Differences between action and actionListener

Question

What is the difference between action and actionListener, and when should I use action versus actionListener?

Blaise

Answer 1

actionListener

Use actionListener if you want have a hook before the real business action get executed, e.g. to log it, and/or to set an additional property (by <f:setPropertyActionListener>), and/or to have access to the component which invoked the action (which is available by ActionEvent argument). So, purely for preparing purposes before the real business action gets invoked.

The actionListener method has by default the following signature:

import javax.faces.event.ActionEvent;
// ...

public void actionListener(ActionEvent event) {
    // ...
}

And it's supposed to be declared as follows, without any method parentheses:

<h:commandXxx ... actionListener="#{bean.actionListener}" />

Note that you can't pass additional arguments by EL 2.2. You can however override the ActionEvent argument altogether by passing and specifying custom argument(s). The following examples are valid:

<h:commandXxx ... actionListener="#{bean.methodWithoutArguments()}" />
<h:commandXxx ... actionListener="#{bean.methodWithOneArgument(arg1)}" />
<h:commandXxx ... actionListener="#{bean.methodWithTwoArguments(arg1, arg2)}" />

public void methodWithoutArguments() {}
public void methodWithOneArgument(Object arg1) {}
public void methodWithTwoArguments(Object arg1, Object arg2) {}

Note the importance of the parentheses in the argumentless method expression. If they were absent, JSF would still expect a method with ActionEvent argument.

If you're on EL 2.2+, then you can declare multiple action listener methods via <f:actionListener binding>.

<h:commandXxx ... actionListener="#{bean.actionListener1}">
    <f:actionListener binding="#{bean.actionListener2()}" />
    <f:actionListener binding="#{bean.actionListener3()}" />
</h:commandXxx>

public void actionListener1(ActionEvent event) {}
public void actionListener2() {}
public void actionListener3() {}

Note the importance of the parentheses in the binding attribute. If they were absent, EL would confusingly throw a javax.el.PropertyNotFoundException: Property 'actionListener1' not found on type com.example.Bean, because the binding attribute is by default interpreted as a value expression, not as a method expression. Adding EL 2.2+ style parentheses transparently turns a value expression into a method expression. See also a.o. Why am I able to bind <f:actionListener> to an arbitrary method if it's not supported by JSF?

---

action

Use action if you want to execute a business action and if necessary handle navigation. The action method can (thus, not must) return a String which will be used as navigation case outcome (the target view). A return value of null or void will let it return to the same page and keep the current view scope alive. A return value of an empty string or the same view ID will also return to the same page, but recreate the view scope and thus destroy any currently active view scoped beans and, if applicable, recreate them.

The action method can be any valid MethodExpression, also the ones which uses EL 2.2 arguments such as below:

<h:commandXxx value="submit" action="#{bean.edit(item)}" />

With this method:

public void edit(Item item) {
    // ...
}

Note that when your action method solely returns a string, then you can also just specify exactly that string in the action attribute. Thus, this is totally clumsy:

<h:commandLink value="Go to next page" action="#{bean.goToNextpage}" />

With this senseless method returning a hardcoded string:

public String goToNextpage() {
    return "nextpage";
}

Instead, just put that hardcoded string directly in the attribute:

<h:commandLink value="Go to next page" action="nextpage" />

Please note that this in turn indicates a bad design: navigating by POST. This is not user nor SEO friendly. This all is explained in When should I use h:outputLink instead of h:commandLink? and is supposed to be solved as

<h:link value="Go to next page" outcome="nextpage" />

See also How to navigate in JSF? How to make URL reflect current page (and not previous one).

---

f:ajax listener

Since JSF 2.x there's a third way, the <f:ajax listener>.

<h:commandXxx ...>
    <f:ajax listener="#{bean.ajaxListener}" />
</h:commandXxx>

The ajaxListener method has by default the following signature:

import javax.faces.event.AjaxBehaviorEvent;
// ...

public void ajaxListener(AjaxBehaviorEvent event) {
    // ...
}

In Mojarra, the AjaxBehaviorEvent argument is optional, below works as good.

public void ajaxListener() {
    // ...
}

But in MyFaces, it would throw a MethodNotFoundException. Below works in both JSF implementations when you want to omit the argument.

