protected int currentOption = null;
public int getCurrentOption(){...}
public void setCurrentOption(int option){...}
Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial |
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A component tag can wire its data to a managed bean by one of the following methods:
Binding its component’s value to a bean property
Binding its component’s instance to a bean property
To bind a component’s value to a managed bean property, a component
tag’s value
attribute uses an EL value expression. To bind a component
instance to a bean property, a component tag’s binding
attribute uses
a value expression.
When a component instance is bound to a managed bean property, the property holds the component’s local value. Conversely, when a component’s value is bound to a managed bean property, the property holds the value stored in the managed bean. This value is updated with the local value during the Update Model Values phase of the lifecycle. There are advantages to both of these methods.
Binding a component instance to a bean property has the following advantages.
The managed bean can programmatically modify component attributes.
The managed bean can instantiate components rather than let the page author do so.
Binding a component’s value to a bean property has the following advantages.
The page author has more control over the component attributes.
The managed bean has no dependencies on the JavaServer Faces API (such as the component classes), allowing for greater separation of the presentation layer from the model layer.
The JavaServer Faces implementation can perform conversions on the data based on the type of the bean property without the developer needing to apply a converter.
In most situations, you will bind a component’s value rather than its
instance to a bean property. You’ll need to use a component binding only
when you need to change one of the component’s attributes dynamically.
For example, if an application renders a component only under certain
conditions, it can set the component’s rendered
property accordingly
by accessing the property to which the component is bound.
When referencing the property using the component tag’s value
attribute, you need to use the proper syntax. For example, suppose a
managed bean called MyBean
has this int
property:
protected int currentOption = null;
public int getCurrentOption(){...}
public void setCurrentOption(int option){...}
The value
attribute that references this property must have this
value-binding expression:
#{myBean.currentOption}
In addition to binding a component’s value to a bean property, the
value
attribute can specify a literal value or can map the component’s
data to any primitive (such as int
), structure (such as an array), or
collection (such as a list), independent of a JavaBeans component.
Table 15-3 lists some example value-binding expressions
that you can use with the value
attribute.
Table 15-3 Examples of Value-Binding Expressions
Value |
Expression |
A Boolean |
|
A property initialized from a context initialization parameter |
|
A bean property |
|
A value in an array |
|
A value in a collection |
|
A property of an object in an array of objects |
|
The next two sections explain how to use the value
attribute to bind a
component’s value to a bean property or other data objects and how to
use the binding
attribute to bind a component instance to a bean
property.
To bind a component’s value to a managed bean property, you specify the
name of the bean and the property using the value
attribute.
This means that the first part of the EL value expression must match the
name of the managed bean up to the first period (.
) and the part of
the value expression after the period must match the property of the
managed bean.
For example, in the Duke’s Bookstore case study, the h:dataTable
tag
in bookcatalog.xhtml
sets the value of the component to the value of
the books
property of the BookstoreBean
backing bean, whose name is
store
:
<h:dataTable id="books"
value="#{store.books}"
var="book"
headerClass="list-header"
styleClass="list-background"
rowClasses="list-row-even, list-row-odd"
border="1"
summary="#{bundle.BookCatalog}">
The value is obtained by calling the backing bean’s getBooks
method,
which in turn calls the BookRequestBean
session bean’s getBooks
method.
If you use the application configuration resource file to configure
managed beans instead of defining them in managed bean classes, the name
of the bean in the value
expression must match the managed-bean-name
element of the managed bean declaration up to the first period (.
) in
the expression. Similarly, the part of the value expression after the
period must match the name specified in the corresponding
property-name
element in the application configuration resource file.
For example, consider this managed bean configuration, which configures
the ImageArea
bean corresponding to the top-left book in the image map
on the index.html
page of the Duke’s Bookstore case study:
<managed-bean eager="true">
...
<managed-bean-name>Book201</managed-bean-name>
<managed-bean-class>dukesbookstore.model.ImageArea</managed-bean-class>
<managed-bean-scope>application</managed-bean-scope>
<managed-property>
...
<property-name>shape</property-name>
<value>rect</value>
</managed-property>
<managed-property>
...
<property-name>alt</property-name>
<value>Duke</value>
</managed-property>
...
This example configures a bean called Book201
, which has several
properties, one of which is called shape
.
Although the bookstore:area
tags on the index.xhtml
page do not bind
to an ImageArea
property (they bind to the bean itself), you could
refer to the property using a value expression from the value
attribute of the component’s tag:
<h:outputText value="#{Book201.shape}" />
See Configuring Managed Beans for information on how to configure beans in the application configuration resource file.
One external data source that a value
attribute can refer to is an
implicit object.
The bookreceipt.xhtml
page of the Duke’s Bookstore case study has a
reference to an implicit object:
<h:outputFormat title="thanks"
value="#{bundle.ThankYouParam}">
<f:param value="#{sessionScope.name}"/>
</h:outputFormat>
This tag gets the name of the customer from the session scope and
inserts it into the parameterized message at the key ThankYouParam
from the resource bundle. For example, if the name of the customer is
Gwen Canigetit, this tag will render:
Thank you, Gwen Canigetit, for purchasing your books from us.
Retrieving values from other implicit objects is done in a similar way
to the example shown in this section. Table 15-4 lists the
implicit objects to which a value attribute can refer. All of the
implicit objects, except for the scope objects, are read-only and
therefore should not be used as values for a UIInput
component.
Table 15-4 Implicit Objects
Implicit Object |
What It Is |
|
A |
|
A |
|
The |
|
A |
|
A |
|
A |
|
A |
|
A |
|
A |
|
A |
|
The root |
A component instance can be bound to a bean property using a value
expression with the binding
attribute of the component’s tag. You
usually bind a component instance rather than its value to a bean
property if the bean must dynamically change the component’s attributes.
Here are two tags from the bookcashier.xhtml
page that bind components
to bean properties:
<h:selectBooleanCheckbox id="fanClub"
rendered="false"
binding="#{cashierBean.specialOffer}" />
<h:outputLabel for="fanClub"
rendered="false"
binding="#{cashierBean.specialOfferText}"
value="#{bundle.DukeFanClub}"/>
</h:outputLabel>
The h:selectBooleanCheckbox
tag renders a check box and binds the
fanClub
UISelectBoolean
component to the specialOffer
property of
the cashier
bean. The h:outputLabel
tag binds the component
representing the check box’s label to the specialOfferText
property of
the cashier
bean. If the application’s locale is English, the
h:outputLabel
tag renders
I'd like to join the Duke Fan Club, free with my purchase of over $100
The rendered
attributes of both tags are set to false
to prevent the
check box and its label from being rendered. If the customer makes a
large order and clicks the Submit button, the submit
method of
CashierBean
sets both components' rendered
properties to true
,
causing the check box and its label to be rendered.
These tags use component bindings rather than value bindings because the
managed bean must dynamically set the values of the components'
rendered
properties.
If the tags were to use value bindings instead of component bindings,
the managed bean would not have direct access to the components and
would therefore require additional code to access the components from
the FacesContext
instance to change the components' rendered
properties.
Writing Properties Bound to Component Instances explains how to write the bean properties bound to the example components.
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