Understanding OGNL expressions in Struts2

Question

The following code displays links using <s:a> of Struts starting from 1 to 10.

<s:set var="currentPage" value="2"/>
<s:set var="begin" value="1"/>
<s:set var="end" value="10"/>

<s:iterator begin="%{begin}" end="%{end}" step="1" var="row" status="loop">
    <s:if test="%{#currentPage eq #row}">    <!--???-->
        <span><s:property value="%{#row}"/></span>
    </s:if>

    <s:else>
        <s:url id="pageURL" action="someAction" escapeAmp="false">
            <s:param name="currentPage" value="%{row}"/>
        </s:url>

        <s:a href="%{pageURL}" name="btnPage" cssClass="paging">
            <s:property value="%{#row}"/>
        </s:a>
    </s:else>
</s:iterator>

When currentPage (which is 2) matches the conditional expression test="%{#currentPage eq #row}", it just displays text using <s:property> inside <span> instead of showing a link. That's fine.

---

When I use these same tags but using appropriate properties in its corresponding action class like so,

<s:iterator begin="%{begin}" end="%{end}" step="1" var="row" status="loop">
    <s:if test="%{currentPage eq #row}">   <!--???-->
        <span class="current"><s:property value="%{#row}"/></span>
    </s:if>

    <s:else>
        <s:url id="pageURL" action="someAction" escapeAmp="false">
            <s:param name="currentPage" value="%{row}"/>
        </s:url>

        <s:a href="%{pageURL}" name="btnPage" cssClass="paging">
            <s:property value="%{#row}"/>
        </s:a>
    </s:else>
</s:iterator>

In this case, currentPage (and all other) is a property of type Long in the action class. Here, the conditional test regarding the previous case which is test="%{#currentPage eq #row}" is evaluated to false.

It requires the omission of # before currentPage. Hence, the expression becomes test="%{currentPage eq #row}" (otherwise, it always evaluates to false).

I don't understand why does the first case require test="%{#currentPage eq #row}" and the second case require test="%{currentPage eq #row}"? Is there anything I might be missing?

Answer 1

When you <s:set> a value it's not on the value stack, but instead in the "value stack context".

Using the bare currentPage reference only searches the actual stack, not the context.

Using #currentPage doesn't check the stack itself, but instead references the stack context.

See these other answers as well:

1. what's the difference between #{} ${} and %{}?
2. Struts 2 "%" sign and '#" sign in OGNL