<h:commandXxx ...>
    <f:ajax execute="@form" listener="#{bean.ajaxListener()}" render="@form" />
</h:commandXxx>

Ajax listeners are not really useful on command components. They are more useful on input and select components <h:inputXxx>/<h:selectXxx>. In command components, just stick to action and/or actionListener for clarity and better self-documenting code. Moreover, like actionListener, the f:ajax listener does not support returning a navigation outcome.

<h:commandXxx ... action="#{bean.action}">
    <f:ajax execute="@form" render="@form" />
</h:commandXxx>

For explanation on execute and render attributes, head to Understanding PrimeFaces process/update and JSF f:ajax execute/render attributes.

---

Invocation order

The actionListeners are always invoked before the action in the same order as they are been declared in the view and attached to the component. The f:ajax listener is always invoked before any action listener. So, the following example:

<h:commandButton value="submit" actionListener="#{bean.actionListener}" action="#{bean.action}">
    <f:actionListener type="com.example.ActionListenerType" />
    <f:actionListener binding="#{bean.actionListenerBinding()}" />
    <f:setPropertyActionListener target="#{bean.property}" value="some" />
    <f:ajax listener="#{bean.ajaxListener}" />
</h:commandButton>

Will invoke the methods in the following order:

1. Bean#ajaxListener()
2. Bean#actionListener()
3. ActionListenerType#processAction()
4. Bean#actionListenerBinding()
5. Bean#setProperty()
6. Bean#action()

---

Exception handling

The actionListener supports a special exception: AbortProcessingException. If this exception is thrown from an actionListener method, then JSF will skip any remaining action listeners and the action method and proceed to render response directly. You won't see an error/exception page, JSF will however log it. This will also implicitly be done whenever any other exception is being thrown from an actionListener. So, if you intend to block the page by an error page as result of a business exception, then you should definitely be performing the job in the action method.

If the sole reason to use an actionListener is to have a void method returning to the same page, then that's a bad one. The action methods can perfectly also return void, on the contrary to what some IDEs let you believe via EL validation. Note that the PrimeFaces showcase examples are littered with this kind of actionListeners over all place. This is indeed wrong. Don't use this as an excuse to also do that yourself.

In ajax requests, however, a special exception handler is needed. This is regardless of whether you use listener attribute of <f:ajax> or not. For explanation and an example, head to Exception handling in JSF ajax requests.

Eri

You're right that exceptions in actionListeners are swallowed by default, but in JSF 2.0 this behavior can be altered. See my answer below for details. – Lenrow

@arjan: you're right that JSF 2.0 allows you to change the default handling of exceptions thrown by actionListener, but that still doesn't make it a good excuse to abuse actionListener for business actions. – Eri

Indeed, business actions are in the main "flow" of the request/response cycle and only the action corresponds with that. actionListener is for secondary stuff. Just wanted to clarify that exceptions from actionListeners can be propagated if so required ;) – Lenrow

Hi BalusC, I try to confirm your answer by writing some code, but the result is a bit odd. Will you please look at this question stackoverflow.com/questions/8683211/…? – Allembracing

Note that the class name action listener signature is void processAction(ActionEvent event). – Stateroom

@Kawy: the method name is free to choice when used in actionListener attribute and it has to be public as well. The processAction name is only mandatory when you're using <f:actionListener type>, simply because the type has to implement ActionListener interface which has exactly that method name processAction definied. – Eri

Yes, by "class name action listener" I meant the <f:actionListener> tag (without the void). – Stateroom

@Eri , may I ask, what would the order of execution be if there's a nested f:ajax with a listener attribute, how would the execution flow go? – Leora

@Muhammed: ajax action listener is invoked before all regular action listeners. Note that even when using <f:ajax>, you'd in case of command components prefer using the action attribute for business actions. E.g. <h:commandButton action="#{bean.businessAction}"><f:ajax/></h:commandButton>. – Eri

This <h:link value="Go to next page" outcome="nextpage" /> just won't work in PrimeFaces. <h:outputLink works – Gravois

@Vlad: that problem is caused elsewhere and completely unrelated to the current Q&A. I believe you intended to post that comment on a different post. – Eri

