Binding Component Values and Instances to External Data Sources

As explained in Backing Bean Management, a component tag can wire its component's data to a back-end data object by doing one of the following:

A component tag's value attribute uses a value-binding expression to bind a component's value to an external data source, such as a bean property. A component tag's binding attribute uses a value-binding expression to bind a component instance to a bean property.

When referencing the property using the component tag's value attribute, you need to use the proper syntax. For example, suppose a backing bean called MyBean has this int property:

The value attribute that references this property must have this value-binding expression:

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 18-8 lists some example value-binding expressions that you can use with the value attribute.

Table 18-8 Example Value-binding Expressions
Value	Expression
A Boolean	`cart.numberOfItems > 0`
A property initialized from a context `init` parameter	`initParam.quantity`
A bean property	`CashierBean.name`
Value in an array	`books[3]`
Value in a collection	`books["fiction"]`
Property of an object in an array of objects	`books[3].price`

The next two sections explain in more detail how to use the value attribute to bind a component's value to a bean property or other external data sources and how to use the binding attribute to bind a component instance to a bean property

Binding a Component Value to a Property

To bind a component's value to a bean property, you specify the name of the bean and the property using the value attribute. As explained in Backing Bean Management, the value-binding expression of the component tag's value attribute must match the corresponding managed bean declaration in the application configuration resource file.

This means that the name of the bean in the value-binding expression must match the managed-bean-name element of the managed bean declaration up to the first . in the expression. Similarly, the part of the value-binding expression after the . 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 North America part of the image map on the chooselocale.jsp page of the Duke's Bookstore application:

This example configures a bean called NA, which has several properties, one of which is called shape.

Although the area tags on the chooselocale.jsp page do not bind to an ImageArea property (they bind to the bean itself), to do this, you refer to the property using a value-binding expression from the value attribute of the component's tag:

Much of the time you will not include definitions for a managed bean's properties when configuring it. You need to define a property and its value only when you want the property to be initialized with a value when the bean is initialized.

If a component tag's value attribute must refer to a property that is not initialized in the managed-bean configuration, the part of the value-binding expression after the . must match the property name as it is defined in the backing bean.

Writing Component Properties explains in more detail how to write the backing bean properties for each of the component types.

Binding a Component Value to an Implicit Object

One external data source that a value attribute can refer to is an implicit object.

The bookreceipt.jsp page of the Duke's Bookstore application includes a reference to an implicit object from a parameter substitution tag:

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:

The name tag on the bookcashier.jsp page has the NameChanged listener implementation registered on it. This listener saves the customer's name in the session scope when the bookcashier.jsp page is submitted. See Implementing Value-Change Listeners for more information on how this listener works. See Registering a Value-Change Listener on a Component to learn how the listener is registered on the tag.

Retrieving values from other implicit objects is done in a similar way to the example shown in this section. Table 18-9 lists the implicit objects that a value attribute can refer to. All of the implicit objects except for the scope objects are read-only and therefore should not be used as a value for a UIInput component.

Table 18-9 Implicit Objects
Implicit Object	What It Is
`applicationScope`	A `Map` of the application scope attribute values, keyed by attribute name
`cookie`	A `Map` of the cookie values for the current request, keyed by cookie name
`facesContext`	The `FacesContext` instance for the current request
`header`	A `Map` of HTTP header values for the current request, keyed by header name
`headerValues`	A `Map` of `String` arrays containing all the header values for HTTP headers in the current request, keyed by header name
`initParam`	A `Map` of the context initialization parameters for this Web application
`param`	A `Map` of the request parameters for this request, keyed by parameter name
`paramValues`	A `Map` of `String` arrays containing all the parameter values for request parameters in the current request, keyed by parameter name
`requestScope`	A `Map` of the request attributes for this request, keyed by attribute name
`sessionScope`	A `Map` of the session attributes for this request, keyed by attribute name
`view`	The root `UIComponent` in the current component tree stored in the `FacesRequest` for this request

Binding a Component Instance to a Bean Property

A component instance can be bound to a bean property using a value-binding 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.jsp page that bind components to bean properties:

The selectBooleanCheckbox tag renders a checkbox and binds the fanClub UISelectBoolean component to the specialOffer property of CashierBean. The outputLabel tag binds the component representing the checkbox's label to the specialOfferText property of CashierBean. If the application's locale is English, the outputLabel tag renders:

The rendered attributes of both tags are set to false, which prevents the checkbox and its label from being rendered. If the customer orders more than $100 (or 100 euros) worth of books and clicks the Submit button, the submit method of CashierBean sets both components' rendered properties to true, causing the checkbox and its label to be rendered.

These tags use component bindings rather than value bindings because the backing bean must dynamically set the values of the components' rendered properties.

If the tags were to use value bindings instead of component bindings, the backing bean would not have direct access to the components, and would therefore require additional code to access the components from the FacesContext to change the components' rendered properties.

Writing Properties Bound to Component Instances explains how to write the bean properties bound to the example components and also discusses how the submit method sets the rendered properties of the components.