You can use the s:set tag for this.
For example this will call getA() from your action and put the value into "avalue", don't use name instead of var
<s:set var="avalue" value="a" />
And then you can reference it like so on the JSP:
<b>Print value defined in set tag :</b> <s:property value="#avalue" /> <br/>
Which will print out the value.
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