Backing Beans
A typical JavaServer Faces application includes one or more backing beans, each of which is a type of JavaServer Faces managed bean that can be associated with the components used in a particular page. This section introduces the basic concepts on creating, configuring, and using backing beans in an application.
Creating a Backing Bean
A backing bean is created with a constructor with no arguments (like all JavaBeansTM components), and also a set of properties and a set of methods that perform functions for a component.
Each of the component properties can be bound to one of the following:
A component’s value
A component instance
A converter instance
A listener instance
A validator instance
The most common functions that backing bean methods perform include the following:
Validating a component’s data
Handling an event fired by a component
Performing processing to determine the next page to which the application must navigate
As with all JavaBeans components, a property consists of a private data field and a set of accessor methods, as shown by this code:
Integer userNumber = null;
...
public void setUserNumber(Integer user_number) {
userNumber = user_number;
}
public Integer getUserNumber() {
return userNumber;
}
public String getResponse() {
...
}When a bean property is bound to a component’s value, it can be any of the basic primitive and numeric types, or any Java object type for which the application has access to an appropriate converter. For example, a property can be of type Date if the application has access to a converter that can convert the Date type to a String and back again. See Writing Bean Properties for information on which types are accepted by which component tags.
When a bean property is bound to a component instance, the property’s type must be the same as the component object. For example, if a SelectBoolean component is bound to the property, the property must accept and return a SelectBoolean object.
Likewise, if the property is bound to a converter, validator, or listener instance, then the property must be of the appropriate converter, validator, or listener type.
For more information on writing beans and their properties, see Writing Bean Properties.
Using the EL to Reference Backing Beans
To bind component values and objects to backing bean properties or to reference backing bean methods from component tags, page authors use the unified expression language (EL) syntax. As explained in Overview of EL, the following are some of the features that EL offers:
Deferred evaluation of expressions
The ability to use a value expression to both read and write data
Method expressions
Deferred evaluation of expressions is important because the JavaServer Faces lifecycle is split into several phases where component event handling, data conversion and validation, and data propagation to external objects are all performed in an orderly fashion. The implementation must be able to delay the evaluation of expressions until the proper phase of the life cycle has been reached. Therefore, its tag attributes always use deferred evaluation syntax, which is distinguished by the #{} delimiter.
In order to store data in external objects, almost all JavaServer Faces tag attributes use lvalue value expressions, which are expressions that allow both getting and setting data on external objects.
Finally, some component tag attributes accept method expressions that reference methods that handle component events, or validate or convert component data.
To illustrate a JavaServer Faces tag using EL, let’s suppose that a tag of an application referenced a method to perform the validation of user input:
<h:inputText id="userNo"
value="#{UserNumberBean.userNumber}"
validator="#{UserNumberBean.validate}" />This tag binds the userNo component’s value to the UserNumberBean.userNumber backing bean property using an lvalue expression. It uses a method expression to refer to the UserNumberBean.validate method, which performs validation of the component’s local value. The local value is whatever the user enters into the field corresponding to this tag. This method is invoked when the expression is evaluated, which is during the process validation phase of the life cycle.
Nearly all JavaServer Faces tag attributes accept value expressions. In addition to referencing bean properties, value expressions can also reference lists, maps, arrays, implicit objects, and resource bundles.
Another use of value expressions is binding a component instance to a backing bean property. A page author does this by referencing the property from the binding attribute:
<inputText binding="#{UserNumberBean.userNoComponent}" />In addition to using expressions with the standard component tags, you can also configure your custom component properties to accept expressions by creating ValueExpression or MethodExpression instances for them.
For information on EL, see Chapter 6, Unified Expression Language.
For information on referencing backing bean methods from component tags, see Referencing a Backing Bean Method.