="x" value="1"/>
${x} <%-- (x == 1) --%>
<my:example>
    ${x} <%-- (x == 2) --%>
</my:example>
${x} <%-- (x == 1) --%>
<%-- example.tag --%>
<%@ variable name-given="x" scope="NESTED" %>
${x} <%-- (x == null) --%>
<c:set var="x" value="2"/>
<jsp:doBody/>
${x} <%-- (x == 2) --%>
<c:set var="x" value="4"/>

AT_END Scope

In this example, the AT_END scope is used to return a value to the page. The body of the tag is not affected.

<%-- callingpage.jsp --%>
<c:set var="x" value="1"/>
${x} <%-- (x == 1) --%>
<my:example>
    ${x} <%-- (x == 1) --%>
</my:example>
${x} <%-- (x == 4) --%>
<%-- example.tag --%>
<%@ variable name-given="x" scope="AT_END" %>
${x} <%-- (x == null) --%>
<c:set var="x" value="2"/>
<jsp:doBody/>
${x} <%-- (x == 2) --%>
<c:set var="x" value="4"/>

AT_BEGIN and name-from-attribute

In this example the AT_BEGIN scope is used to pass an EL variable to the tag’s body and make to it available to the calling page at the end of the tag invocation. The name of the variable is specified by the value of the attribute var. The variable is referenced by a local name, result, in the tag file.

<%-- callingpage.jsp --%>
<c:set var="x" value="1"/>
${x} <%-- (x == 1) --%>
<my:example var="x">
    ${x} <%-- (x == 2) --%>
    ${result} <%-- (result == null) --%>
    <c:set var="result" value="invisible"/>
</my:example>
${x} <%-- (x == 4) --%>
${result} <%-- (result == ”invisible’) --%>
<%-- example.tag --%>
<%@ attribute name="var" required="true" rtexprvalue="false"%>
<%@ variable alias="result" name-from-attribute="var"
    scope="AT_BEGIN" %>
${x} <%-- (x == null) --%>
${result} <%-- (result == null) --%>
<c:set var="x" value="ignored"/>
<c:set var="result" value="2"/>
<jsp:doBody/>
${x} <%-- (x == ”ignored’) --%>
${result} <%-- (result == 2) --%>
<c:set var="result" value="4"/>