Writing Backing Bean Methods - The J2EE 1.4 Tutorial

Writing Backing Bean Methods

Methods of a backing bean perform application-specific functions for components on the page. These functions include performing validation on the component's value, handling action events, handling value-change events, and performing processing associated with navigation.

By using a backing bean to perform these functions, you eliminate the need to implement the Validator interface to handle the validation or the Listener interface to handle events. Also, by using a backing bean instead of a Validator implementation to perform validation, you eliminate the need to create a custom tag for the Validator implementation. Creating a Custom Validator describes implementing a custom validator. Implementing an Event Listener describes implementing a listener class.

In general, it's good practice to include these methods in the same backing bean that defines the properties for the components referencing these methods. The reason is that the methods might need to access the component's data to determine how to handle the event or to perform the validation associated with the component.

This section describes the requirements for writing the backing bean methods.

Writing a Method to Handle Navigation

A backing bean method that handles navigation processing--called an action method--must be a public method that takes no parameters and returns a String, which is the logical outcome string that the navigation system uses to determine what page to display next. This method is referenced using the component tag's action attribute.

The following action method in CashierBean is invoked when a user clicks the Submit button on the bookcashier.jsp page. If the user has ordered more than $100 (or 100 euros) worth of books, this method sets the rendered properties of the fanClub and specialOffer components to true. This causes them to be displayed on the page the next time the page is rendered.

After setting the components' rendered properties to true, this method returns the logical outcome null. This causes the JavaServer Faces implementation to rerender the bookcashier.jsp page without creating a new view of the page. If this method were to return purchase (which is the logical outcome to use to advance to bookcashier.jsp, as defined by the application configuration resource file), the bookcashier.jsp page would rerender without retaining the customer's input. In this case, we want to rerender the page without clearing the data.

If the user does not purchase more than $100 (or 100 euros) worth of books or the thankYou component has already been rendered, the method returns receipt.

The default NavigationHandler provided by the JavaServer Faces implementation matches the logical outcome, as well as the starting page (bookcashier.jsp) against the navigation rules in the application configuration resource file to determine which page to access next. In this case, the JavaServer Faces implementation loads the bookreceipt.jsp page after this method returns.
public String submit() {
  ...
  if(cart().getTotal() > 100.00 && 
    !specialOffer.isRendered())
  {
    specialOfferText.setRendered(true);
    specialOffer.setRendered(true);
    return null;
  } else if (specialOffer.isRendered() && 
    !thankYou.isRendered()){
    thankYou.setRendered(true);
    return null;
  } else {
    clear();
    return ("receipt");
  }
} 
How the Pieces Fit Together provides more detail on this example. Referencing a Method That Performs Navigation explains how a component tag references this method. Binding a Component Instance to a Bean Property discusses how the page author can bind these components to bean properties. Writing Properties Bound to Component Instances discusses how to write the bean properties to which the components are bound. Configuring Navigation Rules provides more information on configuring navigation rules.

Writing a Method to Handle an Action Event

A backing bean method that handles an action event must be a public method that accepts an action event and returns void. This method is referenced using the component tag's actionListener attribute. Only components that implement ActionSource can refer to this method.

The following backing bean method from LocaleBean of the Duke's Bookstore application processes the event of a user clicking one of the hyperlinks on the chooselocale.jsp page:
public void chooseLocaleFromLink(ActionEvent event) {
  String current = event.getComponent().getId();
  FacesContext context = FacesContext.getCurrentInstance();
  context.getViewRoot().setLocale((Locale)
    locales.get(current));
} 
This method gets the component that generated the event from the event object. Then it gets the component's ID. The ID indicates a region of the world. The method matches the ID against a HashMap object that contains the locales available for the application. Finally, it sets the locale using the selected value from the HashMap object.

Referencing a Method That Handles an Action Event explains how a component tag references this method.

Writing a Method to Perform Validation

Rather than implement the Validator interface to perform validation for a component, you can include a method in a backing bean to take care of validating input for the component.

A backing bean method that performs validation must accept a FacesContext, the component whose data must be validated, and the data to be validated, just as the validate method of the Validator interface does. A component refers to the backing bean method via its validator attribute. Only values of UIInput components or values of components that extend UIInput can be validated.

Here is the backing bean method of CheckoutFormBean from the Coffee Break example:
public void validateEmail(FacesContext context, 
  UIComponent toValidate, Object value) {
  
  String message = "";
  String email = (String) value;
  if (email.indexOf('@') == -1) {
    ((UIInput)toValidate).setValid(false);
    message = CoffeeBreakBean.loadErrorMessage(context, 
      CoffeeBreakBean.CB_RESOURCE_BUNDLE_NAME,
      "EMailError");
    context.addMessage(toValidate.getClientId(context),
      new FacesMessage(message));
  }
} 
The validateEmail method first gets the local value of the component. It then checks whether the @ character is contained in the value. If it isn't, the method sets the component's valid property to false. The method then loads the error message and queues it onto the FacesContext instance, associating the message with the component ID.

See Referencing a Method That Performs Validation for information on how a component tag references this method.

Writing a Method to Handle a Value-Change Event

A backing bean that handles a value-change event must be a public method that accepts a value-change event and returns void. This method is referenced using the component's valueChangeListener attribute.

The Duke's Bookstore application does not have any backing bean methods that handle value-change events. It does have a ValueChangeListener implementation, as explained in the Implementing Value-Change Listeners section.

For illustration, this section explains how to write a backing bean method that can replace the ValueChangeListener implementation.

As explained in Registering a Value-Change Listener on a Component, the name component of the bookcashier.jsp page has a ValueChangeListener instance registered on it. This ValueChangeListener instance handles the event of entering a value in the field corresponding to the component. When the user enters a value, a value-change event is generated, and the processValueChange(ValueChangeEvent) method of the ValueChangeListener class is invoked.

Instead of implementing ValueChangeListener, you can write a backing bean method to handle this event. To do this, you move the processValueChange(ValueChangeEvent) method from the ValueChangeListener class, called NameChanged, to your backing bean.

Here is the backing bean method that processes the event of entering a value in the name field on the bookcashier.jsp page:
public void processValueChange(ValueChangeEvent event)
  throws AbortProcessingException {
  if (null != event.getNewValue()) {
    FacesContext.getCurrentInstance().
      getExternalContext().getSessionMap().
        put("name", event.getNewValue());
  }
}   
The page author can make this method handle the ValueChangeEvent object emitted by a UIInput component by referencing this method from the component tag's valueChangeListener attribute. See Referencing a Method That Handles a Value-change Event for more information.