12.3 Writing Managed Bean Methods
Methods of a managed bean can perform several application-specific functions for components on the page. These functions include
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Performing processing associated with navigation
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Handling action events
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Performing validation on the component's value
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Handling value-change events
By using a managed bean to perform these functions, you eliminate the need to implement the javax.faces.validator.Validator
interface to handle the validation or one of the listener interfaces to handle events. Also, by using a managed bean instead of a Validator
implementation to perform validation, you eliminate the need to create a custom tag for the Validator
implementation.
In general, it is good practice to include these methods in the same managed bean that defines the properties for the components referencing these methods. The reason for doing so 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.
The following sections explain how to write various types of managed bean methods.
12.3.1 Writing a Method to Handle Navigation
An action method, a managed bean method that handles navigation processing, must be a public method that takes no parameters and returns an Object
, which is the logical outcome that the navigation system uses to determine the page to display next. This method is referenced using the component tag's action
attribute.
The following action method is from the managed bean CashierBean
, which is invoked when a user clicks the Submit button on the page. If the user has ordered more than $100 worth of books, this method sets the rendered
properties of the fanClub
and specialOffer
components to true
, causing them to be displayed on the page the next time that 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 re-render the page without creating a new view of the page, retaining the customer's input. If this method were to return purchase
, which is the logical outcome to use to advance to a payment page, the page would re-render without retaining the customer's input. In this case, you want to re-render the page without clearing the data.
If the user does not purchase more than $100 worth of books or if the thankYou
component has already been rendered, the method returns bookreceipt
. The JavaServer Faces implementation loads the bookreceipt.xhtml
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 { ... cart.clear(); return ("bookreceipt"); } }
Typically, an action method will return a String
outcome, as shown in the preceding example. Alternatively, you can define an Enum
class that encapsulates all possible outcome strings and then make an action method return an enum
constant, which represents a particular String
outcome defined by the Enum
class.
The following example uses an Enum
class to encapsulate all logical outcomes:
public enum Navigation { main, accountHist, accountList, atm, atmAck, transferFunds, transferAck, error }
When it returns an outcome, an action method uses the dot notation to reference the outcome from the Enum
class:
public Object submit(){ ... return Navigation.accountHist; }
The section Referencing a Method That Performs Navigation explains how a component tag references this method. The section Writing Properties Bound to Component Instances explains how to write the bean properties to which the components are bound.
12.3.2 Writing a Method to Handle an Action Event
A managed 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 javax.faces.component.ActionSource
can refer to this method.
In the following example, a method from a managed bean named ActionBean
processes the event of a user clicking one of the links on the page:
public void chooseBookFromLink(ActionEvent event) { String current = event.getComponent().getId(); FacesContext context = FacesContext.getCurrentInstance(); String bookId = books.get(current); context.getExternalContext().getSessionMap().put("bookId", bookId); }
This method gets the component that generated the event from the event object; then it gets the component's ID, which is a code for the book. The method matches the code against a HashMap
object that contains the book codes and corresponding book ID values. Finally, the method sets the book ID by using the selected value from the HashMap
object.
Referencing a Method That Handles an Action Event explains how a component tag references this method.
12.3.3 Writing a Method to Perform Validation
Instead of implementing the javax.faces.validator.Validator
interface to perform validation for a component, you can include a method in a managed bean to take care of validating input for the component. A managed bean method that performs validation must accept a javax.faces.context.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 managed bean method by using its validator
attribute. Only values of UIInput
components or values of components that extend UIInput
can be validated.
Here is an example of a managed bean method that validates user input, from The guessnumber-cdi CDI Example:
public void validateNumberRange(FacesContext context, UIComponent toValidate, Object value) { if (remainingGuesses <= 0) { ((UIInput) toValidate).setValid(false); FacesMessage message = new FacesMessage("No guesses left!"); context.addMessage(toValidate.getClientId(context), message); return; } int input = (Integer) value; if (input < minimum || input> maximum) { ((UIInput) toValidate).setValid(false); FacesMessage message = new FacesMessage("Invalid guess"); context.addMessage(toValidate.getClientId(context), message); } }
The validateNumberRange
method performs two different validations.
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If the user has run out of guesses, the method sets the
valid
property of theUIInput
component tofalse
. Then it queues a message onto theFacesContext
instance, associating the message with the component ID, and returns. -
If the user has some remaining guesses, the method then retrieves the local value of the component. If the input value is outside the allowable range, the method again sets the
valid
property of theUIInput
component tofalse
, queues a different message on theFacesContext
instance, and returns.
See Referencing a Method That Performs Validation for information on how a component tag references this method.
12.3.4 Writing a Method to Handle a Value-Change Event
A managed bean that handles a value-change event must use a public method that accepts a value-change event and returns void
. This method is referenced using the component's valueChangeListener
attribute. This section explains how to write a managed bean method to replace the javax.faces.event.ValueChangeListener
implementation.
The following example tag comes from Registering a Value-Change Listener on a Component, where the h:inputText
tag with the id
of name
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:
<h:inputText id="name" size="30" value="#{cashierBean.name}" required="true" requiredMessage="#{bundle.ReqCustomerName}"> <f:valueChangeListener type="javaeetutorial.dukesbookstore.listeners.NameChanged" /> </h:inputText>
Instead of implementing ValueChangeListener
, you can write a managed bean method to handle this event. To do this, you move the processValueChange(ValueChangeEvent)
method from the ValueChangeListener
class, called NameChanged
, to your managed bean.
Here is the managed bean method that processes the event of entering a value in the name
field on the page:
public void processValueChange(ValueChangeEvent event) throws AbortProcessingException { if (null != event.getNewValue()) { FacesContext.getCurrentInstance().getExternalContext(). getSessionMap().put("name", event.getNewValue()); } }
To make this method handle the ValueChangeEvent
generated by an input component, reference this method from the component tag's valueChangeListener
attribute. See Referencing a Method That Handles a Value-Change Event for more information.