Store session variable in Action Class:

Map session = ActionContext.getContext().getSession();
session.put("variableName", "variableValue");

Store session variable in View / JSP:

<s:set name="variableName" scope="session" value="'variableValue'"/>

Retrieve session variable in Action Class:

(String) ActionContext.getContext().getSession().get("variableName");

<s:property value="%{#session.variableName}"/>

<s:property value="#session['variableName']"/>

Destroy session variable:

Map session = ActionContext.getContext().getSession();
session.remove("userName"); 

ActionContext.getContext().getSession().clear();