public  void addPreResultListener(PreResultListener listener)
Register a PreResultListener  to be notified after the Action is executed and
before the Result is executed.

The ActionInvocation implementation must guarantee that listeners will be called in
the order in which they are registered.

Listener registration and execution does not need to be thread-safe.

 public Object getAction()
Get the Action associated with this ActionInvocation.

 public ActionContext getInvocationContext()
Gets the ActionContext associated with this ActionInvocation. The ActionProxy is
responsible for setting this ActionContext onto the ThreadLocal before invoking
the ActionInvocation and resetting the old ActionContext afterwards.

 public ActionProxy getProxy()
Get the ActionProxy holding this ActionInvocation.

 public Result getResult() throws Exception
If the ActionInvocation has been executed before and the Result is an instance of ActionChainResult , this method
will walk down the chain of ActionChainResults until it finds a non-chain result, which will be returned. If the
ActionInvocation's result has not been executed before, the Result instance will be created and populated with
the result params.

 public String getResultCode()
Gets the result code returned from this ActionInvocation.

 public ValueStack getStack()
Gets the ValueStack associated with this ActionInvocation.

 public  void init(ActionProxy proxy)

 public String invoke() throws Exception
Invokes the next step in processing this ActionInvocation.

If there are more Interceptors, this will call the next one. If Interceptors choose not to short-circuit
ActionInvocation processing and return their own return code, they will call invoke() to allow the next Interceptor
to execute. If there are no more Interceptors to be applied, the Action is executed.
If the ActionProxy#getExecuteResult()  method returns true, the Result is also executed.

 public String invokeActionOnly() throws Exception
Invokes only the Action (not Interceptors or Results).

This is useful in rare situations where advanced usage with the interceptor/action/result workflow is
being manipulated for certain functionality.

 public boolean isExecuted()
Gets whether this ActionInvocation has executed before.
This will be set after the Action and the Result have executed.

 public  void setActionEventListener(ActionEventListener listener)
Sets the action event listener to respond to key action events.

 public  void setResultCode(String resultCode)
Sets the result code, possibly overriding the one returned by the
action.

The "intended" purpose of this method is to allow PreResultListeners to
override the result code returned by the Action.

If this method is used before the Action executes, the Action's returned
result code will override what was set. However the Action could (if
specifically coded to do so) inspect the ActionInvocation to see that
someone "upstream" (e.g. an Interceptor) had suggested a value as the
result, and it could therefore return the same value itself.

If this method is called between the Action execution and the Result
execution, then the value set here will override the result code the
action had returned.  Creating an Interceptor that implements
PreResultListener  will give you this oportunity.

If this method is called after the Result has been executed, it will
have the effect of raising an IllegalStateException.