ExecuteAndWaitInterceptor org.apache.struts2.interceptor.ExecuteAndWaitInterceptor.doIntercept(ExecuteAndWaitInterceptor.java:256)

public static final String WAIT = "wait";

org.apache.struts2.interceptor.ExecuteAndWaitInterceptor.java
.............
protected String doIntercept(ActionInvocation actionInvocation) throws Exception {
..............
//sync on the real HttpSession as the session from the context is a wrap that is created
//on every request
synchronized (httpSession)
{
BackgroundProcess bp = (BackgroundProcess) session.get(KEY + name);
if ((!executeAfterValidationPass || secondTime) && bp == null)

{ bp = getNewBackgroundProcess(name, actionInvocation, threadPriority); session.put(KEY + name, bp); performInitialDelay(bp); // first time let some time pass before showing wait page secondTime = false; }

if ((!executeAfterValidationPass || !secondTime) && bp != null && !bp.isDone()) {
actionInvocation.getStack().push(bp.getAction());
Map results = proxy.getConfig().getResults();
if (!results.containsKey(WAIT))

{ LOG.warn("ExecuteAndWait interceptor has detected that no result named 'wait' is available. " + "Defaulting to a plain built-in wait page. It is highly recommend you " + "provide an action-specific or global result named '" + WAIT + "'! This requires FreeMarker support and won't work if you don't have it installed"); // no wait result? hmm -- let's try to do dynamically put it in for you! ResultConfig rc = new ResultConfig.Builder(WAIT, "org.apache.struts2.views.freemarker.FreemarkerResult") .addParams(Collections.singletonMap("location", "/org/apache/struts2/interceptor/wait.ftl")) .build(); /************ABOVE CODE causes abend************/ results.put(WAIT, rc); //http://java.sun.com/j2se/1.4.2/docs/api/java/util/Map.html#put(java.lang.Object,%20java.lang.Object) }

if (TokenHelper.getToken() != null)

{ session.put(TokenHelper.getTokenName(), TokenHelper.getToken()); }

return WAIT;
} else if ((!executeAfterValidationPass || !secondTime) && bp != null && bp.isDone()) {
session.remove(KEY + name);
actionInvocation.getStack().push(bp.getAction());

// if an exception occured during action execution, throw it here
if (bp.getException() != null)

{ throw bp.getException(); }

return bp.getResult();
} else

{ // this is the first instance of the interceptor and there is no existing action // already run in the background, so let's just let this pass through. We assume // the action invocation will be run in the background on the subsequent pass through // this interceptor return actionInvocation.invoke(); }

}
}

where
Collections.singletonMap("location", "/org/apache/struts2/interceptor/wait.ftl"))

public static <K,V> Map<K,V> singletonMap(K key, V value)

{ 3343 return new SingletonMap<K,V>(key, value); 3344 }

//Here is applicable SingletonMap

3349 private static class SingletonMap<K,V>
3350 extends AbstractMap<K,V>
3351 implements Serializable {
3352 private static final long serialVersionUID = -6979724477215052911L;
3353
3354 private final K k;
3355 private final V v;
3356
3357 SingletonMap(K key, V value) { 3358 k = key; 3359 v = value; 3360 }

//UnsupportedOperation details
The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw UnsupportedOperationException if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an UnsupportedOperationException if the invocation would have no effect on the map. For example, invoking the #putAll(Map) method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty.

//which calls Builder
http://struts.apache.org/2.1.6/struts2-core/apidocs/com/opensymphony/xwork2/config/entities/ResultConfig.Builder.html

//which calls addparams method which takes 2 String placeholders for Map
ResultConfig.Builder addParams(Map<String,String> params)

//the code calls singletonMap which is an immutable Map

3332 /**
3333 * Returns an immutable map, mapping only the specified key to the
3334 * specified value. The returned map is serializable.
3335 *
3336 * @param key the sole key to be stored in the returned map.
3337 * @param value the value to which the returned map maps <tt>key</tt>.
3338 * @return an immutable map containing only the specified key-value
3339 * mapping.
3340 * @since 1.3
3341 */
3342 public static <K,V> Map<K,V> singletonMap(K key, V value) { 3343 return new SingletonMap<K,V>(key, value); 3344 }

so the fix would be to replace
Collections.SingletonMap
.addParams(Collections.singletonMap("location", "/org/apache/struts2/interceptor/wait.ftl"))

with 2 plain strings for addParams e.g.
.addParams("location", "/org/apache/struts2/interceptor/wait.ftl"))

?
Martin