@BalusC: The problem that I mentioned actually happens inside a <p:treeTable>, in Liferay. Could be related to that. Thanks. – Gravois

@BalusC: returning a null value from an action method loses the request parameters. Could you please explain how can they be kept in these circumstances? Thanks! – Arvonio

@Neil: That's likely a misobservation. Same problem will occur when returning non-null. Head on to https://mcmap.net/q/24326/-retaining-get-request-query-string-parameters-on-jsf-form-submit. – Eri

@BalusC: I'd found that too but was wanting to distinguish between returning null and using void. The post you referenced was for redirecting to another page. If the action needs to return to the same page after a validation failure, how is that best done in order to keep the parameters but not call the PostConstruct method which in my case will reinitialise all the values? – Arvonio

@Eri In the invocation order can we say that the disabled condition is re evaluated before everything else (before 1) and might prevent running everything else? If so, is that a standard? – Jaclyn

@ForguesR: yes but this happens during an earlier phase already, the apply request values phase. See also point 6 of https://mcmap.net/q/17145/-commandbutton-commandlink-ajax-action-listener-method-not-invoked-or-input-value-not-set-updated – Eri

Answer 2

As BalusC indicated, the actionListener by default swallows exceptions, but in JSF 2.0 there is a little more to this. Namely, it doesn't just swallows and logs, but actually publishes the exception.

This happens through a call like this:

context.getApplication().publishEvent(context, ExceptionQueuedEvent.class,                                                          
    new ExceptionQueuedEventContext(context, exception, source, phaseId)
);

The default listener for this event is the ExceptionHandler which for Mojarra is set to com.sun.faces.context.ExceptionHandlerImpl. This implementation will basically rethrow any exception, except when it concerns an AbortProcessingException, which is logged. ActionListeners wrap the exception that is thrown by the client code in such an AbortProcessingException which explains why these are always logged.

This ExceptionHandler can be replaced however in faces-config.xml with a custom implementation:

<exception-handlerfactory>
   com.foo.myExceptionHandler
</exception-handlerfactory>

Instead of listening globally, a single bean can also listen to these events. The following is a proof of concept of this:

@ManagedBean
@RequestScoped
public class MyBean {

    public void actionMethod(ActionEvent event) {

        FacesContext.getCurrentInstance().getApplication().subscribeToEvent(ExceptionQueuedEvent.class, new SystemEventListener() {

        @Override
        public void processEvent(SystemEvent event) throws AbortProcessingException {
            ExceptionQueuedEventContext content = (ExceptionQueuedEventContext)event.getSource();
            throw new RuntimeException(content.getException());
        }

        @Override
        public boolean isListenerForSource(Object source) {
            return true;
        }
        });

        throw new RuntimeException("test");
    }

}

(note, this is not how one should normally code listeners, this is only for demonstration purposes!)

Calling this from a Facelet like this:

<html xmlns="http://www.w3.org/1999/xhtml"
    xmlns:h="http://java.sun.com/jsf/html"
    xmlns:f="http://java.sun.com/jsf/core">
    <h:body>
        <h:form>
            <h:commandButton value="test" actionListener="#{myBean.actionMethod}"/>
        </h:form>
    </h:body>
</html>

Will result in an error page being displayed.

Lenrow

Answer 3

ActionListener gets fired first, with an option to modify the response, before Action gets called and determines the location of the next page.

If you have multiple buttons on the same page which should go to the same place but do slightly different things, you can use the same Action for each button, but use a different ActionListener to handle slightly different functionality.

Here is a link that describes the relationship:

http://www.java-samples.com/showtutorial.php?tutorialid=605

Irony

plus one, The bold letters says almost everything. – Bluegrass

Answer 4

TL;DR:

The ActionListeners (there can be multiple) execute in the order they were registered BEFORE the action

Long Answer:

A business action typically invokes an EJB service and if necessary also sets the final result and/or navigates to a different view if that is not what you are doing an actionListener is more appropriate i.e. for when the user interacts with the components, such as h:commandButton or h:link they can be handled by passing the name of the managed bean method in actionListener attribute of a UI Component or to implement an ActionListener interface and pass the implementation class name to actionListener attribute of a UI Component.

Bulgarian